lecroy 19_LTXXX-OM-E_Ch15 电路图.pdf

LTXXX-OM-E Rev BISSUED: January2002205 CH A P T E RFI F T E E NHowHistogramsWork Theoryof Operation Anunderstandingof statistical variations inparameter values is neededfor manywaveform parameter measurements. Knowledgeof theaverage, minimum, maximum, andstandarddeviationof theparameter may oftenbeenough, but in manycases youmayneedamoredetailedunderstandingof thedistributionof a parameters values. Histograms allowyoutoseehowaparameters values aredistributedover manymeasurements. Theydothis by dividingarangeof parameter values intosub-ranges calledbins. Maintainedfor eachbinis acount of the number of parameter values events that fall withinranges of thebinitself. Whilesuchavaluerangecanbeinfinite, for practical purposes it needonlybedefinedas largeenoughto includeanyrealisticallypossibleparameter value. For example, inmeasuringTTL high-voltagevalues arangeof 50 V is unnecessarilylarge, whereas oneof 4 V 2.5 V is morereasonable. It is the5 V rangethat is then subdividedintobins. Andif thenumber of bins usedwere50, eachwouldhavearangeof 5 V/50 bins or 0.1 V/bin. Events fallingintothefirst bin wouldthenbebetween1.5 V and1.6 V. Whilethenext binwould captureall events between1.6 V and1.7 V, andso on. After aprocess of several thousandevents, thebar graphof thecount for eachbin its histogram provides agoodunderstandingof thedistributionof values. Histograms generallyusethex axis toshowa bins sub-rangevalue, andthey axis for thecount of parameter values withineachbin. Theleftmost bin with anon-zerocount shows thelowest parameter valuemeasurement(s). Theverticallyhighest binshows the greatest number of events fallingwithinits sub-range. Thenumber of events inabin, peakor ahistogram is referredtoas its population. Figure 4.1 on thenext pageshows ahistograms highest populationbinas theonewithasub-rangeof 4.3 to4.4 V (whichis tobe expectedof aTTL signal). 1.5 Volts 3.15 4.35 Range 10 20 30 40 Count 2 34 56 Figure 4.1 RadioFans.CN 收音机爱 好者资料库 H I S T O G R A M S 206ISSUED: January2002LTXXX-OM-E Rev B Thelowest-valuebinwithevents is that withasub-rangeof 3.0 to3.1 V. As TTL highvoltages needtobe greater than2.5 V, thelowest binis withintheallowabletolerance. However, becauseof its proximitytothis toleranceandthedegreeof thebins separationfrom all other values, additional investigationmayberequired. DSO PROCESS TheWaverunner digital storageoscilloscope(DSO)with WAVA generates histograms of theparameter values of input waveforms. But first, youmust definethefollowing: 1.Theparameter tobehistogrammed 2.Thetraceonwhichthehistogram is to bedisplayed 3.Themaximum number of parameter measurement values to beusedincreatingthehistogram 4.Themeasurement rangeof thehistogram 5.Thenumber of bins to beused Oncethesearedefined, theoscilloscopeis readytomakethehistogram. Thesequencefor acquiringhistogram datais as follows: 1.Trigger 2.Waveform acquisition 3.Parameter calculation(s) 4.Histogram update 5.Trigger re-arm. If youset thetimebasefor non-segmentedmode, asingleacquisition occurs prior to parameter calculations. However, inSequencemodeanacquisitionfor eachsegment occurs prior to parameter calculations. If the sourceof histogram datais amemory, savingnewdatatomemoryeffectivelyacts as atrigger andacquisition. Becauseupdatingthescreencantakemuchprocessingtime, it occurs onlyonceasecond, minimizingtrigger deadtime. Under remotecontrol thedisplaycanbeturnedoff tomaximizemeasurement speed. PARAMETER BUFFER Theoscilloscopemaintains acircular parameter buffer of thelast 20 000 measurements made, includingvalues that fall outsidetheset histogram range. If themaximum number of events tobeusedfor thehistogram is a number N less than20 000, thehistogram will becontinuouslyupdatedwith thelast N events as new acquisitions occur. If themaximum number is greater than20 000, thehistogram will beupdateduntil the number of events is equal toN. Then, if thenumber of bins or thehistogram rangeis modified, thescope will usetheparameter buffer values toredrawthehistogram witheither thelast N or 20 000 values acquired whichever is thelesser. Theparameter buffer therebyallows histograms toberedisplayed, usinganacquired set of values andsettings that produceadistribution shapewith themost useful information. RadioFans.CN 收音机爱 好者资料库 CH A P T E RFI F T E E N:How Histograms Work LTXXX-OM-E Rev BISSUED: January2002207 Inmanycases theoptimal rangeis not readilyapparent. Sothescopehas apowerful rangefindingfunction. If requiredit will examinethevalues in theparameter buffer tocalculateanoptimal rangeandredisplaythe histogram usingit. Waverunner will alsogivearunningcount of thenumber of parameter values that fall within, below , or abovetherange. If anyvalues fall belowor abovetherange, therangefinder canthen recalculatetoincludetheseparameter values, as longas theyarestill withinthebuffer. CAPTURE OF PARAMETER EVENTS Thenumber of events capturedper waveform acquisitionor displaysweepdepends ontheparameter type. Acquisitions areinitiatedbytheoccurrenceof atrigger event. Sweeps areequivalent tothewaveform captured anddisplayedonaninput channel (1, 2, or 3 or 4). For non-segmentedwaveforms anacquisitionis identical to asweep. Whereas for segmentedwaveforms anacquisition occurs