Video signals are complexwaveforms comprised of sig-nals representing a picture aswell as the timing informa-tion needed to display thepicture. To capture and mea-sure these complex signals,you need powerful instru-ments tailored for this appli-cation. But, because of thevariety of video standards,you also need a general-pur-
pose instrument that can pro-vide accurate information –quickly and easily. Finally, todisplay all of the video wave-form details, a fast acquisi-tion technology teamed withan intensity-graded displaygive the confidence andinsight needed to detect anddiagnose problems with thesignal.
This application note demon-strates the use of a TektronixTDS 700D-series DigitalPhosphor Oscilloscope tomake a variety of commonbaseband video measure-ments and examines some ofthe critical measurementissues.
Baseband Video Testing With Digital PhosphorOscilloscopes
Application Note
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Video signals come from anumber of sources, includingcameras, scanners, andgraphics terminals. Typically,the baseband video signalbegins as three componentanalog or digital signals rep-resenting the three primarycolor elements – the Red,Green, and Blue (RGB) com-ponent signals. Basebandvideo signals are the signalsthat are not modulated on anRF carrier, such as in analogterrestrial or cable transmis-sion systems.Figure 1 shows a typicalvideo system block diagram.Notice that in the video sig-nal path shown, the signalchanges formats betweensource and destination. Todesign and debug such sys-tems, test equipment must beable to examine signals in avariety of formats.
ConversionThe next step, conversion, iswhere the real differences invideo standards begin. TheRGB signal is converted intothree component signals: • Luminance signal, Y• Two color-difference sig-
nals, often B-Y and R-Y The color difference signalsmay be modified, dependingon the standard or formatused. For example, I and Qfor NTSC systems, U and Vfor PAL systems, PB and PR
for SMPTE systems, etc. Thethree derived component sig-nals can then be distributedfor processing.
ProcessingIn the processing stage, videocomponent signals may becombined to form a singlecomposite video signal (as inNTSC or PAL systems),divided into separate lumi-nance and chrominance sig-nals (as in Y/C systems:S-VHS or Hi-8), or main-tained separately as discretecomponent signals (as in RGBgraphics and HDTV systems).Composite Video Signals. Foranalog broadcast and cableTV applications, the mostcommon signals are compos-ite signals which containmore than one signal compo-nent. In North America andJapan, for example, the NTSCdefines the way that lumi-nance (black and white infor-mation), chrominance (colorinformation), and synchro-nization (timing information)are encoded into the compos-ite video signal. In Europe,the PAL standards providethe same function. In the caseof the NTSC and PAL stan-dards, the chrominance sig-nals are modulated on a pairof color subcarriers. Themodulated chrominance sig-nal is then added to the lumi-nance signal to form the
active portion of the videosignal. Finally, the synchro-nization information isadded. Although complex,this composite signal is a sin-gle signal that can be carriedon a single coaxial cable.Component Video Signals.Component signals have anadvantage of simplicity ingeneration, recording, andprocessing where many com-binations of switching, mix-ing, special effects, color cor-rection, noise reduction, andother functions may beapplied to the signals. Sincethere is no encoding/decod-ing process as in compositevideo, signal integrity is moreeasily maintained in compo-nent video systems andequipment, resulting in ahigher quality image. How-ever, the signals are carriedon separate cables. In prac-tice, this limits the distancesover which the signals can betransmitted and requirescareful matching of signalpaths.Y/C Video Signals. A com-promise solution, imple-mented in systems such asS-VHS and Betacam, modu-lates the chrominance signalson a pair of color subcarriers,but keeps the chrominancesignal separate from the lumi-nance signal. This minimizesthe luminance/chrominanceartifacts of composite systemswhile simplifying the inter-channel timing issues ofcomponent systems. Thispair of signals can be carriedon a single special cable.
DisplayAfter transmission, the objec-tive is to faithfully reproducethe processed image. In com-posite systems, the signal isdecoded to component formand then translated to RGBformat for display on themonitor. Component videosignals go through less pro-cessing, being converteddirectly to an RGB signal fordisplay.
Figure 1. Typical video system block diagram.
