ABCs of Probes

ABCs of ProbesPrimer

ABCs of ProbesPrimerPrecision Measurements Start at

the Probe TipAs you'll learn in this primer,precision measurements start at theprobe right probes matched to your oscilloscope are vital toachieving the greatest signal fidelity and measurement select the right probe for your specific application,please requestthe probe selection CD from your local Tektronix representative,orvisit / continually expanding library of technical briefs,applicationnotes and other resources will help ensure you get the most out

of your probes and other contact your localTektronix representative or visit /accessoriesSafety SummaryWhen making measurements on electrical or electronic systems

or circuitry,personal safety is of paramount surethat you understand the capabilities and limitations of the measuringequipment that you’re ,before making any measurements,become thoroughly familiar with the system or circuitry that you

will be all documentation and schematics for

the system being measured,paying particular attention to the levelsand locations of voltages in the circuit and heeding any and all

cautionary onally,be sure to review the following safety precautions toavoid personal injury and to prevent damage to the measuringequipment or the systems to which it is additionalexplanation of any of the following precautions,please refer toSafety Precautions.— Observe All Terminal Ratings— Use Proper Grounding Procedures— Connect and Disconnect Probes Properly— Avoid Exposed Circuitry— Avoid RF Burns While Handling Probes— Do Not Operate Without Covers— Do Not Operate in Wet/Damp Conditions— Do Not Operate in an Explosive Atmosphere— Do Not Operate with Suspected Failures— Keep Probe Surfaces Clean and Dry— Do Not Immerse Probes in Liquids

ABCs of ProbesPrimerTable of ContentsProbes – The Critical Link To Measurement Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5What is a Probe? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5The Ideal Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6The Realities of Probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7Choosing the Right Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10Some Probing Tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12Different Probes for Different Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Why So Many Probes? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Different Probe Types and Their Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14Floating Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19Probe Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20How Probes Affect Your Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22The Effect of Source Impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22Capacitive Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22Bandwidth Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24What To Do About Probing Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28Understanding Probe Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29Aberrations (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29Amp-Second Product (current probes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29Attenuation Factor (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29Accuracy (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29Bandwidth (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30Capacitance (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30CMRR (differential probes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30CW Frequency Current Derating (current probes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Decay Time Constant (current probes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Direct Current (current probes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Insertion Impedance (current probes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Input Capacitance (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Input Resistance (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Maximum Input Current Rating (current probes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Maximum Peak Pulse Current Rating (current probes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ./accessories3

ABCs of ProbesPrimerMaximum Voltage Rating (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Propagation Delay (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Rise Time (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Tangential Noise (active probes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Temperature Range (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31A Guide to Probe Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32Understanding the Signal Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32Oscilloscope Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34Selecting the Right Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35Advanced Probing Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36Ground Lead Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36Differential Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39Small Signal Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42Explanation of Safety Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44Observe All Terminal Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44Use Proper Grounding Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44Connect and Disconnect Probes Properly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44Avoid Exposed Circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45Avoid RF Burns While Handling Probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45Do Not Operate Without Covers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45Do Not Operate in Wet/Damp Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45Do Not Operate in an Explosive Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45Do Not Operate with Suspected Failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45Keep Probe Surfaces Clean and Dry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45Do Not Immerse Probes in Liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ./accessories

Probes – The Critical Link toMeasurement QualityProbesare vital to oscilloscope understand howvital,disconnect the probes from an oscilloscope and try to make

a can’t be has to be some kind ofelectrical connection,a probe of some sort between the signal to

be measured and the oscilloscope’s input addition to being vital to oscilloscope measurements,probes arealso critical to measurement ting a probe to a circuitcan affect the operation of the circuit,and an oscilloscope can

only display and measure the signal that the probe delivers to theoscilloscope ,it is imperative that the probe have

minimum impact on the probed circuit and that it maintain adequatesignal fidelityfor the desired the probe doesn’t maintain signal fidelity,if it changes the signalin any way or changes the way a circuit operates,the oscilloscopesees a distorted version of the actual result can bewrong or misleading essence,the probe is the first link in the oscilloscope the strength of this measurement chain relies as muchon the probe as the that first link with aninadequate probe or poor probing methods,and the entire chain

is this and following sections,you’ll learn what contributes to thestrengths and weaknesses of probes and how to select the rightprobe for your ’ll also learn some important tips forusing probes Is a Probe?As a first step,let’s establish what an oscilloscope probe lly,a probe makes a physical and electrical connectionbetween a test point or signal

sourceand an ing on your measurement needs,this connection can

be made with something as simple as a length of wire or withsomething as sophisticated as an active

differential this point,it’s enough to say that an oscilloscope probe is somesort of device or network that connects the signal source to theinput of the is illustrated in Figure 1-1,where theprobe is indicated as an undefined box in the measurement er the probe is in reality,it must provide a connection ofadequate convenience and quality between the signal source andthe oscilloscope input (Figure 1-2).The adequacy of connection has

three key defining issues – physical attachment,impact on circuitoperation,and signal of ProbesPrimerFigure 1-1.A probe is a device that makes a physical and electrical connectionbetween the oscilloscope and test probes consist of a probe head,a probe cable,and a

compensation box or other signal conditioning /accessories5

ABCs of ProbesPrimerTo make an oscilloscope measurement,you must first be able tophysically get the probe to the test make this possible,most probes have at least a meter or two of cable associated

with them,as indicated in Figure probe cable allows theoscilloscope to be left in a stationary position on a cart or bench topwhile the probe is moved from test point to test point in the circuitbeing is a tradeoff for this convenience,be cable reduces the probe’s

bandwidth; the longer the cable,the greater the addition to the length of cable,most probes also have a probehead,or handle,with a probe probe head allows you to holdthe probe while you maneuver the tip to make contact with the ,this probe tip is in the form of a spring-loaded hookthat allows you to actually attach the probe to the test ally attaching the probe to the test point also establishes anelectrical connection between the probe tip and the useable measurement results,attaching the probe to acircuit must have minimum affect on the way the circuit operates,and the signal at the probe tip must be transmitted with adequatefidelity through the probe head and cable to the oscilloscope’s three issues – physical attachment,minimum impact on

circuit operation,and adequate signal fidelity – encompass most ofwhat goes into proper selection of a e probing effectsand signal fidelity are the more complex topics,much of this primeris devoted to those r,the issue of physical connectionshould never be ulty in connecting a probe to a testpoint often leads to probing practices that reduce Ideal ProbeIn an ideal world,the ideal probe would offer the following

key attributes:— Connection ease and convenience— Absolute signal fidelity— Zero signal source

loading— Complete

noiseimmunityConnection ease and a physical connection

to the test point has already been mentioned as one of the keyrequirements of the ideal probe,you should also beable to make the physical connection with both ease and miniaturized circuitry,such as high-density surface mount

technology (SMT),connection ease and convenience are promotedthrough subminiature probe heads and various probe-tip adaptersdesigned for SMT a probing system is shown inFigure probes,however,are too small for practical

/accessoriesuse in applications such as industrial power circuitry where highvoltages and larger gauge wires are power applications,physically larger probes with greater margins of safety are s 1-3b and 1-3c show examples of such probes,whereFigure 1-3b is a high-voltage probe and Figure 1-3c is a clamp-oncurrent these few examples of physical connection,it’s clear thatthere’s no single ideal probe size or configuration for all e of this,various probe sizes and configurations have

been designed to meet the physical connection requirements of

various te signal ideal probe should transmit any

signal from probe tip to oscilloscope input with absolute other words,the signal as it occurs at the probe tipshould be faithfully duplicated at the oscilloscope absolute fidelity,the probe circuitry from tip to oscilloscopeinput must have zero

attenuation,infinite bandwidth,and

linearphaseacross all only are these ideal requirementsimpossible to achieve in reality,but they are example,there’s no need for an infinite bandwidth probe,or oscilloscope forthat matter,when you’re dealing with audio frequency isthere a need for infinite bandwidth when 500 MHz will do for coveringmost high-speed digital,TV,and other typical oscilloscope ,within a given bandwidth of operation,absolute signal fidelity isan ideal to be sought signal source circuitry behind a test point

can be thought of as or modeled as a signal externaldevice,such as a probe,that’s attached to the test point can appearas an additional

loadon the signal source behind the test external device acts as a load when it draws signal currentfrom the circuit (the signal source).This loading,or signal currentdraw,changes the operation of the circuitry behind the test point,and thus changes the signal seen at the test ideal probe causes zero signal source other words,itdoesn’t draw any signal current from the signal meansthat,for zero current draw,the probe must have infinite

impedance,essentially presenting an open circuit to the test practice,a probe with zero signal source loading cannot is because a probe must draw some small amount ofsignal current in order to develop a signal voltage at the uently,some signal source loading is to be expectedwhen using a goal,however,should always be to minimizethe amount of loading through appropriate probe selection.

Complete noise scent lights and fan motors arejust two of the many electrical noise sources in our sources can induce their noise onto nearby electrical cablesand circuitry,causing the noise to be added to e ofsusceptibility to induced noise,a simple piece of wire is a less thanideal choice for an oscilloscope ideal oscilloscope probe is completely immune to all a result,the signal delivered to the oscilloscope has nomore noise on it than what appeared on the signal at the test practice,use of

shieldingallows probes to achieve a high level ofnoise immunity for most common signal ,however,canstill be a problem for certain low-level particular,commonmode noise can present a problem for differential measurements,aswill be discussed Realities of ProbesThe preceding discussion of The Ideal Probementioned several

realities that keep practical probes from reaching the rstand how this can affect your oscilloscope measurements,we need to explore the realities of probes ,it’s important to realize that a probe,even if it’s just a simplepiece of wire,is potentially a very complex DC signals

(0 Hz frequency),a probe appears as a simple conductor pair withsome series resistance and a terminating resistance (Figure 1-4a).However,for AC signals,the picture changes dramatically as signalfrequencies increase (Figure 1-4b).The picture changes for AC signals because any piece of wire

has

distributed inductance(L),and any wire pair has distributedcapacitance(C).The distributed inductance reacts to AC signals byincreasingly impeding AC current flow as signal frequency distributed capacitance reacts to AC signals with decreasingimpedance to AC current flow as signal frequency eraction of these reactive elements (L and C),along with theresistive elements (R),produces a total probe impedance that varieswith signal h good probe design,the R,L,and Celements of a probe can be controlled to provide desired degrees

of signal fidelity,attenuation,and source loading over specified

frequency with good design,probes are limited by

the nature of their ’s important to be aware of these

limitations and their effects when selecting and using dth and rise time dth is the range of

frequencies that an oscilloscope or probe is designed

example,a 100 MHz probe or oscilloscope is designed to makemeasurements within specification on all frequencies up to ed or unpredictable measurement results can occur atsignal frequencies above the specified bandwidth (Figure 1-5).ABCs of ProbesPrimerFigure s probes are available for different application technologies

and measurement g SMT -voltage -on Current /accessories7

ABCs of ProbesPrimerFigure are circuits composed of distributed resistance,inductance,and capacitance (R,L,and C).As a general rule,for accurate amplitude measurements,the

bandwidth of the oscilloscope should be five times greater than thefrequency of the waveform being “five-times rule”ensures adequate bandwidth for the higher-frequency componentsof non-sinusoidal waveforms,such as square rly,the oscilloscope must have an adequate

rise timefor

the waveforms being rise time of a oscilloscope orprobe is defined as the rise time that would be measured if an ideal,instantaneous-rise pulse were reasonable accuracyinmeasuring pulse rise or fall times,the rise time of the probe andoscilloscope together should be three to five times faster than thatof the pulse being measured (Figure 1-6).In cases where rise time isn’t specified,you can derive rise time (Tr)from the bandwidth (BW) specification with the following relationship:Tr= 0.35/BWEvery oscilloscope has defined bandwidth and rise time rly,every probe also has its own set of bandwidth and risetime ,when a probe is attached to an oscilloscope,youget a new set of system bandwidth and rise time unately,the relationship between system bandwidth and theindividual oscilloscope and probe bandwidths is not a simple same is true for rise cope with this,manufacturers ofquality oscilloscopes specify bandwidth or rise time to the probe tipwhen the oscilloscope is used with specific probe isimportant because the oscilloscope and probe together form ameasurement system,and it’s the bandwidth and rise time of thesystem that determine its measurement you use aprobe that is not on the oscilloscope’s recommended list of probes,you run the risk of unpredictable measurement /accessoriesFigure and oscilloscopes are designed to make measurements to

specification over an operating frequencies beyond the 3 dB

point,signal amplitudes become overly attenuated and measurement results

may be c range probes have a high-voltage safetylimit that should not be

passive probes,this limit canrange from hundreds of volts to thousands of r,foractive probes,the maximum safe voltage limit is often in the

range of tens of avoid personal safety hazards as well aspotential damage to the probe,it’s wise to be aware of the voltagesbeing measured and the voltage limits of the probes being addition to safety considerations,there’s also the practical

consideration of measurement dynamic oscopes haveamplitude sensitivity example,1 mV to 10 V/division is

a typical sensitivity an eight-division display,this meansthat you can typically make reasonably accurate measurements onsignals ranging from 4 mV peak-to-peak to 40 V assumes,at minimum,a four-division amplitude display of

the signal to obtain reasonable measurement a 1X probe (1-times probe),the dynamic measurement rangeis the same as that of the the example above,thiswould be a signal measurement range of 4 mV to 40 ,what if you need to measure a signal beyond the 40 V range?

