lecroy 12_LTXXX-OM-E_Ch8 电路图.pdf

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1、86ISSUED: January2002LTXXX-OM-E Rev B CH A P T E REI G H T:TriggerSmart More about Edge triggering. And introducing the SMART Triggerrange forcapturing complexwaveformcharacteristics. Inthis chapter, see how Tohold off withEdge Trigger Tocapture rare phenomena withGlitchtrigger Toset upanexclusiontr

2、igger Todetermine triggerlevel, coupling and slope Totriggeronintervals Touse State and Edge Qualified triggers Totriggeronlost signals using Dropout trigger TotriggeronTV signals PatternTrigger RadioFans.CN 收音机爱 好者资料库 LTXXX-OM-E Rev BISSUED: January200287 CH A P T E REI G H TTriggerSmart HoldOff by

3、TimeorEvents Holdoff is anadditional conditionof Edgetrigger (seeChapter 2, “Simply Trigger”). It canbeexpressed either as aperiodof timeor anevent count. Holdoff disables thetrigger circuit for agivenperiodof timeor events after thelast trigger occurred. Events arethenumber of occasions on which th

4、etrigger conditionis met. Thetrigger will again occur whentheholdoff has elapsedandthetriggers other conditions aremet. Use holdoff toobtainastabletrigger for repetitive, compositewaveforms. For example, if thenumber or duration of sub-signals is knownyoucandisablethem bychoosinganappropriateholdoff

5、 value. Qualifiedtriggers operateusingconditions similar toholdoff (seepage100). HOLD OFF BY TIME Sometimes youcanachieveastabledisplayof complex, repetitivewaveforms byplacingacondition onthe timebetweeneachsuccessivetrigger event. This timewouldotherwisebelimitedonlybytheinput signal, the coupling

6、, andWaverunners bandwidth. Select apositiveor negativeslope, andaminimum timebetween triggers. Thetrigger is generatedwhentheconditionis met after theselectedholdoff time, countedfrom the last trigger (Fig.1). Anytimebetween10 ns and20 s can beselected. Thedelayis initializedandstartedoneach trigge

7、r. Hold-off timeHold-off time TriggerSource: Positive Slope Triggercanoccur Generated Trigger Trigger Event Trigger Event Trigger Event Trigger initiates hold-off timer Trigger initiates hold-off timer Figure 1. E dge Trigger with Holdoff by Time. The bold edges on the trigger source indicate that a

8、 positive slope has been selected. The broken upward-pointing arrows indicate potential triggers, which would occur if other conditions are met. The bold arrows indicate where the triggers actually occur when the holdoff time has been exceeded. RadioFans.CN 收音机爱 好者资料库 PA R TTW O:L O O K I N GD E E P

9、 E R 88ISSUED: January2002LTXXX-OM-E Rev B HOLD OFF BY EVENTS Select apositiveor negativeslopeandanumber of events. Anevent is thenumber of times thetrigger conditionis met after thelast trigger. A trigger is generatedwhen theconditionis met after this number, countedfrom thelast trigger. Thecount i

10、s initializedandstartedoneachtrigger. For example, if theselected event number is two(Fig. 2), thetrigger will occur onthethirdevent. From oneto99 999 999 events canbe selected. Hold-off by2 eventsHold-off by2 events TriggerSource: PositiveSlope Triggercanoccur Generated Trigger Event #1 Event #2 Ev

11、ent #2 Event #1 Trigger Event Trigger Event Trigger Event Trigger initiates hold-off timer Trigger initiates hold-off timer Figure 2. E dge Trigger with Holdoff by E vents (in this example, two events). The bold edges on the trigger source indicate that a positive slope has been selected. The broken

12、, upward-pointing arrows indicate potential triggers, while the bold ones show where triggers actually occur after the holdoff expires. RadioFans.CN 收音机爱 好者资料库 CH A P T E REI G H T:TriggerSmart LTXXX-OM-E Rev BISSUED: January200289 TriggerSMART Youhaveseenhowtotrigger on signals usingEdgetrigger, an