Video Basics
Analog Video SynchronizationSignalsLet's take a closer look at anactual analog baseband videosignal. To reproduce animage, both the camera andthe video display are scannedhorizontally and vertically(see Figure 2a). The horizon-tal lines on the screen mightbe scanned alternately – oddnumbered lines first, theneven numbered lines – as in“interlaced” scanning sys-tems, or they might bescanned sequentially, oneafter another, as in “progres-sive” scanning systems. Eachvertical scan is called a field.Two interlaced fields makeup a frame.
Both the camera and receivermust be synchronized to scanthe same part of the image atthe same time. This synchro-nization is handled by thehorizontal sync pulse, whichstarts a horizontal trace. Dur-ing the horizontal blankinginterval, the beam returns tothe left side of the screen andwaits for the horizontal syncpulse before tracing anotherline. This is called “horizon-tal retrace” (see Figure 2b).When the beam reaches thebottom of the screen, it mustreturn to the top to begin thenext field. This is called the“vertical retrace” and is sig-naled by the vertical syncpulse (see Figure 2c). Thevertical retrace takes much
longer than the horizontalretrace, so a longer synchro-nizing interval – the “verticalblanking interval” – isemployed. No information iswritten on the video screenduring the horizontal or ver-tical blanking intervals.Each video standard definesa series of synchronizationsignals that control how thevideo signal is displayed.PAL signals display a videoframe 25 times a second,where a frame contains 625video lines. NTSC signalsdisplay a video frame 30times a second, but with only525 lines. Some high-resolu-tion computer monitors dis-play more than 1000 lineswith a frame rate of 72 timesa second.Note that component signalsneed timing signals too. Thesynchronization is often com-bined with one of the compo-nents (such as the greenchannel).
Serial Digital InterfaceFor digital video applica-tions, the SMPTE and ITUspecify the way that thevideo signal is representedand formed into a serial datastream. For example, themost common serial compos-ite signal is an NTSC signalthat is sampled at 14.3 MS/swith 8 to 10 bits of resolu-tion. The resulting bit stream(143 Mb/s) is encoded withNon-Return-to-Zero-Inverted,or NRZI coding and scram-bled so it can be sent over75 Ω coaxial cable. For stu-dios, the most common stan-dard samples component sig-nals (Y, PR, and PB) at13.5 MS/s with 8 to 10 bits ofresolution. This bit stream(270 Mb/s) is also encodedand scrambled and can besent over 75 Ω coaxial cable.
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Figure 2. The synchronization signals in an analog composite baseband video signal provide the timing signals nec-essary to reproduce a video signal on a display.
Before discussing measure-ments on video signals, let’sreview the requirements forthe test setup. These require-ments include the requiredoscilloscope specificationsand capabilities, signal con-ditioning, and triggering.
Oscilloscope RequirementsMost oscilloscopes aredescribed by a few basicspecifications. The first isusually bandwidth. A goodrule of thumb is to use anoscilloscope with an analogbandwidth at least five timesthe bandwidth of the signalto assure accurate representa-tion of the signal. (A way toestimate the bandwidth ofyour signal is to divide thenumber 0.35 by the 10 to90% risetime of the fastestsignal component.)The sample rate dictateshow fast the signal is sam-pled. In theory, the samplerate must be at least twice thebandwidth of the signal. Inpractice, the sample rate oneach scope channel should be4 to 5 times the bandwidth ofthe signal for accurately cap-turing signals in a singleacquisition and displayingthem with sin(x)/x interpola-tion.Often you will want toacquire signals repetitively tomonitor changes over time.Unfortunately, traditionaldigital storage oscilloscopesactually capture signals at amuch lower repetition ratethan analog oscilloscopes. Tobe sure you have a lively dis-play of the signal, you willwant to look at the oscillo-scope’s waveform capturerate which specifies the rateat which signals are acquired(in waveforms/second). Forexample, if you’re looking atall lines of NTSC or PAL sig-nals, you expect to see more
than 15,000 waveforms asecond.The record length of a digitaloscilloscope indicates howmany sample points theoscilloscope acquires in awaveform record. The resultis a trade-off between detailand record length, or betweensample rate and time dura-tion acquired. You canacquire either a detailed pic-ture of a signal for a shortperiod of time (the oscillo-scope “fills up” on waveformpoints quickly) or a lessdetailed picture for a longerperiod of time.