Figure time measurement error can be estimated from the above chart.A oscilloscope/probe combination with a rise time three times faster than the

pulse being measured (3:1 ratio) can be expected to measure the pulse rise

time to within 5%.A 5:1 ratio would result in only 2% can shift the oscilloscope’s dynamic range to higher voltages byusing an attenuating probe.A 10X probe,for example,shifts thedynamic range to 40 mV to 400 does this by attenuating theinput signal by a factor of 10,which effectively multiplies the

oscilloscope’s scaling by most general-purpose use,10Xprobes are preferred,both because of their high-end voltage rangeand because they cause less signal source r,if youplan to measure a very wide range of voltage levels,you may wantto consider a switchable 1X/10X gives you a dynamicrange of 4 mV to 400 r,in the 1X mode,more care mustbe taken with regard to signal source previously mentioned,a probe must draw somesignal current in order to develop a signal voltage at the places a load at the test point that can change the signalthat the circuit,or signal source,delivers to the test simplest example of source loading effects is to consider

measurement of a battery-driven resistive is shown inFigure Figure 1-7a,before a probe is attached,the battery’sDC voltage is divided across the battery’s internal resistance (Ri)and the load resistance (Ri) that the battery is the

values given in the diagram,this results in an output voltage of:Eo=Eb* RI/( Ri+ RI)=100 V * 100,000/(100 + 100,000)=10,000,000 V/100,100=99.9 VABCs of ProbesPrimerFigure example of resistive Figure 1-7b,a probe has been attached to the circuit,placing

the probe resistance (Rp) in parallel with Rpis 100 kΩ,theeffective load resistance in Figure 1-7b is cut in half to 50 kΩ.The loading effect of this on Eois:Eo= 100 V * 50,000/(100 + 50,000)= 5,000,000 V/50,100= 99.8 VThis loading effect of 99.9 versus 99.8 is only 0.1% and is negligiblefor most r,if Rpwere smaller,say 10 kΩ,theeffect would no longer be minimize such resistive loading,1X probes typically have aresistance of 1 MΩ,and 10X probes typically have a resistance of10 MΩ.For most cases,these values result in virtually no loading should be expected,though,when measuringhigh-resistance y,the loading of greatest concern is that caused by thecapacitance at the probe tip (see Figure 1-8).For low frequencies,this capacitance has a

reactancethat is very high,and there’s littleor no ,as frequency increases,the capacitive result is increased loading at high capacitive loading affects the bandwidth and rise time

characteristics of the measurement system by reducing bandwidthand increasing rise /accessories9

ABCs of ProbesPrimerFigure AC signal sources,probe tip capacitance (Cp) is the greatest

loading signal frequency increases,capacitive reactance (Xc)

decreases,causing more signal flow through the tive loading can be minimized by selecting probes with lowtip capacitance typical capacitance values for variousprobes are provided in the table below:ProbeAttenuationRCP6101B1X1 MΩ100 pFP6106A10X10 MΩ11 pFP6139A10X10 MΩ8 pFP624310X1 MΩ≤1 pFSince the ground lead is a wire,it has some amount of distributedinductance (see Figure 1-9).This inductance interacts with theprobe capacitance to cause

ringingat a certain frequency that isdetermined by the L and C ringing is unavoidable,andmay be seen as a sinusoid of decaying amplitude that is impressedon effects of ringing can be reduced by designing

probe

groundingso that the ringing frequency occurs beyond thebandwidth limit of the probe/oscilloscope avoid grounding problems,always use the shortest ground leadprovided with the tuting other means of grounding cancause ringing to appear on measured are dealing with the realities of oscilloscopeprobes,it’s important to keep in mind that probes are oscilloscope probes are voltage is,they sense

or probe a voltage signal and convey that voltage signal to the

oscilloscope r,there are also probes that allow you tosense phenomena other than voltage example,current probes are designed to sense the current

flowing through a probe converts the sensed current to acorresponding voltage signal which is then conveyed to the input ofthe rly,optical probessense light power andconvert it to a voltage signal for measurement by an /accessoriesFigure probe ground lead adds inductance to the longer

the ground lead,the greater the inductance and the greater the likelihood of

seeing ringing on fast onally,oscilloscope voltage probes can be used with a varietyof other sensors or transducers to measure different phenomena.A vibration transducer,for example,allows you to view machineryvibration signatures on an oscilloscope possibilities

are as wide as the variety of available transducers on the all cases,though,the transducer,probe,and oscilloscope

combination must be viewed as a measurement er,the realities of probes discussed above also extend down to ucers have bandwidth limits as well and cancause loading ng the Right ProbeBecause of the wide range of oscilloscope measurement applicationsand needs,there’s also a broad selection of oscilloscope probes onthe can make probe selection a confusing cut through much of the confusion and narrow the selectionprocess,always follow the oscilloscope manufacturer’s recommen-dations for is important because different oscilloscopesare designed for different bandwidth,rise time,sensitivity,and inputimpedance full advantage of the oscilloscope’smeasurement capabilities requires a probe that matches the

oscilloscope’s design onally,the probe selection process should include considerationof your measurement are you trying to measure?Voltages? Current? An optical signal? By selecting a probe that isappropriate to your signal type,you can get direct measurementresults ,consider the amplitudes of the signals you are they within the dynamic range of your oscilloscope? If not,you’llneed to select a probe that can adjust dynamic lly,this will be through attenuation with a 10X or higher probe.

Make sure that the bandwidth,or rise time,at the probe tip exceedsthe signal frequencies or rise times that you plan to keep in mind that non-sinusoidal signals have important

frequency components or

harmonicsthat extend well above the

fundamental frequency of the example,to fully includethe 5th harmonic of a 100 MHz square wave,you need a measurementsystem with a bandwidth of 500 MHz at the probe rly,your oscilloscope system’s rise time should be three to five timesfaster than the signal rise times that you plan to always take into account possible signal loading by the for high-resistance,low-capacitance most

applications,a 10 MΩprobe with 20 pF or less capacitance shouldprovide ample insurance against signal source r,forsome high-speed digital circuits you may need to move to the lowertip capacitance offered by active finally,keep in mind that you must be able to attach the probeto the circuit before you can make a may requirespecial selection considerations about probe head size and probe tipadaptors to allow easy and convenient circuit Probing TipsSelecting probes that match your oscilloscope and application needsgives you the capability for making the necessary ly making the measurements and obtaining useful results alsodepends on how you use the following probing tips willhelp you avoid some common measurement pitfalls:Compensate your probes are designed to match theinputs of specific oscilloscope r,there are slightvariations from oscilloscope to oscilloscope and even between

different input channels in the same deal with thiswhere necessary,many probes,especially attenuating probes (10Xand 100X probes),have built-in compensation your probe has a compensation network,you should adjust thisnetwork to compensate the probe for the oscilloscope channel thatyou are do this,use the following procedure: the probe to the the probe tip to the probe compensation test point on the

oscilloscope’s front panel (see Figure 1-10). the adjustment tool provided with the probe or other

non-magnetic adjustment tool to adjust the compensation

network to obtain a calibration waveform display that has flat

tops with no overshoot or rounding (see Figure 1-11). the oscilloscope has a built-in calibration routine,run this

routine for increased of ProbesPrimerFigure compensation adjustments are done either at the probe

head or at a compensation box where the box attaches to the oscilloscope uncompensated probe can lead to various measurement errors,especially in measuring pulse rise or fall avoid sucherrors,always compensate probes right after connecting them tothe oscilloscope and check compensation ,it’s wise to check probe compensation whenever you changeprobe tip appropriate probe tip adapters whenever possible.A probetip adapter that’s appropriate to the circuit being measured makesprobe connection quick,convenient,and electrically repeatable unately,it’s not uncommon to see short lengths of wiresoldered to circuit points as a substitute for a probe tip problem is that even an inch or two of wire can cause significantimpedance changes at high effect of this is shownin Figure 1-12,where a circuit is measured by direct contact of theprobe tip and then measured via a short piece of wire between thecircuit and probe ground leads as short and as direct as doingperformance checks or troubleshooting large boards or systems,itmay be tempting to extend the probe’s ground extendedground lead allows you to attach the ground once and freely movethe probe around the system while you look at various test r,the added inductance of an extended ground lead cancause ringing to appear on fast-transition is illustratedin Figure 1-13,which shows waveform measurements made whileusing the standard probe ground lead and an extended ground /accessories11

ABCs of ly es of probe compensation effects on a square probe tip -inch wire at probe a short piece of wire soldered to a test point can cause signal fidelity this case,rise time has been changed from 4.74 ns (a) to 5.67 ns (b).a.6.5-inch probe ground lead.b.28-inch lead attached to probe ing the length of the probe ground lead can cause ringing to appear on yIn this first chapter,we’ve tried to provide all of the basic informationnecessary for making appropriate probe selections and using / the following chapters,we’ll expand on this informationas well as introduce more advanced information on probes andprobing techniques.12

Different Probes for Different NeedsHundreds,perhaps even thousands,of different oscilloscope probesare available on the Tektronix Measurement Productscatalog alone lists more than 70 different probe such a broad selection of probes really necessary?The answer is Yes,and in this chapter you’ll discover the an understanding of those reasons,you’ll be better

prepared to make probe selections to match both the oscilloscopeyou are using and the type of measurements that you need benefit is that proper probe selection leads to enhancedmeasurement capabilities and So Many Probes?The wide selection of oscilloscope models and capabilities is one

of the fundamental reasons for the number of available ent oscilloscopes require different probes.A 400 MHz oscillo-scope requires probes that will support that 400 MHz r,those same probes would be overkill,both in capabilityand cost,for a 100 MHz ,a different set ofprobes designed to support a 100 MHz bandwidth is a general rule,probes should be selected to match the oscilloscope’sbandwidth whenever g that,the selection should bein favor of exceeding the oscilloscope’s dth is just the beginning,oscopes can alsohave different input connector types and different input example,most scopes use a simple BNC-type input may use an SMA still others,as shown inFigure 2-1,have specially designed connectors to support

readout,trace ID,probe power,or other special ,probe selection must also include connector compatibility

with the oscilloscope being can be direct connector

compatibility,or connection through an appropriate t support is a particularly important aspect of probe/oscilloscopeconnector 1X and 10X probes are interchangedon a oscilloscope,the oscilloscope’s vertical scale readout shouldreflect the 1X to 10X example,if the oscilloscope’s

vertical scale readout is 1 V/div (one volt per division) with a 1Xprobe attached and you change to a 10X probe,the vertical readoutshould change by a factor of 10 to 10 V/ this 1X to 10Xchange is not reflected in the oscilloscope’s readout,amplitudemeasurements made with the 10X probe will be ten times lowerthan they should of ProbesPrimerFigure with various connector types are necessary for matching

different oscilloscope input channel generic or commodity probes may not support readout

capability for all a result,extra caution is necessarywhen using generic probes in place of the probes specifically

recommended by the oscilloscope addition to bandwidth and connector differences,various scopesalso have different input resistance and capacitance lly,oscilloscope input resistances are either 50 Ωor 1 MΩ.However,there can be great variations in input capacitance dependingon the oscilloscope’s bandwidth specification and other design

proper signal transfer and fidelity,it’s important that

the probe’s R and C match the R and C of the oscilloscope it is tobe used example,50 Ωprobes should be used with 50 Ωoscilloscope rly,1 MΩprobes should be used on scopeswith a 1 MΩinput exception to this one-to-one resistance matching occurs whenattenuator probesare example,a 10X probe for a 50 Ωenvironment will have a 500 Ωinput resistance,and a 10X probefor a 1 MΩenvironment will have a 10 MΩinput resistance.(Attenuator probes,such as a 10X probe,are also referred to asdivider probes and multiplier probes multiply themeasurement range of the oscilloscope,and they do this by attenu-ating or dividing down the input signal supplied to the oscilloscope.)/accessories13

ABCs of ProbesPrimerIn addition to resistance matching,the probe’s capacitance shouldalso match the nominal input capacitance of the ,this capacitance matching can be done through adjustment of theprobe’s compensation is only possible,though,whenthe oscilloscope’s nominal input capacitance is within the compen-sation range of the ,it’s not unusual to find probes with

different compensation ranges to meet the requirements of differentoscilloscope issue of matching a probe to an oscilloscope has been tremen-dously simplified by oscilloscope oscope manu-facturerscarefully design probes and oscilloscopes as complete

a result,the best probe-to-oscilloscope match is alwaysobtained by using the standard probe specified by the of any probe other than the manufacturer-specifiedprobe may result in less than optimum measurement -to-oscilloscope matching requirements alone generate muchof the basic probe inventory available on the probecount is then added to significantly by the different probes that arenecessaryfor different measurements most basic differ-ences are in the voltage ranges being olt,volt,and kilovolt measurements typically require probes with differentattenuation factors (1X,10X,100X).Also,there are many cases where the signal voltages are is,the signal exists across two points or two wires,neither

of which is at ground or common potential (see Figure 2-2).Suchdifferential signalsare common in telephone voice circuits,computerdisk read channels,and multi-phase power ingthese signals requires yet another class of probes referred to as

differential then there are many cases,particularly in power applications,where current is of as much or more interest than plications are best served with yet another class of probes thatsense current rather than t probes and differential probes are just two special classesof probes among the many different types of available t of this chapter covers some of the more common types ofprobes and their special ent Probe Types and Their BenefitsAs a preface to discussing various common probe types,it’s

important to realize that there’s often overlap in nly avoltage probe senses voltage exclusively,but a voltage probe can

be a passive probe or an active rly,differential probesare a special type of voltage probe,and differential probes can alsobe active or passive appropriate these overlappingrelationships will be pointed /accessoriesSingle-Ended SignalSingle-Ended ential SignalDifferential -ended signals are referenced to ground (a),while differentialsignals are the difference between two signal lines or test points (b).Passive voltage e probes are constructed of wiresand connectors,and when needed for compensation or attenuation,resistors and are no active components – transistorsor amplifiers – in the probe,and thus no need to supply power tothe e of their relative simplicity,passive probes tend to be themost rugged and economical of are easy to use andare also the most widely used type of r,don’t befooled by the simplicity of use or simplicity of construction –

high-quality passive probes are rarely simple to design!Passive voltage probes are available with various attenuation factors– 1X,10X,and 100X – for different voltage these,the10X passive voltage probe is the most commonly used probe,and

is the type of probe typically supplied as a standard accessory

with applications where signal amplitudes are one-volt peak-to-peakor less,a 1X probe may be more appropriate or even there’s a mix of low amplitude and moderate amplitude signals(tens of millivolts to tens of volts),a switchable 1X/10X probe canbe a great should be kept in mind,however,that aswitchable 1X/10X probe is essentially two different probes in only are their attenuation factors different,but their bandwidth,rise time,and impedance (R and C) characteristics are different a result,these probes will not exactly match the oscilloscope’sinput and will not provide the optimum performance achieved with

a standard 10X probe.