for eachsegment andasweepis equivalent toacquisitions for all segments. Onlythesection of awaveform betweentheparameter cursors is usedinthe calculationof parameter values andcorrespondinghistogram events. Thefollowingtableprovides asummaryof thenumber of histogram events capturedper acquisitionor sweep for eachparameter, andfor awaveform sectionbetweentheparameter cursors. PARAME T E RS (PL US OT H E RS,DE PE N DIN G ON OPT ION S) NUMBE R OFEVE N T SCAPT URE D dataAll datavalues intheregionanalyzed. duty, freq, period, width,Upto49 events per acquisition. ampl, area, base, cmean, cmedian, crms, csdev , cycles,delay , dur , first,last, maximum, mean, median, minimum, nbph, nbpw , over+, over, phase,pkpk, points, rms, sdev , dly, t lv Oneevent per acquisition. f level, f8020%, fall, r level, r2080%, riseUpto49 events per acquisition. HISTOGRAM PARAMETERS Onceahistogram is definedandgenerated, measurements canbeperformedonthehistogram itself. Typical of thesearethehistograms averagevalue, standarddeviation most commonvalue(parameter valueof highest count bin) leftmost bin position(representingthelowest measuredwaveform parameter value) rightmost bin (representingthehighest measuredwaveform parameter value) RadioFans.CN 收音机爱 好者资料库 H I S T O G R A M S 208ISSUED: January2002LTXXX-OM-E Rev B Histogram parameters areprovidedtoenablethesemeasurements. Availablethroughselecting“Statistics” from the“Category” menu, theyarecalculatedfor theselectedsectionbetweentheparameter cursors (for afull description of eachparameter, seeChapter 4): avgaverageof datavalues inhistogram fwhmfull width(of largest peak)at half themaximum bin fwxxfull width(of largest peak)at xx% themaximum bin hamplhistogram amplitudebetweentwolargest peaks hbasehistogram baseor leftmost of twolargest peaks highhighest datavalueinhistogram hmedianmediandatavalueof histogram hrmsrms valueof datainhistogram htophistogram topor rightmost of twolargest peaks lowlowest datavalueinhistogram maxppopulationof most populatedbinin histogram modedatavalueof most populatedbinin histogram pctldatavalueinhistogram for whichspecifiedx% of populationis smaller pksnumber of peaks inhistogram rangedifferencebetweenhighest andlowest datavalues sigmastandarddeviationof thedatavalues inhistogram totptotal populationinhistogram xapkx-axis positionof specifiedlargest peak ZOOM TRACES AND SEGMENTED WAVEFORMS Histograms of zoom traces displayall events for thedisplayedportion of awaveform betweentheparameter cursors. Whendealingwithsegmentedwaveforms, andwhenasinglesegment is selected, thehistogram will be recalculatedfor all events inthedisplayedportion of this segment betweentheparameter cursors. But if “All Segments” is selected, thehistogram for all segments will bedisplayed. HISTOGRAM PEAKS Becausetheshapeof histogram distributions is particularlyinteresting, additional parameter measurements are availablefor analyzingthesedistributions. Theyare generallycenteredaroundoneof several peakvaluebins, known, withits associatedbins, as ahistogram peak. CH A P T E RFI F T E E N:How Histograms Work LTXXX-OM-E Rev BISSUED: January2002209 Example: InFigure4.2, ahistogram of thevoltagevalueof afive-volt amplitudesquarewaveis centered aroundtwo peakvaluebins: 0 V and5 V. Theadjacent bins signifyvariationduetonoise. Thegraphof the centeredbins shows bothas peaks. 05 Volts Figure 4.2 Determiningsuchpeaks is veryuseful becausetheyindicatedominant values of asignal. However, signal noiseandtheuseof ahigh number of bins relativetothenumber of parameter values acquired, cangiveajaggedandspikyhistogram, makingmeaningful peaks hardtodistinguish. Thescope analyzes histogram datatoidentifypeaks from backgroundnoiseandhistogram definitionartifacts suchas small gaps, whichareduetoverynarrowbins. For adetaileddescriptiononhowthescopedetermines peaks seethepks parameter descriptioninChapter 4. BINNING AND MEASUREMENT ACCURACY Histogram bins represent asub-rangeof waveform parameter values, or events. Theevents representedbya binmayhaveavalueanywherewithinits sub-range. However, parameter measurements of thehistogram itself, suchas average, assumethat all events inabin haveasinglevalue. Thescopeuses thecenter valueof eachbins sub-rangeinall its calculations. Thegreater thenumber of bins usedto subdivideahistograms range, theless thepotential deviationbetweenactual event values andthosevalues assumedinhistogram parameter calculations. Nevertheless, usingmorebins mayrequirethat youperform agreater number of waveform parameter measurements, inorder topopulatethebins sufficientlyfor theidentification of acharacteristic histogram distribution. Inaddition, veryfinegrainedbinningwill result ingaps betweenpopulatedbins that maymakeit difficult to determinepeaks. H I S T O G R A M S 210ISSUED: January2002LTXXX-OM-E Rev B Theoscilloscopes 20 000-parameter buffer is veryeffectivefor determiningtheoptimal number of bins to be used. Anoptimal binnumber is onewherethechangeinparameter values is insignificant, andthehistogram distributiondoes not haveajaggedappearance. Withthis buffer, ahistogram canbedynamicallyredisplayedas thenumber of bins is modifiedbytheuser. Inaddition, dependingonthenumber of bins selected, thechange inwaveform parameter values canbeseen.