Acquisition and Display ModesThe most critical display issuefor many video engineers isthe intensity-graded display.This display, a familiar char-acteristic of analog scopes andwaveform monitors, showsthe signal’s statistical behav-ior by varying the intensitiesof the displayed samples.(The result is that frequentlyoccurring signals are bright,and relatively infrequentdetails are proportionatelydim.) The TDS 700D-seriesDigital Phosphor Oscillo-scopes provide this intensity-graded display, providing youinsight through qualitativeintensity information andenabling your eyes to assimi-late the subtle details andvariations of the signal. Sincemany digital storage oscillo-scopes are not capable ofacquiring enough data toaccurately represent the videosignal, special acquisition anddisplay modes are made avail-able in DSOs to compensate.The basic acquisition modeof a digitizing oscilloscope isthe Sample mode, where thewaveform is sampled in timeand the amplitude of eachsample is digitized and dis-played. With the use of inter-
polation, these samples canbe connected to create a con-tinuous waveform display.However, a scope can alsodigitally process the signalbefore it is displayed,enabling complex measure-ments to be made easily.For example, you can use thescope’s Average mode toremove the effects of randomnoise to enable you to makeprecise amplitude measure-ments. The averaging func-tion, found in the ACQUIREMENU, smoothes the wave-form by averaging multiplewaveforms together.HiRes mode filters the sam-ples taken during an acquisi-tion to create a higher-resolu-tion, lower-bandwidth signal.On the other hand, you maywant to see and measure arelatively small noise ridingon a relatively large videosignal. For such problems,the TDS Zoom Previewmode allows detailed signalexamination and waveformexpansion. You can expandand position the waveform inboth the horizontal and verti-cal direction for precise com-parison of fine waveformdetail without affectingon-going acquisitions.Other acquisition functionscan make it easy to see noiseanywhere in the video wave-form. The Peak Detect modecaptures and displays theminimum and maximum val-ues of a waveform, whichshows its worst-case ampli-tude excursions. Choosingthe envelope mode causesthe scope to accumulate anddisplay the minimum andmaximum values of a seriesof waveforms over time.
Measurement FeaturesIf you’re working with NTSCor PAL signals, the TDS
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Test Requirements
video graticules help displaythe signal in a familiar for-mat. Graticules for NTSC andPAL signals are availablefrom the DISPLAY menu.When either of these softwaregraticules are selected, theoscilloscope automaticallyscales the video signal to thegraticule you’ve chosen,allowing you to quicklyassess the captured signal.Manual on-screen measure-ments can be easily madeusing the cursors. Controlsfor the cursors are found in
the CURSOR menu. Horizon-tal cursors allow you to mea-sure signal amplitudes, withthe readouts available inunits of volts or IRE (forNTSC signals). Vertical cur-sors allow measurement ofsignal timing, with readoutsin seconds, Hertz, or videoline numbers. Paired cursorsallow you to simultaneouslymeasure relative amplitudeand timing parameters.The processing power of theDigital Phosphor Oscillo-scope can also be used to
automatically measure anumber of signal parameters.For example, measurementssuch as peak-to-peak ampli-tude, sync-pulse width, andinter-channel timing can beeasily made. Automated mea-surements are selected andcontrolled through theMEASURE menu.
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TerminationMost video systems aredesigned to deliver a knownamplitude signal into a speci-fied impedance. Therefore atlow frequencies, the measure-ment accuracy depends on thesignal being terminated in aprecise resistance, usually75 Ω. At higher frequencies,the termination must matchthe impedance of the trans-mission line (usually coaxialcable). In this case, the termi-nation impedance must have aprecise resistance with negli-gible reactance (also known asmaximizing the return lossand minimizing the voltage
standing wave ratio). Anexample of such a terminatoris the Tektronix AMT75,which is specified to 1 GHz.Improper termination canresult in degraded frequencyresponse.
Video ClampingA common signal anomalyencountered in analog videomeasurements is the low-frequency hum produced byAC line voltage. This hum,when not removed from thevideo signal, causes the signalto drift up and down in thedisplay and can cause the trig-ger point to vary. The
TDS 700D video trigger optionincludes a video clamp thateffectively removes AC hum,as well as any DC offset on thesignal. If the signal has beenAC-coupled, the clamp alsoremoves low-frequency varia-tions which result as the aver-age picture level changes. Theclamp pod attaches to theinput BNC connector andserves as a pre-processor ofthe video signal. It provides“back-porch” clamping on allstandard video signals. Thevideo clamp also provides flatfrequency response, allowingaccurate video measurements.