Most passive probes are designed for use with general such,their bandwidths typically range from lessthan 100 MHz to 500 MHz or is,however,a special category of passive probes that providemuch higher are referred to variously as 50 Ωprobes,Zoprobes,and voltage divider probes aredesigned for use in 50 Ωenvironments,which typically are high-speed device characterization,microwave communication,and

time domain reflectometry (TDR).A typical 50 Ωprobe for suchapplications has a bandwidth of several gigaHertz and a rise time

of 100 picoseconds or voltage probes contain or rely on active

components,such as transistors,for their often,theactive device is a

field-effect transistor (FET).The advantage of a FET input is that it provides a very low inputcapacitance,typically a few picoFarads down to less than ultra-low capacitance has several desirable ,recall that a low value of capacitance,C,translates to a

high value of capacitive reactance, can be seen from theformula for Xc,which is:Xc=

21πfCSince capacitive reactance is the primary input impedance elementof a probe,a low C results in a high input impedance over a broaderband of a result,active FET probes will typicallyhave specified bandwidths ranging from 500 MHz to as high as 4 addition to higher bandwidth,the high input impedance of activeFET probes allows measurements at test points of unknown impedancewith much less risk of loading ,longer ground leads canbe used since the low capacitance reduces ground lead t important aspect,however,is that FET probes offer such lowloading,that they can be used on high-impedance circuits thatwould be seriously loaded by passive all of these positive benefits,including bandwidths as wide asDC to 4 GHz,you might wonder:Why bother with passive probes?The answer is that active FET probes don’t have the voltage rangeof passive linear dynamic range of active probes is

generally anywhere from ±0.6 V to ±10 the maximum voltagethat they can withstand can be as low as ±40 V (DC + peak AC).In other words you can’t measure from millivolts to tens of volts likeyou can with a passive probe,and active probes can be damagedby inadvertently probing a higher can even be damageby a static of ProbesPrimerStill,the high bandwidth of FET probes is a major benefit and theirlinear voltage range covers many typical semiconductor ,active FET probes are often used for low signal level applications,including fast logic families such as ECL,GaAs,and ential ential signals are signals that are referencedto each other instead of earth 2-3 illustrates severalexamples of such include the signal developedacross a collector load resistor,a disk drive read channel signal,multi-phase power systems,and numerous other situations wheresignals are in essence “floating”above ential signals can be probed and measured in two basic approaches are illustrated in Figure two probes to make two single-ended measurements,asshown in Figure 2-4a is an often used ’s also usually theleast desirable method of making differential eless,the method is often used because a dual-channeloscilloscope is available with two ing both signals

to ground (single-ended) and using the oscilloscope’s math functionsto subtract one from the other (channel A signal minus channel B)seems like an elegant solution to obtaining the difference it can be in situations where the signals are low frequency

and have enough amplitude to be above any concerns of are several potential problems with combining two single-ended problem is that there are two long andseparate signal paths down each probe and through each delay differences between these paths results in timeskewing of the two high-speed signals,this skew canresult in significant amplitude and timing errors in the computed

difference minimize this,matched probes should be r problem with single-ended measurements is that they don’tprovide adequate common-mode noise low-levelsignals,such as disk read channel signals,are transmitted andprocessed differentially in order to take advantage of common-modenoise mode noise is noise that is impressed onboth signal lines by such things as nearby clock lines or noise fromexternal sources such as fluorescent a differential systemthis common-mode noise tends to be subtracted out of the success with which this is done is referred to as thecommon-mode rejection ratio (CMRR).Because of channel differences,the CMRR performance of

single-ended measurements quickly declines to dismal levels withincreasing results in the signal appearing noisierthan it actually would be if the common-mode rejection of thesource had been /accessories15

ABCs of ProbesPrimerDisk Read HeadDisk Read HeadPreampPreamp33ø∅Figure examples of differential signal sources.A differential probe,on the other hand,uses a differential amplifierto subtract the two signals,resulting in one differential signal formeasurement by one channel of the oscilloscope (Figure 2-4b).This provides substantially higher CMRR performance over a broaderfrequency onally,advances in circuit miniaturizationhave allowed differential amplifiers to be moved down into the

actual probe the latest differential probes,such as theTektronix P6247,this has allowed a 1-GHz bandwidth to beachieved with CMRR performance ranging from 60 dB (1000:1)

at 1 MHz to 30 dB (32:1) at 1 kind of bandwidth/CMRRperformance is becoming increasingly necessary as disk driveread/write data rates reach and surpass the 100 MHz -voltage term “high voltage”is isconsidered high voltage in the semiconductor industry is practicallynothing in the power the perspective of probes,how-ever,we can define high voltage as being any voltage beyond what canbe handled safely with a typical,general-purpose 10X passive lly,the maximum voltage for general-purpose passive probesis around 400 to 500 volts (DC + peak AC).High-voltage probes onthe other hand can have maximum ratings as high as 20,000 example of such a probe is shown in Figure is a particularly important aspect of high-voltage probes accommodate this,many high-voltage probeshave longer than normal l cable lengths are 10 is usually adequate for locatingthe oscilloscope outside of asafety cage or behind a safety s for 25-foot cablesare also available for those cases where oscilloscope operationneeds to be further removed from the high-voltage /accessoriesCH1 — CH2CH1 – ential ProbeDifferential ential signals can be measured using the invert and

add feature of a dual-channel oscilloscope (a),or preferably by using a

differential probe (b).Current t flow through a conductor causes an

electromagnetic flux field to form around the tprobes are designed to sense the strength of this field and convertit to a corresponding voltage for measurement by an allows you to view and analyze current waveforms with used in combination with an oscilloscope’s

voltage measurement capabilities,current probes also allow you tomake a wide variety of power ing on thewaveform math capabilities of the oscilloscope,these measurementscan include instantaneous power,true power,apparent power,and

are basically two types of current probes for current probes,which usually are passive probes,and AC/DCcurrent probes,which are generally active types usethe same principle of transformer action for sensing alternating

current (AC) in a transformer action,there must first be alternating current flowthrough a alternating current causes a flux field tobuild and collapse according to the amplitude and direction of a coil is placed in this field,as shown in Figure 2-6,thechanging flux field induces a voltage across the coil through simpletransformer transformer action is the basis for AC current ACcurrent probe head is actually a coil that has been wound to precisespecifications on a magnetic this probe head is heldwithin a specified orientation and proximity to an AC current carryingconductor,the probe outputs a linear voltage that is of known pro-portion to the current in the current-related voltagecan be displayed as a current-scaled waveform on an oscilloscope.

Figure P6015A can measure DC voltages up to 20 kV and pulses up

to 40 kV with a bandwidth of 75 bandwidth for AC current probes depends on the design of theprobe’s coil and other dths as high as 1 GHz r,bandwidths under 100 MHz are more all cases,there’s also a low-frequency cutoff for AC current includes direct current (DC),since direct currentdoesn’t cause a changing flux field and,thus,cannot cause trans-former at frequencies very close to DC,0.01 Hz forexample,the flux field still may not be changing fast enough forappreciable transformer ally,though,a low frequencyis reached where the transformer action is sufficient to generate ameasurable output within the bandwidth of the ,depending on the design of the probe’s coil,the low-frequency endof the bandwidth might be as low as 0.5 Hz or as high as 1.2 probes with bandwidths that begin near DC,a

Hall Effectdevice canbe added to the probe design to detect result is an AC/DCprobe with a bandwidth that starts at DC and extends to the specifiedupper frequency 3 dB type of probe requires,at minimum,a power source for biasing the Hall Effect device used for DC ing on the probe design,a current probe amplifier may alsobe required for combining and scaling the AC and DC levels to

provide a single output waveform for viewing on an ’s important to keep in mind that a current probe operates inessence as a closely coupled concept is illustratedin Figure 2-7,which includes the basic transformer ndard operation,the sensed current conductor is a one-turnwinding (N1).The current from this single winding transforms to amulti-winding (N2) probe output voltage that is proportional to theturns ratio (N2/N1).At the same time,the probe’s impedance istransformed back to the conductor as a series insertion of ProbesPrimerFigure 2-6.A voltage is induced across any coil that is placed in the changing

flux field around a conductor which is carrying alternating current (AC).This insertion impedance is frequency dependent with it’s 1-MHzvalue typically being in the range of 30 to 500 MΩ,depending onthe specific most cases,the small insertion impedance

of a current probe imposes a negligible ormer basics can be taken advantage of to increase probesensitivity by looping the conductor through the probe multipletimes,as shown in Figure loops doubles the sensitivity,and three loops triples the r,this also increasesthe insertion impedance by the square of the added 2-8 also illustrates a particular class of probe referred to asa split core windings of this type of probe are on a “U”shaped core that is completed with a ferrite slide that closes the topof the “U”.The advantage of this type of probe is that the ferriteslide can be retracted to allow the probe to be conveniently clippedonto the conductor whose current is to be themeasurement is completed the slide can be retracted and the probecan be moved to another are also available with solid-core current transformers completely encircle the conductor being

a result,they must be installed by disconnecting

the conductor to be measured,feeding the conductor through thetransformer,and then reconnecting the conductor to its ef advantages of solid-core probes is that they offer small sizeand very high frequency response for measuring very fast,lowamplitude current pulses and AC -core current probes are by far the most common are available in both AC and AC/DC versions,and there

are various current-per-division display ranges,depending on theamp-second /accessories17

ABCs of ProbesPrimerFigure h AC transformer action,the single turn of a current carryingconductor (N1) induces a current in the AC probe’s coil (N2),resulting in a

current proportional voltage across the probe’s termination (Rterm).The amp-second product defines the maximum limit for linear

operation of any current product is defined for currentpulses as the average current amplitude multiplied by the the amp-second product is exceeded,the core materialof the probe’s coil goes into a saturated core

cannot handle any more current-induced flux,there can no longerbe constant proportionality between current input and voltage result is that waveform peaks are essentially “clipped off”inareas where the amp-second product is saturation can also be caused by high levels of direct currentthrough the conductor being combat core saturation andeffectively extend the current measuring range,some active currentprobes provide a bucking bucking current is set bysensing the current level in the conductor under test and then feedingan equal but opposite current back through the h thephenomenon that opposing currents are subtractive,the buckingcurrent can be adjusted to keep the core from going into e of the wide range of current measuring needs from

milliamps to kiloamps,from DC to MHz there’s a correspondinglywide selection of current ng a current probe for aparticular application is similar in many respects to selecting t handling capability,sensitivity ranges,insertionimpedance,connectability,and bandwidth/rise time limits are

some of the key selection onally,current handlingcapability must be derated with frequency and the probe’s specifiedamp-second product must not be /accessoriesFigure example of a split core AC current g n turns of the

conductor through the probe increases effective sensitivity n in digital systems can occur for a variety a logic analyzer is the primary tool for identifying andisolating fault occurrences,the actual cause of the logic fault isoften due to the analog attributes of the digital idth

jitter,pulse amplitude

aberrations,and regular old analognoise and crosstalk are but a few of the many possible analog

causes of digital ing the analog attributes of digital waveforms requires use ofan r,to isolate exact causes,digital designersoften need to look at specific data pulses occurring during specificlogic requires a logic triggering capability that ismore typical of a logic analyzer than an logictriggering can be added to most oscilloscopes through use of aword recognizer trigger probe such as shown in Figure particular probe shown in Figure 2-9 is designed for TTL andTTL-compatible can provide up to 17 data-channel probes(16 data bits plus qualifier),and is compatible with both synchronousand asynchronous trigger word to be recognized isprogrammed into the probe by manually setting miniature switcheson the probe a matching word is recognized,the probeoutputs a Hi (one) trigger pulse that can be used to trigger oscilloscopeacquisition of related data waveforms or l the advent and spread of fiber-optic basedcommunications,there’s a rapidly expanding need for viewing andanalyzing optical waveforms.A variety of specialized optical systemanalyzers have been developed to fill the needs of communicationsystem troubleshooting and r,there’s also anexpanding need for general-purpose optical waveform measurementand analysis during optical component development and l probes fill this need by allowing optical signals to be viewedon an oscilloscope.

The optical probe is an optical-to-electrical the opticalside,the probe must be selected to match the specific optical

connector and fiber type or optical mode of the device that’s the electrical side,the standard probe-to-oscilloscopematching criteria are probe addition to all of the above “fairly standard”probe types,there’s also a variety of specialty probes and include:— Environmental probes,which are designed to operate over a verywide temperature range.— Temperature probes,which are used to measure the temperatureof components and other heat generating items.— Probing stations and articulated arms (Figure 2-10) for probing

fine-pitch devices such as multi-chip-modules,hybrid circuits,and ng MeasurementsFloating measurementsare measurements that are made betweentwo points,neither of which is at ground this sounds alot like differential measurements described previously with regardto differential probes,you’re right.A floating measurement is a

differential measurement,and,in fact,floating measurements canbe made using differential lly,however,the term “floating measurement”is used inreferring to power system es are switchingsupplies,motor drives,ballasts,and uninterruptible power sourceswhere neither point of the measurement is at ground (earth potential),and the signal “common”may be elevated (floating) to hundreds ofvolts from ,these measurements require rejection ofhigh common-mode signals in order to evaluate low-level signalsriding on eous ground currents can also add hum to

the display,causing even more measurement example of a typical floating measurement situation is shown inFigure this motor drive system,the three phase AC line isrectified to a floating DC bus of up to 600 ground-referencedcontrol circuit generates pulse modulated gate drive signals throughan isolated driver to the bridge transistors,causing each output toswing the full bus voltage at the pulse modulation te measurement of the gate-to-emitter voltage requiresrejection of the bus onally,the compact design ofthe motor drive,fast current transitions,and proximity to the rotatingmotor contribute to a harsh EMI ,connecting theground lead of a oscilloscope’s probe to any part of the motor drivecircuit would cause a short to of ProbesPrimerFigure 2-9.A word recognizer probes allow oscilloscopes to be

used to analyze specific data waveforms during specific logic e of a probing station designed for probing small geometrydevices such as hybrid circuits and this three-phase motor drive,all points are above ground,making floating measurements a /accessories19

ABCs of addition to being dangerous,floating an oscilloscope can result in significant ringing on

measurements (a) as compared to the safer method of using a probe isolator (b).OscilloscopeRather than floating the oscilloscope,the probe isolator floats justthe isolation of the probe can be done via either a trans-formeror optical coupling mechanism,as shown in Figure case,the oscilloscope remains grounded,as it should,and thedifferential signal is applied to the tip and reference lead of the

isolated isolator transmits the differential signal throughthe isolator to a receiver,which produces a ground-referenced

signal that is proportional to the differential input makesthe probe isolator compatible with virtually any meet different needs,various types of isolators are include multi-channel isolators that provide two or morechannels with independent reference ,fiber-optic basedisolators are available for cases where the isolator needs to bephysically separated from the instrument by long distances (e.g.100 meters or more).As with differential probes,the key isolatorselection criteria are bandwidth and onally,maximumworking voltage is a key specification for isolation lly,this is 600 V RMS or 850 V (DC+peak AC).Probe AccessoriesFigure e of probe isolation for making floating To get around this direct short to ground,some oscilloscope users haveused the unsafe practice of defeating the oscilloscope’s ground lows the oscilloscope’s ground lead to float with the motor drive circuit sothat differential measurements can be unately,this practicealso allows the oscilloscope chassis to float at potentials that could be adangerous or deadly shock hazard to the oscilloscope only is “floating”the oscilloscope an unsafe practice,but the resulting

measurements are often impaired by noise and other is illus-trated in Figure 2-12a,which shows a floated oscilloscope measurement ofone of the gate-to-emitter voltages on the motor drive bottom tracein Figure 2-12ais the low-side gate-emitter voltage and the top trace is

the high-side the significant ringing on both of these ringing is due to the large parasitic capacitance from the oscilloscope’schassis to earth 2-12b shows the results of the same measurement,but this timemade with the oscilloscope properly grounded and the measurement madethrough a probe only has the ringing been eliminatedfrom themeasurement,but the measurement can be made in far greater safetybecause the oscilloscope is no longer floating above probes come with a package of standard ccessories often include a ground lead clip that attaches to theprobe,a compensation adjustment tool,and one or more probe tipaccessories to aid in attaching the probe to various test 2-14 shows an example of a typical general- purpose voltageprobe and its standard /accessories