13、dtheconditions of level, couplingandslope, andholdoff. Waverunner also offers arangeof sophisticatedtriggers that enableyoutotrigger onthese conditions, as well as manyother complexwaveform characteristics. UsetheSMART Trigger rangetoset additional qualifications beforeatrigger is generated. Catchra

14、rephenomenasuchas glitches or spikes, specific logic states, or missingbits. Captureintervals, abnormal signals, or TV signals. Trigger onstateor edgequalified events anddropouts. CATCH A GLITCH Findingandcapturingelusiveglitches abnormallywidepulses inasignal is simplewith theGlitchtrigger. 1.Conne

15、ct your signal totheWaverunner. 2.Simultaneouslypress thesecondandfifthmenubuttonsfrom thetop, and 1 toreturnthescopetoits default power-upsettings. Andturnoff anyunwanteddisplayedtrace. 3.Press thebuttonforandset Couplingtomatchthesourcesignals impedance. 4.Press AUTO SETUP andthen STOP todisplayan

16、ormal waveform. Viewingthis signal duringseveral captures wouldreveal theoccasional glitch. Thegoal of themeasurement is tocatchthis event bysettingatrigger adaptedtoit. 5.Press TRIGGER SETUP andthenthebuttontoselect 6.Press thebuttonforanddisplaythesemenus: PA R TTW O:L O O K I N GD E E P E R 90ISS

17、UED: January2002LTXXX-OM-E Rev B Usethesemenus toset uptotrigger onaglitch, or tocreateanexclusiontrigger (seepage93). 7.Select Glitchtocatchpulses of achosenwidth. Capturenarrow pulses less than or equal to, or greater than or equal to, agiven timelimit: thepulses width. Youcanalsoset uptoexcludeor

18、 includecertainevents (seepage93). 8.Select thetrigger source. This couldbeasignal onachannel, the linevoltagethat powers theWaverunner or theEXT connector. 9.Select thecouplingfor thetrigger source. 10. Placethetrigger point at theendof apositiveor negativeslope. See N NOOT TE E this page. 11. Pres

19、s thebuttontoset toOnandtotrigger if the pulseis less than or equal tothevalueset withthe knob(range: 2.5 ns to20 s). Useincombination with “width” below . 12. Press thebuttontoset toOnandtotrigger if the pulseis greater than or equal tothevalueset with theknob(range: 2.5 ns to20 s). Useincombinatio

20、n with “width,” combinedtotarget glitches within (“ DC levels are rejected and frequencies below 50 Hz attenuated. LF RE J:The signal is coupled through a capacitive high-pass filter network, DC is rejected and signal frequencies below 50 kHz are attenuated. For stable triggering on medium to high f

21、requency signals. HF RE J:Signals are DC coupled to the trigger circuit, and a low-pass filter network attenuates frequencies above 50 kHz. It is used for triggering on low frequencies. Slopedetermines the direction of the trigger voltage transition used for generating a particular trigger event. Yo

22、u can choose a positive or negative slope. L ike coupling, the selected slope is associated with the chosen trigger source. PA R TTW O:L O O K I N GD E E P E R 96ISSUED: January2002LTXXX-OM-E Rev B TRIGGER ON INTERVALS WhileGlitchtrigger performs over thewidthof apulse, Interval trigger performs ove

23、r thewidth of aninterval thesignal duration(theperiod)separatingtwoconsecutiveedges of thesamepolarity: positiveto positiveor negativeto negative. UseInterval trigger tocaptureintervals that fall short of, or exceed, agiventimelimit. In addition, youcandefineawidthrangetocaptureanyinterval that is i

24、tself insideor outsidethespecifiedrange anExclusiontrigger byInterval. Usethesemenus toset upfor triggeringonaninterval. 1.Select Interval. 2.Select thetrigger source. 3.Select thecouplingfor thetrigger source. 4.Placethetrigger point onthepositiveor negativeedgeof the selectedpulse. 5.Select Ontotr

25、igger if thepulseis less thanor equal tothevalueset with theknob(range: 2.5 ns to20 s). Useincombination with“width.” 6.Select Ontotrigger if thepulseis greater thanor equal tothevalueset withtheknob (range: 2.5 ns to 20 s). Useincombination with“width,” combined totarget intervals within (“ T2=rang