Signal Conditioning
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The first step toward measur-ing video waveforms is get-ting a stable waveform. Toenable you to capture andanalyze the signal, you mustfirst trigger the oscilloscopeon the signal. There are anumber of advanced triggermodes in the TDS oscillo-scopes to make your job eas-ier.
Analog Composite VideoTriggeringThe TDS video trigger isselected by pressing theTRIGGER button on the frontpanel and choosing “Video”from the on-screen triggertype menu. By default, thisselection automatically setsthe scope to trigger on 525-line, 60 Hz NTSC video sig-nals. It also directs the instru-ment to lock on the inter-laced color field 1 using neg-ative sync pulse polarity (seeFigure 3).Use the menus to alter thesedefault settings. Using the“Standard” option, you canalso direct the scope to trig-ger on PAL/SECAM, HDTV,and a variety of custom videosignals. Or select “SyncPolarity” and change to posi-tive sync if the portion of thecircuit you are debugging hasinverted the video signal.Select “Field” in the mainmenu and choose all, odd,
even, or numeric video fieldson the side menu.Since much of the informa-tion of interest in a video sig-nal is on specific video lines,you can choose which partic-ular line to display. Selectthe “Line” option in the sidemenu and turn the general-purpose knob or use the key-pad to specify the line ofinterest. The line numberappears on the screen to helpyou keep track.
FlexFormat TriggeringThere are a variety of high-definition video systemsunder development aroundthe world. These include the787.5/60, 1050/60, 1125/60,and 1250/50 formats. How-ever, new formats are stillbeing experimented with.Certain markets have createdtheir own high-definition for-mats and established theirown standards. For example,the medical imaging marketand the military have devel-oped HDTV standards to fittheir immediate needs. Thiscan add to the confusionwhen searching for video testand measurement instrumen-tation.The TDS video trigger optionprovides a solution for cus-tomized HDTV triggeringneeds. With the Flex-Format™ triggering mode,
you can specify the timing ofcustomized tri-level syncpulses (see Figure 4), selectany field rate between 20 and200 Hz with up to two digitresolution, and define thenumber of lines and fields inyour customized format.
Single-Pixel TriggeringWith more of the video moni-tor market moving to flat-panel displays, design anddebug applications need sin-gle pixel triggering and analy-sis capabilities. A TDS scopewith the video trigger and the“Delay By Events” triggerallows you to define eachpulse of the device-under-test’s system clock as anevent. Each event then corre-sponds to a pixel, and succes-sive events equate to succes-sive pixels.First, connect the video sig-nal of interest into Channel 1.Set up Channel 1, main trig-ger, to trigger on the videosignal. Press the TRIGGERMENU button on the frontpanel and select VIDEO trig-ger. Select appropriate stan-dard and parameters to trig-ger on the interesting sectionof the signal.Connect the system referenceclock to Channel 2. Set thedelay trigger to use Channel 2as its source by pressing theSHIFT and TRIGGER MENU
Figure 4. The FlexFormat triggering mode allows you to define the start andstop times of tri-level sync pulses for both odd and even fields.
Triggering
Figure 3. The TDS video trigger allows convenient selection of video stan-dard, channel, sync polarity, and field and line.
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buttons on the front paneland select Channel 2 as thesource of the delay trigger.Now select Delay by Events.Turn on the Delay Trigger bygoing to the Horizontal menuand selecting the DelayedOnly time base.Now, you can go back to theDelay Trigger menu and dialin the event you want to see,or enter the appropriate num-ber on the keypad (seeFigure 5).