Retractable HookTip AdapterClip-On GroundLeadAdjustmentToolFigure 2-14.A typical general-purpose voltage probe with its standard that are designed for specific application areas,such asprobing surface mount devices,may include additional probe tipadapters in their standard accessories ,various

special purpose accessories may be available as options for 2-15 illustrates several types of probe tip adaptorsdesigned for use with small geometry ’s important to realize that most probe accessories,especiallyprobe tip adaptors,are designed to work with specific probe ing adaptors between probe models or probe manufacturersis not recommended since it can result in poor connection to thetest point or damage to either the probe or probe selecting probes for purchase,it’s also important to take intoaccount the type of circuitry that you’ll be probing and any adaptorsor accessories that will make probing quicker and manycases,less expensive commodity probes don’t provide a selection

of adaptor the other hand,probes obtained through anoscilloscope manufacturer often have an extremely broad selectionof accessories for adapting the probe to special exampleof this is shown in Figure 2-16,which illustrates the variety ofaccessories and options available for a particular class of accessories and options will,of course,vary between

different probe classes and of ProbesPrimerFigure examples of probe tip adaptors for small geometry adapters make probing of small circuitry significantly easier and can

enhance measurement accuracy by providing high integrity probe to test

point example of the various accessories that are available for

a 5-mm (miniature) probe probe families will have differing

accessories depending on the intended application for that family of /accessories21

ABCs of ProbesPrimerHow Probes Affect

Your MeasurementsTo obtain an oscilloscope display of a signal,some portion of thesignal must be diverted to the oscilloscope’s input isillustratedin Figure 3-1,where the circuitry behind the test point,Tp,is represented by a signal source,Es,and the associated circuitimpedances,Zs1and Zs2,that are the normal load on

an oscilloscope is attached to the test point,the probe impedance,Zp,and oscilloscope input impedance,Zi,become part of the loadon the signal ing on the relative values of the impedances,addition of theprobe and oscilloscope to the test point causes various chapter explores loading effects,as well as other

probing effects,in Effect of Source ImpedanceThe value of the

source impedancecan significantly influence thenet effect of any probe example,with low sourceimpedances,the loading effect of a typical high-impedance 10Xprobe would be hardly is because a high impedanceadded in parallel with a low impedance produces no significantchange in total r,the story changes dramatically with higher source er,for example,the case where the source impedances inFigure 3-1 have the same value,and that value equals the total ofthe probe and oscilloscope situation is illustratedin Figure equal values of Z,the source load is 2Z without the probe andoscilloscope attached to the test point (see Figure 3-2a).Thisresults in a signal amplitude of 0.5ESat the unprobed test r,when the probe and oscilloscope are attached (Figure 3-2b),the total load on the source becomes 1.5Z,and the signal amplitudeat the test point is reduced to two-thirds of its unprobed this latter case,there are two approaches that can be taken toreduce the impedance loading effects of approach isto use a higher impedance other is to probe the signalsomewhere else in the circuit at a test point that has a example,cathodes,emitters,and sources usuallyhave lower impedances than plates,collectors,or tive LoadingAs signal frequencies or transition speeds increase,the capacitiveelement of the impedances becomes uently,capacitive loading becomes a matter of increasing

particular,capacitive loading will affect the rise and fall times onfast-transition waveforms and the amplitudes of high-frequencycomponents in /oscope and signal being measured at the test point (TP) can be represented

by a signal source and associated load impedances (a).Probing the test point

adds the probe and oscilloscope impedances to the source load,resulting in

some current draw by the measurement system (b).oscope and higher the source impedances,the greater the loading caused

by this case,the impedances are all equal and probing causes a

more than 30% reduction in signal amplitude at the test point.

Figure rise time of a pulse generator is determined by its RC on rise illustrate capacitive loading,let’s consider apulse generator with a very fast rise is shown in Figure 3-3,where the pulse at the ideal generator’s output has a rise time ofzero (tr = 0).However,this zero rise time is modified through

integration by the associated resistance and capacitance of thesource impedance Tip CapacitanceProbeAttenuationTip CapacitanceP6101B1X54 pFP6105A10X11.2 pFP5100100X2.75 pFThe RC integration network always produces a 10 to 90% rise timeof is derived from the universal time-constant curve ofa value of 2.2 is the number of RC time constantsnecessary for C to charge through R from the 10% value to the90% amplitude value of the the case of Figure 3-3,the 50 Ωand 20 pF of the source

impedance results in a pulse rise time of 2.2 2.2RC

value is the fastest rise time that the pulse can the pulse generator’s output is probed,the probe’s inputcapacitance and resistance are added to that of the pulse is shown in Figure 3-4,where the 10 MΩand 11 pF of a typicalprobe have been the probe’s 10 MΩresistance is somuch greater than the generator’s 50 Ωresistance,the probe’sresistance can be r,the probe’s capacitance is inparallel with the load capacitance and adds to it directly for a totalload capacitance of 31 increases the value of 2.2RC andresults in an increase in the measured rise time to 3.4 ns versusthe 2.2 ns previous to can estimate the effect of probe tip capacitance on rise time bytaking the ratio of the probe’s specified capacitance to the known

or estimated source the values from Figure 3-4

ABCs of added capacitance of a probe increases RC the value and

increases the measured rise would result in the following estimate of percentage change inrise time:Cprobe tip/C1x 100%=11 pF/20 pF x 100%=55%From the above,it’s clear that probe choice,especially with regardto probe capacitance,can affect your rise time sive probes,the greater the attenuation ratio,the lower the tipcapacitance in is indicated in Table 3-1 which listssome probe capacitance examples for various passive smaller tip capacitance is needed,active FET-input probesshould be ing on the specific active probe model,tipcapacitances of 1 pF and less are on amplitude and addition to affecting rise time,capacitive loading also affects the amplitude and phase of the high-frequency components in a regard to this,it is importantto keep in mind that all waveforms are composed of sinusoidalcomponents.A 50 MHz square wave will have harmonic componentsof significance beyond 100 ,it’s important to not onlyconsider loading effects at a waveform’s fundamental frequency

but also at frequencies several multiples above the /accessories23

ABCs of ProbesPrimerFigure l input impedance versus frequency for the Tektronix P6205

Active g is determined by the total impedance at the probe designated as Zp,and Zpis composed of a resistive component,Rp,and reactive component reactive component is predomi-nantly capacitive,although inductive elements may be designed intothe probe to partially offset capacitive a rule,Zpdecreases with increasing probe instructionmanuals document probe Rpwith curves showing Zpversus 3-5 is an example of such a curve for the Tektronix P6205Active that the 1 MΩimpedance magnitude is

constant to nearly 100 was done by careful design of theprobe’s associated resistive,capacitive,and inductive 3-6 shows another example of a probe this case

Rpand Xpversus frequency are shown for a typical 10 MΩ dotted line (Xp) shows capacitive reactance versus

that Xpbegins decreasing at DC,but Rpdoesn’tstart rolling off significantly until 100 ,the total loadinghas been offset by careful design of the associated R,C,and

L you don’t have access to a probe’s impedance curves,you canmake a worst-case loading estimate using the following formula:Xp= 1/2πfCwhere:Xp= capacitive reactancef = frequencyC = probe tip /accessoriesFigure Rpversus frequency for a typical 10 MΩpassive example,a standard passive 10 MΩprobe with a tip capacitanceof 11 pF has a capacitive reactance (Xp) of about 290 Ωat 50 ing on the signal source impedance,this loading could havea major effect on the signal amplitude (by simple divider action),and it could even affect the operation of the circuit being dth ConsiderationsBandwidth is a measurement system issue that involves both thebandwidth of the probe and the oscilloscope’sbandwidth should exceed the predominate frequencies of the

signals you want to measure,and the bandwidth of the probe usedshould equal or exceed the bandwidth of the a measurement system perspective,the real concern is thebandwidth at the probe ,manufacturers will specify band-width at the probe tip for certain oscilloscope/probe is not always the case,uently,you should beaware of the major bandwidth issues of an oscilloscope and aprobe,both individually and in oscope bandwidth and rise dth is defined asthe point on an amplitude versus frequency plot where the measurementsystem is 3 dB down from the reference is illustrated inFigure 3-7 which shows a response curve with the 3 dB point ’s important to note that the measurement system is 3 dB down inamplitude at its rated means that you can expect30% error in amplitude measurements for frequencies at the

bandwidth y you won’t be using an oscilloscope at its full bandwidth r,if amplitude accuracy is of paramount importance,you shouldbe prepared to

deratethe oscilloscope’s bandwidth accordingly.

Figure dth is defined as the frequency in the response curve whereamplitude has decreased by -3 an example,consider the expanded view of bandwidth roll-offshown in Figure horizontal scale in this figure shows thederating factor necessary to obtain amplitude accuracies better than30%.With no derating (a factor of 1.0),a 100 MHz oscilloscope

will have up to a 30% amplitude error at 100 you wantamplitude measurements to be within 3%,the bandwidth of thisoscilloscope must be derated by a factor of 0.3 to 30 ngbeyond 30 MHz will have an amplitude error in excess of 3%.The above example points out a general rule of thumb for amplitude measurements within 3%,select an

oscilloscope with a specified bandwidth that’s three to five timesgreater than the highest frequency waveform that you’ll be rise time or fall time are the measurements of primary interest,you can convert an oscilloscope’s bandwidth (BW) specification to arise time specification with the following formula:Tr ≈0.35/BWor,for convenience:Tr (nanoseconds) ≈350/BW (MHz)As with bandwidth,you should select an oscilloscope with a risetime that’s three to five time greater than the fastest rise time

that you expect to measure.(It should be noted that the abovebandwidth to rise time conversion assumes that the oscilloscope’sresponse has a Gaussian oscilloscopes are designedto have a Gaussian roll-off.)Probe probes,like other electronic circuits,have abandwidth ,like oscilloscopes,probes are typically rankedor specified by their ,a probe with a 100 MHzbandwidth will have an amplitude response that is 3 dB down at

the 100 MHz of ProbesPrimerFigure dth derating rly,probe bandwidth can also be expressed in terms of risetime by the same formula used for oscilloscopes (Tr ≈0.35/BW).Also,for active probes,the oscilloscope and probe rise times can be

combined by the following formula to obtain an approximateprobe/oscilloscope system rise time:Trsystem2≈Trprobe2+ Trscope2For passive probes,the relationship is more complex,and the aboveformula should not be a rule,probe bandwidth should always equal or exceed thebandwidth of the oscilloscope that it will be used aprobe of lesser bandwidth will limit the oscilloscope to less than itsfull measurement is illustrated further in Figure 3-9,which shows the same pulse transition being measured with threeprobes of different first measurement,shown in Figure 3-9a,was made using

a matched 400 MHz oscilloscope and probe

probe used was a 10X probe with 10 MΩresistance and 14.1 that the pulse rise time was measured as 4.63 is well within the 875 ps rise time range of the 400 MHz

oscilloscope/probe look what happens when a 10X,100 MHz probe is used tomeasure the same pulse with the same is shownin Figure 3-9b,where the measured rise time is now 5.97 ’snearly a 30% increase over the previous measurement of 4.63 ns!As would be expected,the pulse’s observed rise time becomes evenlonger with a lower bandwidth extreme case is shown inFigure 3-9c,where a 1X,10 MHz probe was used on the same the rise time has slowed from the original 4.63 ns to 27 /accessories25

ABCs of s on rise time of three different probes:(a) 400 MHz,10X probe,(b) 100 MHz,10X probe,and (c) 10 MHz,1X measurements were made withthe same 400 MHz key point made by Figure 3-9 is:Just any probe will not do!To get maximum performance from any oscilloscope – the

performance that you paid for – be sure to use the manufacturer’srecommended dth to the probe general,the issues of probe bandwidthand resulting probe/oscilloscope system bandwidth should beresolved by following manufacturer’s specifications and nix,for example,specifies the bandwidth over whicha probe will perform within specified limits includetotal aberrations,rise time,and swept ,when used with a compatible oscilloscope,a Tektronix probeextends the oscilloscope’s bandwidth to the probe example,a Tektronix 100 MHz probe provides 100 MHz performance (–3 dB)

at the probe tip when used with a compatible 100 MHz industry recognized test setup for verifying bandwidth to theprobe tip is illustrated by the equivalent circuit in Figure t signal source is specified to be a 50 Ωsource terminated in 50 Ω,resulting in an equivalent 25 Ωsource onally,the probe must be connected to the source by a probe-tip-to-BNCadaptor or its latter requirement for probe connectionensures the shortest possible ground the above described test setup,a 100 MHz oscilloscope/probe

combination should result in an observed rise time of ≤3.5 3.5 ns rise time corresponds to a 100 MHz bandwidth according tothe previously discussed bandwidth/rise time relationship (Tr ≈0.35/BW).Figure lent circuit for testing bandwidth to the probe a

100 MHz system,the displayed rise time should be 3.5 ns or manufacturers of general-purpose oscilloscopes that includestandard accessory probes promise and deliver the advertised

oscilloscope bandwidth at the probe r,it’s important to remember that bandwidth at the probe tipis determined by the test method of Figure real-worldsignals rarely originate from 25 Ωsources,somewhat less thanoptimum response and bandwidth should be expected in real-worlduse — especially when measuring higher impedance lead making ground-referenced measurements,two connections to the circuit or device under test are connection is made via the probe which senses the voltage orother parameter being other necessary connection

is a ground return through the oscilloscope and back to the

circuit under ground return is necessary to complete

the measurement current /accessories

Figure ECB to probe-tip cases where the circuit under test and the oscilloscope areplugged into the same power outlet circuit,the common of thepower circuit provides a ground return signal return

path through the power grounds is typically indirect and uently it should not be relied on as a clean,low-inductiveground a rule,when making any kind of oscilloscope measurement,you should use the shortest possible grounding ultimategrounding system,is an in-circuit ECB (etched circuit board) toprobe-tip is shown in Figure ECB adaptorallows you to plug the probe tip directly into a circuit test point,and the outer barrel of the adaptor makes a direct and short groundcontact to the ground ring at the probe’s critical amplitude and timing measurements,it’s recommendedthat circuit board designs include ECB/probe-tip adaptors for

established test only does this clearly indicate test

point locations,but it ensures the best possible connection to thetest point for the most reliable oscilloscope unately,the ECB/probe-tip adaptor isn’t practical for manygeneral-purpose measurement d of using an

adaptor,the typical approach is to use a short ground lead that’sclipped to a grounding point in the circuit under is farmore convenient in that it allows you to quickly move the probeABCs of ProbesPrimerFigure lent circuit of a typical passive probe connected to a

signal point to point in the circuit under ,the short groundlead that most probe manufacturers supply with their probes providesan adequate ground return path for most measurement r,it’s wise to be aware of the possible problems that canarise from improper set the stage for this,notice

that there’s an inductance (L) associated with the ground lead in

the equivalent circuit shown in Figure ground-lead

inductance increases with increasing lead ,notice that the ground lead L and Cinforms a series resonantcircuit with only Rinfor this series resonant circuit

is hit with a pulse,it will only will there be ringing,butexcessive ground-lead L will limit the charging circuit to Cinand,thus,will limit the rise time of the t going into the mathematics,an 11 pF passive probe with