26、es upper limit. The broken upward pointing arrow indicates a potential trigger, while the bold one indicates where the actual trigger occurs on the positive edge within the selected range. PA R TTW O:L O O K I N GD E E P E R 100ISSUED: January2002LTXXX-OM-E Rev B QUALIFY A SIGNAL Useasignals transit

27、ionaboveor belowagivenlevel its validation as anenabling(qualifying)condition for asecondsignal that is thetrigger source. TheseareQualifiedtriggers. WithStateQualifiedtrigger, the amplitudeof thefirst signal must remaininthedesiredstateuntil thetrigger occurs. Whilefor EdgeQualified trigger thevali

28、dationis sufficient andnoadditional requirement is placedonthefirst signal. A Qualifiedtrigger canoccur immediatelyafter thevalidationor withinaset timeafter it. Or it canoccur followinga predeterminedtimedelayor number of potential trigger events. Thetimedelayor trigger count is restarted withevery

29、validation. Usethesemenus toset upan Edgeor StateQualifiedtrigger. 1.Select Qualified. 2.Select Edge or State as thequalifier. (Set conditions suchas the coupling, slope, andholdoff usingtheEdgetrigger menus.) 3.Select thetrigger source. This couldbeachannel, EXT, or EXT 10. 4.Select thequalifier so

30、urce. This couldbeachannel, EXT or EXT 10, dependingonyour choiceof trigger source. 5.Turntoadjust thequalifier threshold, andpress to determinewhether thequalifier signal is validonceit “has gone” (EdgeQualified)or “goes andstays” (StateQualified)aboveor belowthat threshold. 6.Specifythetimelimit (

31、“within” T )or number of trigger events (“wait” Evs)after avalidtransitionhas occurred. A trigger canonlybeacceptedafter this delay. Anysubsequent qualifier event restarts thecount.Thetimevaluecan beset intherange10 ns20 s. Thetrigger event count canbeset intherange199999999. CH A P T E REI G H T:Tr

32、iggerSmart LTXXX-OM-E Rev BISSUED: January2002101 HOWQUAL IFIE DTRIGGE RSWORK State Qualified and Wait(Fig. 7) is determined by the parameters of Time or E vents. Timedetermines a delay from the start of the desired pattern. A fter the delay (timeout) and while the pattern is present, a trigger can

33、occur. The timing for the delay is restarted when the selected pattern begins. E ventsdetermines a minimum number of events of the trigger source. A n event is generated when a trigger source meets its trigger conditions. On the selected event of the trigger source and while the pattern is present,

34、a trigger can occur. The count is initialized and started whenever the selected pattern begins, and continues while the pattern remains. When the selected count is reached, the trigger occurs. Generated Trigger Wait TriggerSource: Positive Slope Triggercanoccur Qualifier: PatternPresent Wait Figure

35、7. State Qualified and Wait: Trigger after timeout. The broken upward pointing arrows indicate potential triggers, while the bold arrows show where the actual triggers occurs. PA R TTW O:L O O K I N GD E E P E R 102ISSUED: January2002LTXXX-OM-E Rev B E dge Qualified and Wait(Fig. 8) is also conditio

36、ned by either Time or E vents: Timedetermines a delay from the start of the desired pattern. A fter the delay (timeout) and before the end of the pattern, a trigger can occur. The timing for the delay is restarted when the selected pattern begins. E ventsdetermines a minimum number of events for the

37、 trigger source. A n event is generated when a trigger source meets its trigger conditions. A trigger can occur on the selected event of the trigger source and before the end of the pattern. The count is initialized and started whenever the selected pattern begins. It continues while the pattern rem

38、ains. When the selected count is reached, the trigger occurs. Selec ted Tim e Selec ted Tim e TriggerSource: PositiveSlope Qualifier: PatternPresent Triggercanoccur Generated Trigger Figure 8. E dge Qualified and Wait: Trigger after timeout. The broken upward pointing arrows indicate potential trigg

