數(shù)字式示波器TektronixTDS1002初步使用

第四章常用儀器簡介示波器最主要的功能就是把測量點的電壓隨時間變化曲線直觀地顯示在屏幕上。示波器是最重要的電子測量儀器之一，也是使用最頻繁的電子儀器之一。要正確使用一臺示波器，使用說明書中的主要性能指標(biāo)和基本操作方法列于本節(jié)之后。下面所介紹的僅僅是實驗中使用該示波器所所涉及到的最基本的內(nèi)容。1.功能簡介放大，然后采樣，再將采樣數(shù)據(jù)對應(yīng)的波形記錄，最后將波形顯示在屏幕上。同時，在示波功能選擇狀態(tài)選擇擇信號波形顯示屏幕菜單選時間擇信號波形顯示屏幕菜單選時間擇按鈕擇按鈕選擇增增益和位置模式和時基計算機接口觸發(fā)器流線示波器輸入等效電路信號源等路示波器輸入等效電路信號源等路若干問題并聯(lián)。并聯(lián)電容是為了抑制高頻干擾。示波器探頭有×1、×10轉(zhuǎn)換開關(guān)。當(dāng)探頭開關(guān)置于×1時，示波器為方波，當(dāng)信號源輸出上升沿時，由于信號輸出功率有限，給電容充電需要時間，所以，示波器輸入回路向示波器內(nèi)部電路輸出的電壓信號的前沿變緩，上升時間延長，如圖中右上方為其示意圖。這樣一來，在示屏幕上看到的方波的上升沿將大于實際輸入方波的開關(guān)與可變電容調(diào)節(jié)探頭中的補探頭中的補償10”的說法由此而來。在數(shù)字示波器中沿用了這一說法，當(dāng)在通道菜單中將探頭設(shè)為×10可大大改善方波的上升沿，試分析如下。設(shè)方波幅值為E，上升時刻為t=0。由克?；舴螂妷憾煽芍藭r刻兩個電容上源電壓，v(0+)+v(0+)=EC2C1(1)Cv2(0+)+Cv2(0+)=12E222C221C12C+C由(1)、(2)式可解出v(0+)=v(0+)=C2EC1oC+Cv()=R1EoR+R12(2)(3)(4)由RC電路過渡過程三要素法可得電容C1上的電壓變化的過程，即示波器輸入回路向示波RCCRR21212其中，T為電路的時間常數(shù)。由(5)式可知，若=，稱此為臨界補償，1212則在t=0+時刻有v(0+)=R1E，示波器輸入回路向示波器內(nèi)部電路輸出信號的上oR+R121212形成過補償或欠補償，如圖6。實際中，由于每臺儀器的輸入回路不可能完全一樣，所以需要通過調(diào)整C2使其達到臨界補償。又由于輸入回路中還存在著分布參數(shù)，實際中可能得到用探頭×10檔，使示波器輸入回路向示波器內(nèi)部電路輸出的電壓信號衰減了十倍。示波器自身的噪聲是一定的，所以使用×10檔，在改善輸入信號上升沿的同時，輸入信號的信噪比降低了。對于幅值較大的方波信號，上升沿的重要性大于幅較大的方波，探頭應(yīng)使用×10檔。其上升速率慢于示波器探頭×1檔時輸入回路的上升速率，所以應(yīng)使用×1。若使用VoEERC=RC1221t0償時示波器的階躍響應(yīng)的波形示意圖。對于正確測量正弦波小信號，示波器輸入回路向示波器內(nèi)z示波器探頭×1檔時輸入回路的上升速率，所以應(yīng)使用×10。若使用×1檔將使輸入信號的幅值減小。TTTT水平觸發(fā)位置觸發(fā)電平顯示屏?xí)r間坐標(biāo)為水平坐標(biāo)，”時，“波形記錄”電路將此時的電壓發(fā)位置”與“觸發(fā)電平”兩條虛線的交點上，為便于敘述，記錄圖7(a)中低電平和高電平時有較小的波紋起伏，顯示過程中的迭加將使顯示屏上波形的對應(yīng)的線條變粗。后，將示波器狀態(tài)恢復(fù)到初始狀態(tài)，再做下一項。不與所設(shè)置的“觸發(fā)電平”相等，所以示波器不觸發(fā)，顯示屏上就沒有波形顯示。示時間T之間原先不被顯示的波形也被顯示在顯示屏上。這時在顯示屏上顯示的是快速左通道2的電壓相比較。由于通道2沒有電壓波形輸入，所以電路不觸發(fā)，顯示屏沒有波形列做處理。所以，正確地采集數(shù)據(jù)是正確地使用數(shù)字示波器的十分重要。首先敘述采樣定理：設(shè)被采樣的時域連續(xù)量x(t)中包含的最高頻率分量為fimax，若以采f2f(6)simax必須避免發(fā)生頻率混迭。首先應(yīng)知道輸入時域窗口的時間寬度，由此可知道示波器2500個時域等間隔的離散數(shù)據(jù)，由此可計算出采樣頻率ffkHz(7)提高采樣頻率，或者外接低通濾波器，減小輸入量的最高頻率fimax，以滿足采樣定理。本數(shù)字示波器有三種數(shù)據(jù)采集方式：取樣、峰值檢測和平均。“取樣”是開機默認數(shù)據(jù)采集模式。在這種方式下，對輸入的連續(xù)量直接采樣。秒/格”或更快時，示波器會自動的將數(shù)據(jù)采集模式改為“取16、64、128。例x(n)=i=1in=1,2…2500(8)av14x(n)=i=2in=1,2…2500(9)av24在采集到的原始序列中，通常包含信號、噪聲和干擾。(通常稱需要的量是信號。稱有很多原因但沒有一個原因明顯突出的形成的不需要的量是噪聲。稱有有明顯原因形成的不需要的量是干擾。)對于周期信號，在觸發(fā)同步時，平均運算使周期信號不變?？烧J為噪聲是之間是相互統(tǒng)計獨立或統(tǒng)計上相關(guān)性較弱或很弱的。通過N次統(tǒng)計平均，噪聲的功率會減小。從而提高在示波器顯示的時域波形的信噪比。圖9是做統(tǒng)計平均的作用的示意圖。圖圖9做統(tǒng)計平均的作用的示意圖對采集到的原始序列做統(tǒng)計平均，具體能減小多少噪聲功率與隨機變量的統(tǒng)計分布和加一次平均所減少的噪聲功率量越來越小，例如，若隨機變量為零均值正態(tài)分布N(0,σ2)，序列統(tǒng)計平均)，噪聲功率為σ2/2，平均每次統(tǒng)計平均運算減少二分之一的噪聲功率；做三次統(tǒng)計平均后(即用四個2500個數(shù)據(jù)點的原始序列統(tǒng)計平均)，噪聲功率為σ2/4平均每次應(yīng)適當(dāng)。