6-inch ground lead will ring at about 140 MHz when excited by afast a 100 MHz oscilloscope,this ringing is well abovethe bandwidth of the oscilloscope and may not be seen at ,with a faster oscilloscope,say 200 MHz,the ground lead inducedringing will be well within the oscilloscope’s bandwidth and will beapparent on the display of the you see ringing on a pulse display,try shortening the length ofyour ground lead.A shorter ground lead has less inductance andwill cause a higher frequency you see the ringing frequencychange on the pulse display,you’ll know that it’s ground-lead ning the ground lead further should move the ring frequencybeyond the bandwidth of the oscilloscope,thereby minimizing itseffect on your the ringing doesn’t change whenyou change ground-lead length,then the ringing is likely beinginduced in the circuit under 3-13 illustrates ground lead induced ringing

Figure 3-13a,a matched oscilloscope/probe combination was usedto acquire a fast ground lead used was the standard6.5-inch probeground clip,and it was attached to a common nearthe test /accessories27

ABCs of lead length and placement can dramatically affect Figure 3-13b,the same pulse transition is time,however,the probe’s standard ground lead was extended with a

28-inch clip ground lead extension might be done,forexample,to avoid having to move the ground clip each time differentpoints are probed in a large unately,this practicelengthens the ground loop and can cause severe ringing,as shownin Figure 3-13c shows the results of another variation of lengtheningthe ground this case,the probe’s ground lead wasn’t

connected at d,a separate,28-inch clip lead was run

from the circuit common to the oscilloscope created adifferent,and apparently longer,ground loop,resulting in the lowerfrequency ringing seen in Figure the examples in Figure 3-13,it’s clear that grounding practiceshave tremendous impact on measurement ically,probeground leads need to be kept as short and direct as to do About Probing Effects— Always keep your probe ground leads as short and direct as

ive ground loops can cause ringing on pulses.— Select the probes that best match your application’s needs in

terms of both measurement capabilities and mechanical

attachment to test finally,always be aware of the possible probe loading effects onthe circuit being many cases,loading can be controlledorminimized through probe following summarizes someof the probe loading considerations to be aware of:Passive probes.1X passive probes typically have a lower resistanceand higher capacitance than 10X passive a result,1Xprobes are more prone to cause loading,and whenever possible10X probes should be used for general-purpose e divider (Zo) probes have very low tip capacitance,but at the expense of relatively high resistive ’re intendedfor use where impedance matching is required in 50 Ωr,because of their very high bandwidth/rise time capabilities,voltage divider probes are often used in other environments forhigh-speed timing amplitude measurements,theeffect of the probe’s low input R should be taken into -offset probes.A bias-offset probe is a special type of voltagedivider probe with the capability of providing a variable offset voltageat the probe probes are useful for probing high-speed ECLcircuitry,where resistive loading could upset the circuit’s operating probes can provide the best of both worldswith very low resistive loading and very low tip de-off is that active probes typically have a low dynamic r,if your measurements fit within the range of an activeprobe,this can be the best choice in many the preceding examples and discussion,we’ve seen that thesignal source impedance,the probe,and the oscilloscope form aninteractive optimum measurement results,you need todo everything possible to minimize the oscilloscope/probe affects

on the signal following general rules will help you indoing this:— Always match your oscilloscope and probes according to the

oscilloscope manufacturer’s recommendations.— Make sure that your oscilloscope/probe has adequate bandwidthor rise time capabilities for the signal you’re trying to lly,you should select a oscilloscope/probe combination

with a rise time specification that’s three to five times faster

than the fastest rise time you plan to /accessories

Figure example of measuring aberrations relative to 100% pulse tanding Probe SpecificationsMost of the key probe specifications have been discussed in precedingchapters,either in terms of probe types or in terms of how probesaffect chapter gathers all of those key probespecification parameters and terms into one place for easier following list of specifications is presented in alphabeticalorder; not all of these specifications will apply to any given example,Insertion Impedance is a specification that applies tocurrent probes specifications,such as bandwidth,areuniversal and apply to all tions (universal)An aberration is any amplitude deviation from the expected or idealresponse to an input practice,aberrations usually occurimmediately after fast waveform transitions and appear as what’ssometimes referred to as “ringing.”Aberrations are measured,or specified,as a ±percentage deviationfrom the final pulse response level (see Figure 4-1).This specificationmight also include a time window for the example ofthis would be:Aberrations should not exceed ±3% or 5% peak-to-peak within thefirst When excessive aberrations are seen on a pulse measurement,be sureto consider all possible sources before assuming that the aberrations arethe fault of the example,are the aberrations actually part of thesignal source? Or are they the result of the probe grounding technique?One of the most common sources of observed aberrations isneglecting to check and properly adjust the compensation of voltageprobes.A severely over-compensated probe will result in significantpeaks immediately following pulse edges (see Figure 4-2).Amp-Second Product (current probes)For current probes,amp-second product specifies the energy handlingcapability of the current transformer’s the product of theaverage current and pulse width exceeds the amp-second rating,theABCs of ProbesPrimerFigure tions from over compensating a core saturation results in a clipping off or sup-pression of those portions of the waveform occurring during the amp-second product is not exceeded,the signal voltage

output of the probe will be linear and the measurement ation Factor (universal)All probes have an attenuation factor,and some probes may have selectableattenuation l attenuation factors are 1X,10X,and attenuation factor is the amount by which the probe reducessignal amplitude.A 1X probe doesn’t reduce,or attenuate,the signal,while a 10X probe reduces the signal to 1/10th of its probe attenuation factors allow the measurement rangeof an oscilloscope to be example,a 100X probeallows signals of 100 times greater amplitude to be 1X,10X,100X designations stem from the days when oscilloscopesdidn’t automatically sense probe attenuation and adjust scale 10X designation,for example,reminded you that

all amplitude measurements needed to be multiplied by dout systems on today’s oscilloscopes automatically sense probeattenuation factors and adjust the scale factor readouts e probe attenuation factors are achieved using resistive voltagedivider uently,probes with higher attenuationfactors typically have higher input the dividereffect splits probe capacitance,effectively presenting lower probetip capacitance for higher attenuation cy (universal)For voltage-sensing probes,accuracy generally refers to the probe’sattenuation of a DC calculations and measurements of probeaccuracy generally should include the oscilloscope’s input ,a probe’s accuracy specification is only correct or applicablewhen the probe is being used with an oscilloscope having the assumedinput example accuracy specification would be:10X within 3% (for oscilloscope input of 1 MΩ±2%)/accessories29

ABCs of ProbesPrimerFor current-sensing probes,the accuracy specification refers to theaccuracy of the current-to-voltage depends on thecurrent transformer turns ratio and the value and accuracy of theterminating t probes that work with dedicatedamplifiers have outputs that are calibrated directly in amps/div andhave accuracy specifications that are given in terms of attenuatoraccuracy as a percentage of the current/division dth (universal)All probes have bandwidth.A 10 MHz probe has a 10 MHz bandwidth,and a 100 MHz probe has a 100 MHz bandwidth ofa probe is that frequency where the probe’s response causes outputamplitude to fall to 70.7% (–3 dB),as indicated in Figure should also be noted that some probes have a low-frequencybandwidth limit as is the case,for example,with AC e of their design,AC current probes cannot pass DCor low-frequency ,their bandwidth must be specifiedwith two values,one for low frequency and one for high oscilloscope measurements,the real concern is the overallbandwidth of the oscilloscope and probe system

performanceis what ultimately determines measurement unately,attaching a probe to an oscilloscope results in somedegradation of bandwidth example,using a 100 MHzgeneric probe with a 100 MHz oscilloscope results in a measurementsystemwith a bandwidth performance that is something less than100 avoid the uncertainty of overall system bandwidth

performance,Tektronix specifies its passive voltage probes to

provide a specified measurement system bandwidth at the probe tipwhen used with the designated oscilloscope selecting oscilloscopes and oscilloscope probes,it’s important torealize that bandwidth has several implications for measurement terms of amplitude measurements,a sine wave’s amplitudebecomes increasingly attenuated as the sine wave frequencyapproaches the bandwidth the bandwidth limit,the sine wave’samplitude will be measured as being 70.7% of its actual ,for greater amplitude measurement accuracy,it’s necessary toselect oscilloscopes and probes with bandwidths several times greaterthan the highest frequency waveform that you plan to same holds true for measuring waveform rise and fall rm transitions,such as pulse and square wave edges,aremade up of high-frequency ation of these high-frequency components by a bandwidth limit results in the transitionappearing slower than it really accurate rise- and fall-timemeasurements,it’s necessary to use a measurement system withadequate bandwidth to preserve the high frequencies that make upthe waveform’s rise and fall is most often stated in /accessoriesFigure dth is that frequency in the response curve where a sine

wave’s amplitude is decreased by 70.7% (–3 dB).of a measurement system rise time,which should typically be fourto five times faster than the rise times that you are trying to tance (universal)Generally,probe capacitance specifications refer to the capacitanceat the probe is the capacitance that the probe adds to thecircuit test point or device under tip capacitance is important because it affects how pulses aremeasured.A low tip capacitance minimizes errors in making risetime ,if a pulse’s duration is less than five timesthe probe’s RC time constant,the amplitude of the pulse is also present a capacitance to the input of the oscilloscope,and this probe capacitance should match that of the 10X and 100X probes,this capacitance is referred to as a

compensation range,which is different than tip be matching,the oscilloscope’s input capacitance should bewithin the compensation range of the (differential probes)Common-mode rejection ratio (CMRR) is a differential probe’s abilityto reject any signal that is common to both test points in a is a key figure of merit for differential probesand amplifiers,and it is defined by:CMRR = |Ad/Ac|where:Ad= the voltage gain for the difference = the voltage gain for common-mode y,Adshould be large,while Acshould equalize to zero,resultingin an infinite practice,a CMRR of 10,000:1 is consideredquite this means is that a common-mode input signal of5 volts will be rejected to the point where it appears as 0.5 millivoltsat the rejection is important for measuring differencesignals in the presence of CMRR decreases with increasing frequency,the frequency atwhich CMRR is specified is as important as the CMRR value.A

differential probe with a high CMRR at a high frequency is betterthan a differential probe with the same CMRR at a lower frequency.

CW Frequency Current Derating (current probes)Current probe specifications should include amplitude versus frequencyderating curves that relate core saturation to increasing effect of core saturation with increasing frequency is that awaveform with an average current of zero amps will experience

clipping of amplitude peaks as the waveform’s frequency or

amplitude is Time Constant (current probes)The decay time constant specification indicates a current probe’spulse supporting time constant is the secondaryinductance (probe coil) divided by the terminating ay time constant is sometimes called the probe L/R larger L/R ratios,longer current pulses can be representedwithout significant decay or droop in smaller L/Rratios,long- duration pulses will be seen as decaying to zero beforethe pulse is actually Current (current probes)Direct current decreases the permeability of a current probe’s coil decreased permeability results in a decreased coil inductanceandL/R time result is reduced coupling performance for lowfrequencies and loss of measurement response for low-frequency AC current probes offer current-bucking options that null the effects of ion Impedance (current probes)Insertion impedance is the impedance that is transformed from thecurrent probe’s coil (the secondary) into the current carrying conductor(the primary) that’s being lly,a current probe’sreflected impedance values are in the range of milliOhms and

present an insignificant effect on circuits of 25 Ωor more Capacitance (universal)The probe capacitance measured at the probe Resistance (universal)A probe’s input resistance is the impedance that the probe placeson the test point at zero Hertz (DC).Maximum Input Current Rating (current probes)The maximum input current rating is the total current (DC plus peakAC) that the probe will accept and still perform as ACcurrent measurements,peak-to-peak values must be derated versusfrequency to calculate the maximum total input m Peak Pulse Current Rating

(current probes)This rating should not be takes into account core saturationand development of potentially damaging secondary maximumpeak pulse current rating is usually stated as an amp-second of ProbesPrimerMaximum Voltage Rating (universal)Voltages approaching a probe’s maximum rating should be imum voltage rating is determined by the breakdown voltage ratingof the probe body or the probe components at the measuring ation Delay (universal)Every probe offers some small amount of time delay or phase shiftthat varies with signal is a function of the probecomponents and the time it takes for the signal to travel throughthese components from probe tip to oscilloscope y,the most significant shift is caused by the probe mple,a 42-inch section of special probe cable has a 5 ns a 1 MHz signal,the 5 ns delay results in a two-degree phaseshift.A longer cable results in correspondingly longer signal ation delay is usually only a concern when comparativemeasurements are being made between two or more mple,when measuring time differences between two waveforms,the waveforms should be acquired using matched probes so thateach signal experiences the same propagation delay through the r example would be making power measurements by using avoltage probe and a current probe in voltage andcurrent probes are of markedly different construction,they will havedifferent propagation r or not these delays will havean effect on the power measurement depends on the frequencies ofthe waveforms being Hz and kHz signals,the delaydifferences will generally be r,for MHz signalsthe delay differences may have a noticeable Time (universal)A probe’s 10 to 90% response to a step function indicates the fastesttransition that the probe can transmit from tip to accurate rise- and fall-time measurements on pulses,the meas-urement system’s rise time (oscilloscope and probe combined) shouldbe three to five times faster than the fastest transition to be tial Noise (active probes)Tangential noise is a method of specifying probe-generated noise inactive tial noise figures are approximately two timesthe RMS ature Range (universal)Current probes have a maximum operating temperature that’s theresult of heating effects from energy induced into the coil’s sing temperature corresponds to increased e of this,current probes have a maximum amplitude versusfrequency derating ator voltage probes (i.e.,10X,100X,etc.) may be subject toaccuracy changes due to changes in /accessories31