39、ers, while the bold ones show where the actual trigger occurs. CH A P T E REI G H T:TriggerSmart LTXXX-OM-E Rev BISSUED: January2002103 TRIGGER ON LOST SIGNALS UseDropout trigger whenever your signal disappears for aset periodof time. Thetrigger is generatedat the endof thetimeout periodfollowingthe

40、“last” trigger sourcetransition(Fig. 9, page105). Timeouts of between 25 ns and20 s canbeselected. Dropout trigger is usedessentiallyfor single-shot applications usuallywitha pre-trigger delay. 1.Connect thesignal to bemeasuredtoChannel 1. 2.Press thebuttonforandset Couplingtomatchthesourceimpedance

41、. 3.Press AUTO SETUP twicetodisplaythewaveform. Thefollowingsteps set theDropout trigger tocaptureonly the“last normal” periodof thesignal andtransient signal. 4.Press TRIGGER SETUP andthenthebuttontoselect 5.Press thebuttonfortodisplaythemenus shownonthenext page. PA R TTW O:L O O K I N GD E E P E

42、R 104ISSUED: January2002LTXXX-OM-E Rev B Usethesemenus toset uptheDropout trigger. 6.Select Dropout. T TI IP P: : Use Dropout Triggerwhenviewing signals onmore than one channel(you canviewthe signalcharacteristics ona channel otherthanthe triggersource). Totriggerthe scope whena signal disappears, m

43、ake the timeout longerthanthe signalperiod. The Waverunnerwillnot triggerwhentherepetitive signalis active, because twosuccessive edges occurina single period. 7.Select thetrigger source. 8.Select theslope. 9.Turntoset thetimeout (25 ns bydefault). 10. Turn DELAY toset thetrigger point toallowdispla

44、yof thesignals “last normal” period. Whenthesignal disappears, theWaverunner triggers. CH A P T E REI G H T:TriggerSmart LTXXX-OM-E Rev BISSUED: January2002105 HOWDROPOUTTRIGGE RWORKS Generated Trigger TriggerSource Triggercanoccur Wait Tim e- out Wait Tim e- out Figure 9. Dropout Trigger: occurs wh

45、en the timeout has expired. The bold upward-pointing arrows show where the trigger occurs. PA R TTW O:L O O K I N GD E E P E R 106ISSUED: January2002LTXXX-OM-E Rev B TRIGGER ON TV SIGNALS Waverunners TV triggers providestabletriggeringonstandardor custom compositevideosignals. Usethem onPAL, SECAM,

46、or NTSC systems. A compositevideosignal onthetrigger input is analyzedtoprovidea signal for thebeginningof thechosenfield “any,” “odd,” or “even” andfor asignal at thebeginningof eachline. Thefieldsignal provides thestartingtransition, andthebeginnings of linepulses arecountedto allowthefinal trigge

47、r onthechosenline. Eachfield, thenumber of fields, thefieldrate, interlacefactor, and number of lines per picturemust bespecified althoughtherearestandardsettings for themost common types of TV signals. TV Trigger canalsofunctioninasimpleany-linemode. Usethesemenus toset upaTV trigger. 1.Select TV-P

48、os or TV-Neg polarity. 2.Select thetrigger source. 3.Specifythenumber of fields or definethefieldnumber. For the LT364 series, youhavethechoiceof 1, 2, 4, or 8fields. For all other models: 1=odd, 2=even. 4.Select Standard or CustomTV decoding. 5.Standard:Select either 625/50/2:1(PAL,SECAM)or 525/60/

49、2:1(NTSC)standard.Custom:Specifythe number of linesandcycles,andset theinterlacing factor for non-standardTV signals. 6.Definethelineonwhichto trigger with theknob: anyallows youtotrigger onanylinenumber. CH A P T E REI G H T:TriggerSmart LTXXX-OM-E Rev BISSUED: January2002107 TOUSETV TRIGGE RS Most TV systems have more than two fields. Waverunners enhanced field counting capability (FIE L DL OCK) allows you to trigger consistently on a chosen line within a signal field. The field numbering system is relative: the oscilloscope will trigger on odd or even fields for