Serial Digital (NRZ) TriggeringThe most common way tocharacterize a serial digitalsignal is by examining an eyediagram. This display is acomposite display of manywaveform acquisitions, over-
laid upon another, to form aconsolidated image of thedata pulses which resemblesan eye. In general, the largerthe opening of the center ofthe eye, the better the perfor-mance of the system undertest. A wider vertical openingshows a greater noise toler-ance, while a wider horizon-tal opening indicates morejitter tolerance. In otherwords, excessive amplitudenoise or timing jitter willtend to close the eye.The oscilloscope may triggeron the rising edge of theserial system clock and cap-ture the data that coincideswith the clock edge. Thismethod requires that theclock and the data signals be
correlated. Or, the oscillo-scope may trigger on the dataitself, wait for a few unitintervals, and then acquireenough waveforms to build adisplay. This can be donewith a delayed timebase withdelay by time or events. An easier method is to use aneye diagram trigger. Selectthe COMM trigger type fromthe TDS 700D TRIGGER typemenu and NRZ from the codemenu. Then when you selectthe serial digital video stan-dard from the list, the oscillo-scope is automatically set upto display an eye diagram ofthe signal (see Figure 6).
Figure 5. The system clock (bottom waveform) serves as the Delay Triggerfor the video signal (top waveform). With Delayed by Events, each event cor-responding to a pixel, you can observe the video signal at each pixel.
Figure 6. Setting up an eye diagram is easy using the NRZ communicationsignal trigger.
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Video Signal MonitoringWhether you are monitoringanalog or digital video sig-nals, an oscilloscope with anintensity-graded displaywhich is tailored for videoapplications can be yourmost valuable debug tool.Subtle variations in the sig-nal, which are not visible ona DSO display, can spell thedifference between a videosystem that works and onethat doesn’t.
H-rate Intensity-graded Displaysof Live VideoThe most basic analog videodisplay is the horizontal-ratedisplay of the signal ampli-tude vs. time. This can bedone most easily by edge-triggering on the leading edgeof sync. As shown in Figure7, a Digital Phosphor Oscillo-scope with an intensity-graded display (and a wave-form capture rate highenough to capture every line)provides the familiar wave-
form monitor H-ratedisplay.
XY Displays ofChrominanceThe Digital PhosphorOscilloscope’s XYdisplay mode allowsyou to display onesignal against anotherin a manner similar toa vectorscope. PressFORMAT selection inthe DISPLAY menu
and select the XY mode. If aB-Y signal is connected toChannel 1 and an R-Y signalis connected to Channel 2,the scope will imitate a famil-iar vectorscope display. Also,the intensity-graded displayshows details in the signalwhich are not visible on ordi-nary DSOs.
Intensity-graded Displays ofDigital Video Eye DiagramsIntensity-grading is alsoimportant for monitoring eyediagram displays, where youwant to qualitatively examinethe signal variations overtime, whether the variationsare due to noise or timing jit-ter. Intensity-graded displays,available with analog oscillo-scopes and Digital PhosphorOscilloscopes, combinedwith a high waveform cap-ture rate, give you the bestmethod of capturing andidentifying infrequentanomalies.
Figure 7. A horizontal-rate waveform monitor display, showing the effect ofan intensity-graded display on the oscilloscope.
Video Signal Measurements
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Amplitude MeasurementsAmplitude measurementscan be made a number ofways with an oscilloscope.For example, to measure thepeak-to-peak amplitude ofthe NTSC burst signal, youcan simply compare the sig-nal to the TDS 700D’s IREvideo graticule (see Figure 8.)You can also use the
TDS 700D’s video cursors tomake the same measurement.Finally, if you want to ana-lyze variations over time, thescope can make a number ofmeasurements automatically,and accumulate the measure-ment statistics.
Timing MeasurementsTiming measurements areespecially critical for compo-
nent analog systemsbecause they requireprecise inter-channeltiming. The mostimportant use of amulti-channel oscillo-scope can be to dis-play the relative tim-ing differencesbetween channels.Before you can accu-rately display themultiple channels,you need to matchthe probe path delays.This can be donewith the deskew fea-ture, found in theTDS 700D’s VERTI-CAL menu. Connectboth probes to a com-
mon signal and adjust thechannel deskew with the gen-eral-purpose knob until thetraces line up on the display.Now, connect the signals ofinterest to the scope channelsand adjust the channel timingcontrols to match the signals(see Figure 9).The oscilloscope can alsomake timing measurementsautomatically and accumu-late statistics on those mea-surements. For example, tomeasure sync width, triggeron the leading edge of sync,turn on HiRes acquisitionmode, and adjust the hori-zontal and vertical controlsso the sync pulse fills most ofthe display. This optimizesthe accuracy of the measure-ment system. Now turn onthe negative pulse widthmeasurement in the MEA-SURE menu. To monitor themean (µ) and standard devia-tion (σ) of the pulse widthmeasurement, enable themeasurement statistics (seeFigure 10).