ABCs of ProbesPrimerFigure s probe categories based on the signal type to be measured.A Guide to Probe SelectionThe preceding chapters have covered a wide range of topics regardingoscilloscope probes in terms of how probes function,the various types ofprobes,and their effects on the most part,the focushas been on what happens when you connect a probe to a test this chapter,the focus changes to the signal source and how totranslate its properties into criteria for appropriate probe goal,as always,is to select the probe that delivers the bestrepresentation of the signal to the r,it doesn’tstop oscilloscope imposes certain requirements that mustalso be considered as part of the probe selection apter explores the various selection requirements,beginning withunderstanding the requirements imposed by the signal tanding the Signal SourceSignal first step in probe selection is to assess the type

of signal to be this purpose,signals can be categorizedas being:— Voltage Signals— Current Signals— Logic Signals— Other SignalsVoltage signals are the most commonly encountered signal type inelectronic accounts for the voltage-sensingprobe as being the most common type of oscilloscope ,itshould be noted that,since oscilloscopes require a voltage signal attheir input,other types of oscilloscope probes are,in essence,transducers that convert the sensed phenomenon to a correspondingvoltage signal.A common example of this is the current probe,which transforms a current signal into a voltage signal for viewingon an signals are actually a special category of voltage a logic signal can be viewed with a standard voltage probe,it’smore often the case that a specific logic event needs to be can be done by setting a logic probe to provide a trigger signalto the oscilloscope when a specified logic combination are four fundamental signal source issues to be considered inselecting a are the signal type,the signal frequencycontent,the source impedance,and the physical attributes of thetest of these issues is covered in the following /accessories

This allows specific logic events to be viewed on the addition to voltage,current,and logic signals,there are numerousother types of signals that may be of can includesignals from optical,mechanical,thermal,acoustic,and s transducers can be used to convert such signals

to corresponding voltage signals for oscilloscope display and

this is done,the transducer becomes the

signal source for the purposes of selecting a probe to convey thetransducer signal to the 5-1 provides a graphical categorization of probes based onthe type of signal to be that under each categorythere are various probe subcategories that are further determinedby additional signal attributes as well as oscilloscope frequency signals,regardless of their type,have frequency signals have a frequency of 0 Hz,andpure sinusoids have a single frequency that is the reciprocal of the

sinusoid’s other signals contain multiple frequencieswhose values depend upon the signals example,asymmetrical square wave has a fundamental frequency (fo) that’sthe reciprocal of the square wave’s period and additional harmonicfrequencies that are odd multiples of the fundamental (3fo,5fo,7fo,...).The fundamental is the foundation of the waveshape,andthe harmonics combine with the fundamental to add structural

detail such as the waveshape’s transitions and a probe to convey a signal to an oscilloscope while maintainingadequate signal fidelity,the probe must have enough bandwidth topass the signal’s major frequency components with minimum

the case of square waves and other periodic signals,this generally means that the probe bandwidth needs to be three tofive times higher than the signal’s fundamental lows the fundamental and the first few harmonics to be passedwithout undue attenuation of their relative higherharmonics will also be passed,but with increasing amounts ofattenuation since these higher harmonics are beyond the probe’s

3-dB bandwidth r,since the higher harmonics are

still present at least to some degree,they’re still able to contributesomewhat to the waveform’s of major frequency components of a signal are beyond the

measurement system bandwidth (a),they experience a higher degree of

result is loss of waveform detail through rounding of corners

and lengthening of transitions (b).The primary effect of bandwidth limiting is to reduce signal closer a signal’s fundamental frequency is to the probe’s 3-dBbandwidth,the lower the overall signal amplitude seen at the the 3-dB point,amplitude is down 30%.Also,those

harmonics or other frequency components of a signal that extendbeyond the probe’s bandwidth will experience a higher degree ofattenuation because of the bandwidth result of higherattenuation on higher frequency components may be seen as arounding of sharp corners and a slowing of fast waveform transitions(see Figure 5-2).It should also be noted that probe tip capacitance can also limit

signal transition rise r,this has to do with signalsource impedance and signal source loading,which are the nexttopics of /accessories33

ABCs of ProbesPrimerSignal source discussion of source impedance canbe distilled down to the following key points*1: probe’s impedance combines with the signal source

impedance to create a new signal load impedance that has

some effect on signal amplitude and signal rise the probe impedance is substantially greater than the signalsource impedance,the effect of the probe on signal amplitude

is tip capacitance,also referred to as input capacitance,has theeffect of stretching a signal’s rise is due to the time requiredto charge the input capacitance of the probe from the 10% to

90% level,which is given by:tr = 2.2 x Rsourcex CprobeFrom the above points,it’s clear that high-impedance,low-capacitanceprobes are the best choice for minimizing probe loading of the

signal ,probe loading effects can be further minimizedby selecting low-impedance signal test points whenever al connection location and geometry ofsignal test points can also be a key consideration in probe itenough to just touch the probe to the test point and observe the signalon the oscilloscope,or will it be necessary to leave the probe attached tothe test point for signal monitoring while making various circuit adjust-ments? For the former situation,a needle-style probe tip is appropriate,while the latter situation requires some kind of retractable hook size of the test point can also impact probe rdsize probes and accessories are fine for probing connector pins,resistor leads,and back r,for probing surface mountcircuitry,smaller probes with accessories designed for surfacemount applications are goal is to select probe sizes,geometries,and accessories thatbest fit your particular allows quick,easy,and solidconnection of probes to test points for reliable oscope IssuesBandwidth and rise ’s important to realize that the

oscilloscopeand its probes act together as a measurement ,the oscilloscope used should have bandwidth and rise timespecificationsthat equal or exceed those of the probe used and thatare adequate for the signals to be general,the bandwidth and rise time interactions between probesand oscilloscopes are e of this complexity,mostoscilloscope manufacturers specify oscilloscope bandwidth and risetime to the probe tip for specific probe models designed for usewith specific ensure adequate oscilloscope

system bandwidth and rise time for the signals that you plan toexamine,it’s best to follow the oscilloscope manufacturer’s

probe resistance and oscilloscopes have inputresistance and input maximum signal transfer theinput R and C of the oscilloscope must match the R and C present-ed by the probe’s output as follows:RscopeCscope= RprobeCprobe= Optimum Signal TransferMore specifically,50 Ωoscilloscope inputs require 50 Ωprobes,and 1 MΩoscilloscope inputs require 1 MΩprobes.A 1 MΩoscilloscope can also be used with a 50 Ωprobe when the

appropriate 50 Ωadapter is -to-oscilloscope capacitances must be matched as done through selection of probes designed for use with specificoscilloscope onally,many probes have a compensationadjustment to allow precise matching by compensating for minorcapacitance er a probe is attached to an

oscilloscope,the first thing that should be done is to adjust theprobe’s compensation (see Compensation in Chapter 1).Failing toproperly match a probe to the oscilloscope – both through properprobe selection and proper compensation adjustment – can result

in significant measurement oscope issues have as much bearing on probe selection assignal source the probe doesn’t match the oscilloscope,signal fidelity will be impaired at the oscilloscope end of the probe.*1Refer to the section titled “Different Probes for Different Needs”for more detail regarding signal source impedance and the effects of its interaction with probe /accessories

oscilloscope’s vertical sensitivity range determinesthe overall dynamic range for signal amplitude mple,an oscilloscope with a 10-division vertical display rangeand a sensitivity range from 1 mV/division to 10 V/division has apractical vertical dynamic range from around 0.1 mV to 100 thevarious signals that you intend to measure range in amplitude from0.05 mV to 150 V,the base dynamic range of the example oscilloscopefalls short at both the low and high r,this shortcomingcan be remedied by appropriate probe selection for the various

signals that you’ll be dealing high-amplitude signals,the oscilloscope’s dynamic range can beextended upwards by using attenuator example,a 10Xprobe effectively shifts the oscilloscope’s sensitivity range upwardby a decade,which would be 1 mV/division to 100 V/division for theexample only does this provide adequate range foryour 150-volt signals,it gives you a top-end oscilloscope displayrange of 1000 r,before connecting any probe to a

signal make sure that the signal doesn’t exceed the probe’s

maximum voltage nAlways observe the probe’s maximum specified voltage ing the probe to a voltage in excess of those capabilities mayresult in personal injury as well as damage to low-amplitude signals,it’s possible to extend the range of theoscilloscope to lower sensitivities through use of a probe typically is a differential amplifier,which could providea sensitivity of 10 µV/division for probe amplifier systems are highly specialized and are designedto match specific oscilloscope a result,it’s important inmaking an oscilloscope selection to always check the manufacturer’slist of recommended accessories for available differential probe

systems that meet your small-signal application nDifferential probe systems often contain sensitive components thatmay be damaged by overvoltages,including static d damage to the probe system,always follow the manufacturer’srecommendations and observe all t modern oscilloscopes provide on-screenreadouts of their vertical and horizontal sensitivity settings(volts/division and seconds/division).Often these oscilloscopes alsoprovide probe sensing and readout processing so that the readoutproperly tracks the type of probe being example,if a 10Xprobe is used,the oscilloscope should appropriately reflect that byadjustingthe vertical readout by a 10X if you’re using acurrent probe,the vertical readout is changed from volts/division toamps/division to reflect the proper units of of ProbesPrimerTo take advantage of such readout capability,it’s important to useprobes that are compatible with the oscilloscope’s readout ,this means following the manufacturer’s recommendationsregarding probe usages with specific is

especially important for newer oscilloscopes which may haveadvanced readout features that may not be fully supported by

many generic or commodity ing the Right ProbeFrom all of the preceding signal source and oscilloscope issues,it’s clear that selecting the right probe can be a daunting processwithout some fact,since some key selection criteria –such as probe rise time and oscilloscope input C – are not alwaysspecified,the selection process may be reduced to guesswork insome avoid guesswork,it’s always best to select an oscilloscope thatincludes a wide selection of probes in the recommended ,when you encounter new measurement requirements,besure to check with the manufacturer of your oscilloscope for newlyintroduced probes that may extend your oscilloscope’s finally,keep in mind that there really is no “right”probe selec-tion for any given are only “right”oscilloscope/probe combination selections,and they rely on firstdefining your signal measurement requirements in terms of:— Type of signal (voltage,current,optical,etc.)— Signal frequency content (bandwidth issues)— Signal rise time— Source impedance (R and C)— Signal amplitudes (maximum,minimum)— Test point geometries (leaded component,surface mount,etc.)By considering the above issues and filling in the blanks with

information specific to your applications,you’ll be able to specifythe oscilloscope and various compatible probes that will meet all ofyour application /accessories35

ABCs of 6-1.A fast step (1 ns Tr) has aberrations impressed on it due to use of a six-inch probe ground lead (a).These aberrations can be changed by moving the probe cable or placing a hand over the cable (b).Advanced Probing TechniquesThe preceding chapters have covered all of the basic informationthat you should be aware of concerning oscilloscope probes andtheir most measurement situations,the standard probesprovided with your oscilloscope will prove more than adequate aslong as you keep in mind the basic issues of:— Bandwidth/rise time limits— The potential for signal source loading— Probe compensation adjustment— Proper probe groundingEventually,however,you’ll run into some probing situations that gobeyond the chapter explores some of the advancedprobing issues that you’re most likely to encounter,beginning withour old friend the ground Lead following discussion provides information and guidelinesfor determining if aberrations are part of the measurement processand,if so,how to address the lead probe ground lead has some inductance,and the longer the ground lead the greater the mbined with probe tip capacitance and signal source capacitance,ground lead inductance forms a resonant circuit that causes ringingat certain order to see ringing or other aberrations caused by poor

grounding,the following two conditions must exist: oscilloscope system bandwidth must be high enough to

handle the high-frequency content of the signal at the probe input signal at the probe tip must contain enough

high-frequency information (fast rise time) to cause the ringing

or aberrations due to poor 6-1 shows examples of ringing and aberrations that can beseen when the above two conditions are waveforms shownin Figure 6-1 were captured with a 350 MHz oscilloscope whileusing a probe having a six-inch ground actual waveformat the probe tip was a step waveform with a 1 ns rise 1 ns rise time is equivalent to the oscilloscope’s bandwidth(BW ≈0.35/Tr) and has enough high-frequency content to causeringing within the probe’s ground ringing signal isinjected in series with the step waveform,and it’s seen as

aberrations impressed on top of the step,as shown in Figure 6-1aand Figure lead issues continue to crop up in oscilloscope measurementsbecause of the difficulty in determining and establishing a trueground reference point for difficulty arises fromthe fact that ground leads,whether on a probe or in a circuit,haveinductance and become circuits of their own as signal effect of this was discussed and illustrated inChapter 1,where a long ground lead caused ringing to appear on addition to being the source of ringing and other waveformaberrations,the ground lead can also act as an antenna for ion is the first defense against ground-lead be suspicious of any noise or aberrations being observed on anoscilloscope display of a noise or aberrations may bepart of the signal,or they may be the result of the /accessories

ABCs of ProbesPrimerFigure 1 ns rise time step waveform as acquired through an

ECB-to-Probe Tip l ECB-to-Probe Tip Adaptor es of ground lead effects for passive probes versus active three traces on the left

show the effects on the waveform of 1/2-inch,6-inch,and 12-inch ground leads used on a passive

three traces on the right show the same waveform acquired using the same ground leads,but with an active

FET of the waveform displays in Figure 6-1 were obtained whileacquiring the same step waveform with the same oscilloscope ,however,that the aberrations are slightly different inFigure 6-1b,as compared to Figure difference seen inFigure 6-1b was obtained by repositioning the probe cable slightlyand leaving a hand placed over part of the probe

repositioning of the cable and the presence of a hand near thecable caused a small change in the capacitance and high-frequencytermination characteristics of the probe grounding circuitry and thusa change in the fact that the probe ground lead can cause aberrations on awaveform with fast transitions is an important point to ’salso just as important to realize that aberrations seen on a waveformmight just be part of the waveform and not a result of the probegrounding distinguish between the two situations,movethe probe cable placing your hand over the probe or

moving the cable causes a change in the aberrations,the aberrationsare being caused by the probe grounding system.A correctlygrounded (terminated) probe will be completely insensitive to

cable positioning or /accessories37

ABCs of ProbesPrimerTo further illustrate the above points,the same waveform was

again acquired with the same oscilloscope and thistime,the six-inch probe ground lead was removed,and the step

signal was acquired through an ECB-to-Probe Tip Adaptor installation(see Figure 6-2).The resulting display of the aberration-free stepwaveform is shown in Figure ation of the probe’s groundlead and direct termination of the probe in the ECB-to-Probe TipAdaptor has eliminated virtually all of the aberrations from thewaveform display is now an accurate portrayal of thestep waveform at the test are two main conclusions to be drawn from the above first is that ground leads should be kept as short as possiblewhen probing fast second is that product designers canensure higher effectiveness of product maintenance and troubleshootingby designing in product includes using ECB-to-ProbeTip Adaptors where necessary to better control the test environmentand avoid misadjustment of product circuitry during installation

or you’re faced with measuring fast waveforms where an ECB-to-Probe Tip Adaptor hasn’t been installed,remember to keep theprobe ground lead as short as many cases,this can bedone by using special probe tip adaptors with integral another alternative is to use an active FET bes,because of their high input impedance and extremely low tipcapacitance (often less than 1 pF),can eliminate many of theground lead problems often experienced with passive isillustrated further in Figure Lead Noise is another type of signal distor-tionthat can appear on oscilloscope waveform withringing and aberrations,noise might actually be part of the signal atthe probe tip,or it might appear on the signal as a result of impropergrounding difference is that the noise is generallyfrom an external source and its appearance is not a function of thespeed of the signal being other words,poor groundingcan result in noise appearing on any signal of any are two primary mechanisms by which noise can beimpressed on signals as a result of is by ground

loop noise other is by inductive pickup through theprobe cable or probe ground mechanisms are discussedindividually loop noise injection into the grounding

system can be caused by unwanted current flow in the ground loopexisting between the oscilloscope common and test circuit powerline grounds and the probe ground lead and ly,all ofthese points are,or should be,at zero volts,and no ground current

/accessoriesDevice Under TestSupplyPowerFigure complete ground circuit,or ground loop,for an oscilloscope,probe,and test circuit on two different power r,if the oscilloscope and test circuit are on differentbuilding system grounds,there could be small voltage differences ornoise on one of the building ground systems (see Figure 6-5).Theresulting current flow will develop a voltage drop across the probe’souter cable noise voltage will be injected into the

oscilloscope in series with the signal from the probe resultis that you’ll see noise riding on the signal of interest,or the signalof interest may be riding on ground loop noise injection,the noise is often line frequencynoise (60 Hz).Just as often,though,the noise may be in the form

of spikes or bursts resulting from building equipment,such as airconditioners,switching on and are various things that can be done to avoid or minimizeground loop noise first approach is to minimizeground loops by using the same power circuits for the oscilloscopeand circuit under onally,the probes and their cablesshould be kept away from sources of potential

particular,don’t allow probe cables to lie alongside or across

equipment power ground loop noise problems persist,you may need to open theground loop by one of the following methods: a ground isolation a power line isolation transformer on either the test circuit oron the an isolation amplifier to isolate the oscilloscope probes from

the differential probes to make the measurement

(rejects common-mode noise).

Figure example of circuit board induced noise in the probe ground loop(tip shorted to the ground clip).In no case should you attempt to isolate the oscilloscope or test

circuit by defeating the safety three-wire ground it’s

necessary to float the measurements,use an approved isolationtransformer or preferably a ground isolation monitor specificallydesigned for use with an nTo avoid electrical shock,always connect probes to the oscilloscopeor probe isolator before connecting the probe to the circuit under d can enter a common ground system by inductioninto probe cables,particularly when probes with long cables are ity to power lines or other current-carrying conductors can inducecurrent flow in the probe’s outer cable circuit is completedthrough the building system common minimize this potentialsource of noise,use probes with shorter cables when possible,andalways keep probe cables away from possible sources of can also be induced directly into the probe ground is the result of typical probe ground leads appearing as a

single-turn loop antenna when connected to the test ound lead antenna is particularly susceptible to electromagneticinterference from logic circuits or other fast changing theprobe ground lead is positioned too close to certain areas on thecircuit board under test,such as clock lines,the ground lead maypick up signals that will be mixed with the signal at the probe you see noise on an oscilloscope display of a signal,thequestion is:Does the noise really occur as part of the signal at theprobe tip,or is it being induced into the probe ground lead?To answer this question,try moving the probe ground lead noise signal level changes,the noise is being induced into theground of ProbesPrimer+Differential+A–VVOModeVDM+CommonModeV=CMVO

AV •

VDMFigure 6-7.A differential amplifier has two signal lines which are differenced

into a single signal that is referenced to r very effective approach to noise source identification is todisconnect the probe from the circuit and clip the probe’s groundlead to the probe pass this probe-tip/ground-lead loopantenna back and forth over the loop antenna will pickup areas of strong radiated noise in the 6-6 shows anexample of what can be found on a logic circuit board by searchingwith the probe ground lead connected to the probe minimize noise induced into the probe ground,keep ground leadsaway from noise sources on the board under onally,ashorter ground lead will reduce the amount of noise ential MeasurementsStrictly speaking,all measurements are differential measurements.A standard oscilloscope measurement where the probe is attachedto a signal point and the probe ground lead is attached to circuitground is actually a measurement of the signal difference betweenthe test point and that sense,there are two signal lines –the ground signal line and the test signal practice,however,differential measurements refers to the

measurement of two signal lines,both of which are above requires use of some sort of differential amplifier so that thetwo signal lines (the double-ended signal source) can be algebraicallysummed into one signal line reference to ground

(single-ended signal)for input to the oscilloscope,as shown in Figure differentialamplifier can be a special amplifier that is part of the probing system,or if the oscilloscope allows waveform math,each signal line can

be acquired on separate oscilloscope channels and the two channelsalgebraically either case,rejection of the common-mode signal is a key concern in differential measurement /accessories39

ABCs of ProbesPrimer++–TP1TP1-+175 V+175 VTP2 (VTP2 (Vs8))–175 V-175 VVV+175 VTP2+175 VV+7 VTP2VGSGS (TP1-TP2)(TP1-TP2)+7 V–175 V-175 V-7 V–7 VFigure ential amplifier used to measure gate to source voltage ofupper transistor in an inverter that the source potential changes 350volts during the tanding difference and common-mode ideal

differential amplifier amplifies the “difference”signal,VDM,betweenits two inputs and completely rejects any voltage which is commonto both inputs, result is an output voltage given by:Vo– Av(V+in- V-in)where:Av= the amplifier’s gainVo= the output signal referenced to earth groundThe voltage of interest,or difference signal,is referred to as the

differential voltage or differential mode signal and is expressed as:VDMwhere:VDM= the V+in– V-interm in the equation aboveNotice that the common mode voltage,VCM,is not part of the ’s because the ideal differential amplifier rejects all of thecommon-mode component,regardless of its amplitude or 6-8 provides an example of using a differential amplifier tomeasure the gate drive of the upper

MOSFETdevice in an the MOSFET switches on and off,the source voltageswings from the positive supply rail to the negative rail.A /accessories+45 V+ 45 V0.10.1ΩΩVVOUT80 V80Vp-pp-p35 mV35mVp-pp-p_-+Measured voltageMeasured voltagecould be as high

could be as highas 42 mV

as 42 mV– 45 V-45 V(22% error)Figure -mode error from a differential amplifier with 10,000:1 the gate signal to be referenced to the differentialamplifier allows the oscilloscope to measure the true VGSsignal (afew volt swing) at sufficient resolution,such as 2 V/division,while

rejecting the several-hundred-volt transition of the source to real-life,differential amplifiers cannot reject all of the common-mode signal.A small amount of common-mode voltage appears asan error signal in the common-mode error signal isindistinguishable from the desired differential ability of a differential amplifier to minimize undesirable com-mon-mode signals is referred to as common-mode rejection ratio orCMRR for true definition of CMRR is “differential-modegain divided by common-mode gain referred to the input”:CMRR = ADM/ACMFor evaluation purposes,CMRR performance can be assessed withno input CMRR then becomes the apparent VDMseen

at the output resulting from the common-mode isexpressed either as a ratio – such as 10,000:1 – or in dB:dB = 20 log (ADM/ACM)For example,a CMRR of 10,000:1 is equivalent to 80 see theimportance of this,suppose you need to measure the voltage in theoutput damping resistor of the audio power amplifier shown inFigure full load,the voltage across the damper (VDM) shouldreach 35 mV with an output swing (VCM) of 80 differentialamplifier being used has a CMRR specification of 10,000:1 at 1 the amplifier driven to full power with a 1 kHz sine wave,oneten thousandth of the common-mode signal will erroneously appearas VDMat the output of the differential amplifier,which would be

80 V/10,000 or 8 8 mV of residual commonmode signalrepresents up to a 22% error in the true 35 mV signal!

It’s important to note that the CMRR specification is an doesn’t specify polarity or degrees of phase shift of ore,you cannot simply subtract the error from the

displayed ,CMRR generally is highest (best) at DCand degrades with increasing frequency of differentialamplifiers plot the CMRR specification as a function of frequency;others simply provide CMRR specifications at a few key either case,it’s important in comparing differential amplifiers orprobes to be certain that your CMRR comparisons are at the samefrequency or ’s also important to realize that CMRR specifications assume thatthe common-mode component is is often not thecase in example,the common-mode signal in theinverter of Figure 6-8 is a 30 kHz square the squarewave contains energy at frequencies considerably higher than

30 kHz,the CMRR will probably be worse than that specified at the30 kHz er the common-mode component is not sinusoidal,anempirical test is the quickest way to determine the extent of theCMRR error (see Figure 6-10).Temporarily connect both input leadsto the oscilloscope is now displaying only the common-mode can now determine if the magnitude of the errorsignal is er,the phase difference between

VCMand VDMis not ore subtracting the displayedcommon-mode error from the differential measurement will notaccurately cancel the error test illustrated by Figure 6-10 is handy for determining theextent of common-mode rejection error in the actual r there’s one effect this test will not both inputs connected to the same point,there’s no differencein driving impedance as seen by the situation producesthe best CMRR when the two inputs of a differentialamplifier are driven from significantly different source impedances,the CMRR will be zing differential measurement ting thedifferential amplifier or probe to the signal source is generally thegreatest source of maintain the input match,both pathsshould be as identical as cabling should be of thesame length for both individual probes are used for each signal line,they should be thesame model and cable measuring low-frequency signalswith large common-mode voltages,avoid the use of high gains,they simply cannot be used as it’s impossibleto precisely balance their attenuation is neededfor high-voltage or high-frequency applications,special passiveABCs of ProbesPrimer+175 V+175 V++-–-175 V–175 VFigure cal test for adequate common-mode inputs

are driven from the same al common-mode appears at the test will not catch the effect of differential source impedances.+–Figure the input leads twisted together,the loop area is very small,hence less field passes through induced voltage tends to be in the VCMpath which is rejected by the differential designed specifically for differential applications should probes have provisions for precisely trimming DC

attenuation and AC get the best performance,a setof probes should be dedicated to each specific amplifier and calibratedwith that amplifier using the procedure included with the cabling that’s spread apart acts as a transformer magnetic field passing through the loop induces a voltage whichappears to the amplifier input as differential and will be faithfullysummed into the output! To minimize this,it’s common practice totwist the + and – input cables together in a reduces linefrequency and other noise pick the input leads twistedtogether,as indicated in Figure 6-11,any induced voltage tends tobe in the VCMpath,which is rejected by the differential /accessories41

ABCs of 6-12.A noisy signal (a) can be cleaned up by signal averaging (b).-frequency measurements subject to excessive common-modecan be improved by winding both input leads through a ferrite

attenuates high-frequency signals which are commonto both e the differential signals pass through thecore in both directions,they’re input connectors of most differential amplifiers are BNC

connectors with the shell using probes or coaxialinput connections,there’s always a question of what to do with e measurement applications vary,there are no hardand fast measuring low-level signals at low frequencies,it’s generallybest to connect the grounds only at the amplifier end and leaveboth unconnected at the input provides a return path forany currents induced into the shield,but doesn’t create a groundloop which may upset the measurement or the higher frequencies,the probe input capacitance,along with thelead inductance,forms a series resonant “tank”circuit which single-ended measurements,this effect can be minimized byusing the shortest possible ground lowers the inductance,effectively moving the resonating frequency higher,hopefully beyondthe bandwidth of the ential measurements are madebetween two probe tips,and the concept of ground does not enterinto the r,if the ring is generated from a fastrise of the common-mode component,using a short ground leadreduces the inductance in the resonant circuit,thus reducing thering some situations,a ring resulting from fast

differential signals may also be reduced by attaching the is the case if the common-mode source has very lowimpedance to ground at high frequencies, bypassed this isn’t the case,attaching the ground lead maymake the situation worse! If this happens,try grounding the probestogether at the input lowers the effective inductancethrough the course,connecting the probe ground to the circuit may generatea ground usually doesn’t cause a problem when measuringhigher-frequency best advice when measuring high

frequencies is to try making the measurement with and without theground lead; then use the setup which gives the best connecting the probe ground lead to the circuit,remember toconnect it to ground! It’s easy to forget where the ground connectionis when using differential amplifiers since they can probe anywherein the circuit without the risk of Signal MeasurementsMeasuring low-amplitude signals presents a unique set of st of these challenges are the problems of noise and

adequate measurement t noise levels that would be considerednegligible when measuring signals of a few hundred millivolts ormore are no longer negligible when measuring signals of tens ofmillivolts or uently,minimizing ground loops and keepingground leads short are imperatives for reducing noise pick up by

the measurement the extreme,power-line filters and ashielded room may be necessary for noise-free measurement ofvery low amplitude /accessories

However,before resorting to extremes,you should consider signalaveraging as a simple and inexpensive solution to noise the signal you’re trying to measure is repetitive and the noise thatyou’re trying to eliminate is random,signal averagingcan provideextraordinary improvements in the

SNR (signal-to-noise ratio)of theacquired example of this is shown in Figure averaging is a standard function of most digital storage

oscilloscopes (DSOs).It operates by summing multiple acquisitionsof the repetitive waveform and computing an average waveformfrom the multiple random noise has a long-termaverage value of zero,the process of signal averaging reduces

random noise on the repetitive amount of improvementis expressed in terms of y,signal averaging improvesSNR by 3 dB per power of two ,averaging just twowaveform acquisitions (21) provides up to 3 dB of SNR improvement,averaging four acquisitions (22) provides 6 dB of improvement,eightaverages (23) provides 9 dB of improvement,and so sing measurement oscilloscope’s measure-ment sensitivity is a function of it’s input input circuitryeither amplifies or attenuates the input signal for an amplitude

calibrated display of the signal on the oscilloscope unt of amplification or attenuation needed for displaying a

signal is selected via the oscilloscope’s vertical sensitivity setting,which is adjusted in terms of volts per display division (V/div).In order to display and measure small signals,the oscilloscope

input must have enough gain or sensitivity to provide at least a fewdivisions of signal display example,to provide a two-division high display of a 20 mV peak-to-peak signal,the oscilloscopewould require a vertical sensitivity setting of 10 mV/ thesame two-division display of a 10 mV signal,the higher sensitivitysetting of 5 mV/div would be that a low volts-per-division setting corresponds to high sensitivity and vice addition to the requirement of adequate oscilloscope sensitivityfor measuring small signals,you’ll also need an adequate lly,this will not be the usual probe supplied as a standardaccessory with most rd accessory probes areusually 10X probes,which reduce oscilloscope sensitivity by a factorof other words,a 5 mV/div oscilloscope setting becomes a 50mV/div setting when a 10X probe is uently,to maintainthe highest signal measurement sensitivity of the oscilloscope,you’llneed to use a non-attenuating 1X of ProbesPrimerHowever,as discussed in previous chapters,remember that 1X

passive probes have lower bandwidths,lower input impedance,andgenerally higher tip means that you’ll need to beextra cautious about the bandwidth limit of the small signals you’remeasuring and the possibility of signal source loading by the any of these appear to be a problem,then a better approach is totake advantage of the much higher bandwidths and lower loadingtypical of 1X active cases where the small signal amplitude is below the oscilloscope’ssensitivity range,some form of preamplification will be e of the noise susceptibility of the very small signals,differential preamplification is generally differential

preamplification offers the advantage of some noise immunitythrough common-mode rejection,and the advantage of amplifyingthe small signal so that it will be within the sensitivity range of differential preamplifiers designed for oscilloscope use,sensitivities on the order of 10 µV/div can be

specially designed preamplifiers have features that allow useableoscilloscope measurements on signals as small as 5 µV,even inhigh noise environments!