Analog Signal Measurements
Figure 8. An example of amplitude measurements on an NTSC signal. Thepeak-to-peak amplitude of the burst packet can either be measured visuallywith the graticule, or with the video cursors (note cursor readout in upperright corner).
Figure 10. Automatic timing measurements provide an easy and accuratemethod of repetitively measuring basic signal parameters.
Figure 9. Inter-channel timing is of critical importance in component analogvideo systems. The display shows the relative timing of the luminance andone of the color-difference signals (after the cable delays were equalized withthe channel deskew controls).
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Jitter MeasurementsTiming jitter on a signal canaffect a receiver’s ability todecode a video data stream.The effects are readily seenon an eye diagram becausejitter shrinks the opening ofthe eye. As the jitterincreases, the data transitionpoints move closer and closerto the decision point of thereceiver, eventually increas-ing the bit-error rate of thesystem.Jitter comes in two types:deterministic and random.Deterministic, or data-depen-dent, jitter is caused by thepattern of data bits precedingthe current bit in the datastream. By triggering onrepetitive data patterns andmeasuring the variation inedge placement, you cancharacterize deterministic jit-ter components. Such ananalysis can be time-consum-ing but useful for detectingproblems early in the designprocess.Random jitter, on the otherhand, is due to random noisein a system and is not corre-lated to the data. It can becharacterized and measured
by statistically analyzing thewaveform, using the DigitalPhosphor Oscilloscope’s his-togram capability. Displayand draw a histogram boxaround the rising edge,falling edge, or eye crossingwhere the jitter is to be mea-sured, and then have theoscilloscope draw a his-togram of the delay of theedge from the trigger point. Ifthe histogram of the place-ment of the signal edge is anormally distributed curve,the standard deviation isequal to the RMS jitter of thewaveform. You can also turnon the observed RMS jitter(standard deviation) or otherhistogram measurements tofurther characterize the jitter(see Figure 11).
Mask TestingAs discussed before, an eyediagram reveals a lot about aserial digital signal, espe-cially about the relative mar-gin available for noise and jit-ter. It represents the mostimportant time-domain signalcharacteristics in one display:rise time and fall time, pulseovershoot and undershoot,ringing, duty cycle, jitter, andnoise.
To determine if a serial digi-tal video signal complieswith the standard, all rele-vant parameters must beexamined to see whether theyare within specifications.Measuring the parametersindividually would be atedious business and couldeasily result in errors. Tosimplify the verification task,the video standards specifythe shape of compliant sig-nals by defining a mask. Yousimply overlay the mask onthe eye diagram and canimmediately see if the signalcomplies by fitting into theallotted areas of the mask (seeFigure 12).Advanced communicationoscilloscopes have built-instandard masks, which youcan select from a menu.These oscilloscopes also pro-vide calibrated, variable timedelay and voltage scales, canautomatically adjust the sig-nal to fit the mask, and caneven count the number ofwaveforms acquired and thenumber of mask violations,or “hits,” for faster and moreaccurate testing.
Figure 12. Mask testing provides a convenient and reliable method for verify-ing the compliance of serial video signals to industry standards. In thisexample, a minimum of 100 waveforms was compared to the mask, with noerrors (0 “hits”).
Figure 11. Characterize random jitter on a digital video signal with a his-togram. Notice the bi-modal nature of the histogram. Also, measurements onthe histogram are shown at the right of the screen, indicating such character-istics as the observed peak-to-peak jitter.
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ConclusionIn this application note,we’ve demonstrated the useof a Tektronix TDS 700D-series Digital Phosphor Oscil-loscope to quickly and easily
make a variety of commonbaseband video measure-ments on a variety of com-plex video signals. With thepower of the intensity-gradeddisplay, high waveform cap-
ture rate, and abundance ofwaveform data, this general-purpose instrument is thetool of choice to debug, char-acterize, and verify yourvideo circuits and systems.