Remember,though,taking full advantage of a differential preamplifierrequires use of a matched set of high-quality passive g to use matched probes will defeat the common-mode noiserejection capabilities of the differential ,in cases where you need to make single-ended rather thandifferential measurements,the negative signal probe can beattached to the test circuit ,in essence,is a differentialmeasurement between the signal line and signal r,in doing this,you lose common-mode noise rejection since therewill not be noise common to both the signal line and a final note,always follow the manufacturer’s recommended

procedures for attaching and using all probes and probe ,with active probes in particular,be extra cautious about

over-voltages that may damage voltage-sensitive probe /accessories43

ABCs of ProbesPrimerExplanation of Safety PrecautionsReview the following safety precautions to avoid injury and to

prevent damage to your test equipment or any product that it isconnected avoid potential hazards,use your test equipmentonly as specified by the in mind that all voltagesand currents are potentially dangerous,either in terms of personalhazard or damage to equipment or e All Terminal Ratings— To avoid fire or shock hazard,observe all ratings and markings

on the t the product manual for further ratings

information before making connections to the product.— Do not apply a potential to any terminal that exceeds the

maximum rating of that terminal.— Connect the ground lead of probes to earth ground r those scopes that are specifically designed and specified tooperate in a floating oscilloscope application (e.g.,the TektronixTHS700 Series battery powered Digital Storage Oscilloscopes),thesecond lead is a common lead and not a ground this case,follow the manufacturer’s specification for maximum voltage levelthat this can be connected to.— Check probe and test equipment documentation for,and observeany derating example,the maximum input

voltage rating may decrease with increasing Proper Grounding Procedures— Probes are indirectly grounded through the grounding conductor

of the oscilloscope power avoid electric shock,the

grounding conductor must be connected to earth

making connections to the input or output terminals of the

product,ensure that the product is properly grounded.— Never attempt to defeat the power cord grounds of any

test equipment.— Connect probe ground leads to earth ground only.— Isolation of an oscilloscope from ground that is not specifically

designed and specified for this type of operation,or connecting

a ground lead to anything other than ground could result in

dangerous voltages being present on the connectors,controls,or other surfaces of the oscilloscope and /accessoriesFigure e Derating values and ranges will vary with

specific is is true for most scopes,but there are some scopes that aredesigned and specified to operate in floating

example is the Tektronix THS700 Series battery powered DigitalStorage t and Disconnect Probes Properly— Connect the probe to the oscilloscope properly groundthe probe before connecting the probe to any test point.— Probe ground leads should be connected to earth ground only.— When disconnecting probes from the circuit under test,remove

the probe tip from the circuit first,then disconnect the ground lead.— Except for the probe tip and the probe connector center

conductor,all accessible metal on the probe (including the

ground clip) is connected to the connector shell.

Avoid Exposed Circuitry— Avoid touching exposed circuitry or components with your handsor any other part of your body.— Make sure that probe tips and ground lead clips are attached

such that they do not accidentally brush against each other or

other parts of the circuit under RF Burns While Handling Probes— To avoid RF (radio frequency) burns,do not handle the probe

leads when the leads are connected to a source that’s above thevoltage and frequency limits specified for RF burn risk (see

Example Derating Curve in Figure A-1).— There’s always a risk of RF burns when using non-grounded

probes and lead sets to measure signals,normally above

300 volts and 1 MHz.— If you need to use a probe within the risk area for RF burn,turn power off to the source before connecting or disconnecting

the probe not handle the input leads while the circuit

is Not Operate Without Covers— Oscilloscopes and probes should not be operated with any coveror protective housing ng covers,shielding,probe bodies,or connector housings will expose conductors or

components with potentially hazardous Not Operate in Wet/Damp Conditions— To avoid electrical shock or damage to equipment,do not

operate measurement equipment in wet or damp of ProbesPrimerDo Not Operate in an Explosive Atmosphere— Operating electrical or electronic equipment in an explosive

atmosphere could result in an ially explosive

atmospheres may exist wherever gasoline,solvents,ether,propane,and other volatile substances are in use,have been in

use,or are being ,some fine dusts or powders

suspended in the air may present an explosive Not Operate with Suspected Failures— If you suspect there’s damage,either electrical or physical,to

an oscilloscope or probe,have it inspected by qualified service

personnel before continuing Probe Surfaces Clean and Dry— Moisture,dirt,and other contaminants on the probe surface can

provide a conductive safe and accurate measurements,keep probe surfaces clean and dry.— Probes should be cleaned using only the procedures specified inthe probe’s Not Immerse Probes in Liquids— Immersing a probe in a liquid could provide a conductive path

between internal components or result in damage to or corrosionof internal components or the outer body and shielding.— Probes should be cleaned using only the procedures specified inthe probe’s /accessories45

ABCs of ProbesPrimerGlossaryaberrations– Any deviation from the ideal or norm; usually associatedwith theflat tops and bases of waveforms or s may have aberrationscaused by the circuit conditions of the signal source,and aberrations may beimpressed upon a signal by the measurement

any measurement where aberrations are involved,it is important

to determine whether the aberrations are actually part of the signal or theresult of the measurement lly,aberrations are specified as apercentage deviation from a flat probe – A probe containing transistors or other active devices aspart of the probe’s signal conditioning ation –The process whereby the amplitude of a signal is ator probe –A probe that effectively multiplies the scale factorrange of an oscilloscope by attenuating the example,a 10X probeeffectively multiplies the oscilloscope display by a

factor of probes achieve multiplication by attenuating the signalapplied to the probe tip; thus,a 100 volt peak-to-peak signal is attenuatedto 10 volts peak-to-peak by a 10X probe,and then is displayed on the

oscilloscope as a 100 volts peak-to-peak signal through 10X multiplicationof the oscilloscope’s scale dth –The continuous band of frequencies that a network or circuitpasses without diminishing power more than 3-dB from the mid-band tance – An electrical phenomenon whereby an electric charge

is -mode rejection ratio (CMRR) –A differential probe’s ability

to reject any signal that is common to both test points in a is a key figure of merit for differential probes and

amplifiers,and is defined by:CMRR = |Ad/Ac|where:Ad= the voltage gain for the difference = the voltage gain for common-mode t probe –A device to sense current flow in a wire and convert

the sensed current to a corresponding voltage signal for measurement by

an –To reduce the rating of a component or system based on one ormore operating variables; for example,amplitude measurement accuracymay be derated based on the frequency of the signal being /accessoriesdifferential probe –A probe that uses a differential amplifier to subtracttwo signals,resulting in one differential signal for measurement by onechannel of the ential signals –Signals that are referenced to each other instead ofearth buted elements (L,R,C) –Resistance and reactance that arespread out over the length of a conductor; distributed element values aretypically small compared to lumped component -effect transistor (FET) –A voltage-controlled device in which thevoltage at the gate terminal controls the amount of current through the ng measurements –Measurements that are made between twopoints,neither of which is at ground ing –Since probes must draw some current from the signal sourcein order for a measurement to be made,there must be a return path for return path is provided by a probe ground lead that is attachedto the circuit ground or Effect –Generation of an electric potential perpendicular to both

an electric current flowing along a conducting material and an externalmagnetic field applied at right angles to the current upon application of

the magnetic ics –Square waves,sawtooth waveforms,and other periodic

non-sinusoidal waveforms contain frequency components that consist of

the waveform’s fundamental frequency (1/period) and frequencies that areinteger multiples (1x,2x,3x,...) of the fundamental which are referred to asharmonic frequencies; the second harmonic of a waveform has a frequencythat is twice that of the fundamental,the third harmonic frequency is threetimes the fundamental,and so nce –The process of impeding or restricting AC signal nce is expressed in Ohms and consists of a resistive component (R)and a reactive component (X) that can be either capacitive (XC) or inductive(XL).Impedance (Z) is expressed in a complex form as:Z = R + jXor as a magnitude and phase,where the magnitude (M) is:M = √R2+ X2and phase o is:o = arctan(X/R)inductance – A property of an electric circuit by which an electromotiveforce is induced in it by a variation of current either in the circuit itself or ina neighboring –The short-term variations of a digital signal's significant instantsfrom their ideal positions in phase –The characteristic of a network whereby the phase of anapplied sine wave is shifted linearly with increasing sine wave frequency; anetwork with linear phase shift maintains the relative phase relationships ofharmonics in non-sinusoidal waveforms so that there’s no phase-relateddistortion in the waveform.

load – The impedance that’s placed across a signal source; an open circuitwould be a “no load”g – The process whereby a load applied to a source draws currentfrom the -capacitance probe –A passive probe that has very low

input –Metal-oxide semiconductor field-effect transistor,one of twomajor types of –A type of signal distortion that can appear on oscilloscope

waveform l probe –A device to sense light power and convert to a

corresponding voltage signal for measurement by an e probe –A probe whose network equivalent consists only of

resistive (R),inductive (L),or capacitive (C) elements; a probe that containsno active –A means of expressing the time-related positions of waveforms

or waveform components relative to a reference point or mple,a cosine wave by definition has zero phase,and a sine wave is acosine wave with 90-degrees of phase –A device that makes a physical and electrical connection betweena test point or signal source and an power –Power that’s supplied to the probe from some source suchas the oscilloscope,a probe amplifier,or the circuit under thatrequire power typically have some form of active electronics and,thus,arereferred to as being active nce –An impedance element that reacts to an AC signal by

restricting its current flow based on the signals frequency.A capacitor (C)presents a capacitive reactance to AC signals that is expressed in Ohms

by the following relationship:XC= 1/2πfCwhere:XC= capacitive reactance in Ohmsπ= f = frequency in HzC = capacitance in FaradsABCs of ProbesPrimerAn inductor (L) presents an inductive reactance to AC signals that’sexpressed in Ohms by the following relationship:XL= 2πfLwhere:XL= inductive reactance in Ohmsπ= f = frequency in HzL = inductance in Henrysreadout – Alphanumeric information displayed on an oscilloscope screen

to provide waveform scaling information,measurement results,or

other g – Oscillations that result when a circuit resonates; typically,thedampedsinusoidal variations seen on pulses are referred to as time –On the rising transition of a pulse,rise time is the time it takesthe pulse to rise from the 10% amplitude level to the 90% amplitude ing –The practice of placing a grounded conductive sheet of materialbetween a circuit and external noise sources so that the shielding materialintercepts noise signals and conducts them away from the averaging –Summing multiple acquisitions of the repetitive wave-form and computing an average waveform from the multiple fidelity –The signal as it occurs at the probe tip is duplicated atthe oscilloscope -ended signals –Signals that are referenced to (signal-to-noise ratio) –The ratio of signal amplitude to noiseamplitude; usually expressed in dB as follows:SNR = 20 log (Vsignal/Vnoise)source –The origination point or element of a signal voltage or current;also,one of the elements in a FET (field effect transistor).source impedance –The impedance seen when looking back into a domain reflectometry (TDR) –A measurement technique wherein afast pulse is applied to a transmission path and reflectionsof the pulse areanalyzed to determine the locations and types of discontinuities (faults ormismatches) in the transmission ID –When multiple waveform traces are displayed on an oscilloscope,a trace ID feature allows a particular waveform trace to be identified ascoming from a particular probe or oscilloscope arily

pressing the trace ID button on a probe causes the corresponding waveformtrace on the oscilloscope to momentarily change in some manner as ameans of identifying that /accessories47

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United Kingdom & Eire +44 (0) 1344 392400USA 1 (800) 426-2200USA (Export Sales) 1 (503) 627-1916For other areas contact Tektronix,:1 (503) 627-7111Updated November 3,2004XYZs of OscilloscopesUnderstanding High-performance Oscilloscope SpecificationsA Guide to Understanding and Characterizing Timing JitterDigital Designer's Guide to Signal IntegrityHigh-speed Differential Data Signaling and MeasurementsBasics of Serial Data Compliance and Validation MeasurementsXYZs of Signal SourcesXYZs of Logic AnalyzersIntroduction to Logic Analysis:A Hardware Debug TutorialUMTS Protocols and Protocol TestingGPRS Protocol Testing in the Wireless WorldTroubleshooting cdmaOne™ BTS Transmitters in the FieldInterference TestingSDH Telecommunications StandardsSONET Telecommunications StandardsT1 Testing:Technology and ApplicationsFiber Optic Cable and Test Equipment BasicsA Guide to Maintaining Video Quality of Service for Digital Television ProgramsMulti-layer Confidence Monitoring in Digital Television BroadcastingA Guide to Standard and High-definition Digital Video MeasurementsA Guide to Picture Quality Measurements for Modern Television SystemsA Guide to MPEG Fundamentals and Protocol AnalysisPCR MeasurementsCustomer Service illoscopesLogic AnalyzersSignal SourcesSignaling Test EquipmentRF/Wireless Test EquipmentOptical Communications Test EquipmentVideo Test EquipmentProbesAccessoriesOther Test and Measurement Equipment48For Further InformationTektronix maintains a comprehensive,constantly expanding collec-tion of application notes,technical briefs and other resources to helpengineers working on the cutting edge of yright © 2005,Tektronix, rights nix products are covered by

foreign patents,issued and ation in this publication supersedes that in all previously

published ication and price change privileges NIX and TEK

are registered trademarks of Tektronix, other trade names referenced are the service marks,trademarks or registered trademarks of their respective companies.01/05 TN//accessories