這里對數(shù)據(jù)采集只是就示波器的使用做最粗略的介紹，對數(shù)據(jù)采集問題的學(xué)習(xí)應(yīng)閱讀有關(guān)的信號處理的書籍。為周期而重復(fù)，則有以下離散付立葉變換和離散反付立葉變換NnnNNnn(10)(11)到的離散付立葉變換以fs/2為中心，兩邊共軛對稱，所以只顯示1024個點的離散付立葉幅=1VRMS)為單位的垂直刻度；3是以“頻率/分度”為單位的水平刻度；4是以“采樣數(shù)/秒”T采集不能保證被采集的時域信號正好是正弦波周期的整數(shù)倍，如圖11(a)中下方所示，付么由此得到的付立葉頻譜如圖11(a)下方所示的頻譜，它不再是一條線譜，其在頻域上被葉變換結(jié)果(a)(b)展寬，幅值略有減小。稱這種現(xiàn)象為因截斷引起的頻譜泄漏展寬。在本示波器中，減小因截斷引起的頻譜泄漏展寬的辦法是加窗口函數(shù)，這是一種常用的方法。圖11(b)所示的是加窗口函數(shù)(Hanning窗)后的結(jié)果，由圖可見，因截斷引起的頻譜泄漏展寬被大大地減小了。表達式為(12)宜用于分析周期信號，與Flattop窗比，其付立葉幅值譜的主瓣較窄，但第一旁瓣的幅值較當(dāng)被分析的信號不是正弦波時，在離散付立葉變換中，信號的時域波形可被看作由多本示波器是對顯示屏上所示的波形做FFT。在做FFT前，應(yīng)將波形的零電平線移到示波器顯示屏的零位線處(過顯示屏垂直方向中點的水平線)，這樣才能在離散付立葉頻譜中比降低，相應(yīng)地頻域的信噪比也降低了。在本示波器中離散付立葉變換的結(jié)果是付立葉幅值譜。付立葉幅值譜以分貝(dB)的形式L=20lgViRMSn(dB)n1V(13)讀有關(guān)的信號處理的書籍。3.關(guān)于本示波器的具體性能指標(biāo)和基本操作使用方法AllspecificationsapplytotheTDS1000-seriesoscilloscopes.P2200probespecificationsappearattheendofthischapter.Toverifythattheoscilloscopemeetsspecifications,theoscilloscopemustfirstmeetthefollowingconditions:＊Theoscilloscopemusthavebeenoperatingcontinuouslyfortwentyminuteswithinthespecifiedoperatingtemperature.＊YoumustperformtheDoSelfCaloperation,accessiblethroughtheUtilitymenu,iftheoperatingtemperaturechangesbymorethan5℃.＊TheoscilloscopemustbewithinthefactorycalibrationintervalAllspecificationsareguaranteedunlessnoted“typical”.OscilloscopeSpecificationsAcquisitionSampleSample,PeakDetect,andAverageUpto180waveformspersecond,perchannel(Sampleacquisitionmode,nomeasurements)AcquisitionModeSample,PeakDetectAverageAcquisitionStopsAfterSingleacquisition,allchannelssimultaneouslyNacquisitions,allchannelssimultaneously,Nisselectablefrom4,16,64,and128AcquisitionModesAcquisitionRate,typicalSingleSequenceCACorGND1M?±2%inparallelwith20pF±3pF1X,10X,100X,1000XOvervoltageCategoryMaximumVoltageCATIandCATII300VRMS,InstallationCategoryIIInputCouplingInputImpedance,DCCoupledAttenuationSupportedAttenuationFactorsMaximumVoltageBetweenSignalandCommonatinputBNCCATIII150VRMSInstallationCategoryII;decadeat20dB/decadeabove100kHzto13VpeakACat3MHz1andabove.Fornon-sinusoidalwaveforms,peakvaluemustbelessthan450V.Excursionabove300Vshouldbelessthan100msduration.RMSsignallevelincludinganyDCcomponentremovedthroughACcouplingmustbelimitedto300V.Ifthesevaluesareexceeded,damagetotheinstrumentmayresult.Channel-to-Channel100:1at60Hz,20:1at30MHz1CommonModeMeasuredonMATHCh1-Ch2waveform,withtestsignalRejection,typicalappliedbetweensignalandcommonofbothchannels,andwiththesameVOLTS/DIVandcouplingsettingsoneachchannelChannel-to-Channel≥100:1at30MHz1CrosstalkMeasuredononechannel,withtestsignalappliedbetweensignalandcommonoftheotherchannel,andwiththesameVOLTS/DIVandcouplingsettingsoneachchannelcalzersVOLTS/DIVRangePositionRangeAnalogBandwidthinSampleandAverage8-bitresolution(exceptzersVOLTS/DIVRangePositionRangeAnalogBandwidthinSampleandAverage2mV/divto5V/divatinputBNC2mV/divto200mV/div,±2V>200mV/divto5V/div,±50V60MHz1Whenverticalscaleissetto<5mVmodesatBNCorwith20MHz1P2200modesatBNCorwith20MHz1P2200probe,DCCoupledAnalogBandwidthinPeakDetectmode(50s/divAnalogBandwidthinPeakDetectmode(50s/divto5μs/div2)typicalSelectableAnalog50MHz120MHz120MHz1Whenverticalscaleissetto≥5mV.BandwidthLimit,typicalLowerFrequencyBandwidthLimit,typicalLowerFrequencyLimit,ACCoupledseTimeatBNCtypicalPeakDetectResponse2DCGainAccuracyDCMeasurement≤1Hzwhenusinga10Xpassiveprobe<5.8nsCaptures50%orgreateramplitudeofpulses≥12nswidetypical(50s/divto5_s/div)inthecenter8verticaldivisions±3%forSampleorAverageacquisitionmode,5V/divto10mV/div±4%forSampleorAverageacquisitionmode,5mV/divand2mV/divMeasurementTypeAccuracyAccuracy,AverageAcquisitionModeVoltsMeasurementRepeatability,AverageAcquisitionModeontalAverageof≥16waveformswithverticalpositionatzeroAverageof≥16waveformswithverticalpositionnotatzeroDeltavoltsbetweenanytwoaveragesof≥16waveformsacquiredundersamesetupandambientconditions±(3%×reading+0.1div+1mV)when10mV/divorgreaterisselected±[3%×(reading+verticalposition)+1%ofverticalposition+0.2div]Add2mVforsettingsfrom2mV/divto200mV/divAdd50mVforsettingsfrom>200mV/divto5V/div(3%×reading+0.05div)55S/sto1GS/snxx2500samplesforeachchannel5ns/divto50s/div,ina1,2.5,5sequence±50ppmoverany≥1mstimeintervalConditionsSingle-shot,Sample>16averagesSampleinterval=s/div÷2505ns/divto10ns/div(-4div×s/div)to20ms25ns/divto100_s/div(-4div×s/div)to50ms250μs/divto50s/div(-4div×s/div)to50sAccuracy±(1sampleinterval+100ppm×reading+0.6ns)±(1sampleinterval+100ppm×reading+0.4ns)SampleRateRangeWaveformInterpolationRecordLengthSEC/DIVRangeSampleRateandDelayTimeAccuracyDeltaTimeMeasurementAccuracyFullBandwidth)PositionRangeriggerTriggerTriggerSensitivity,CouplingSensitivityEdgeTriggerTypeDCCH1,1divfromDCto10MHz1,CH2,1.5divfrom10MHz1toFullEXT200mVfromDCto100MHz1,350mVfrom100MHzto200MHz1EXT/51VfromDCto100MHz1,1.5Vfrom100MHzto200MHz1TriggerSensitivity,CouplingSensitivityEdgeTriggerType,ACSameasDCat50HzandaboveEJFREJceCH1,CH2ReducestheDC-coupledtriggersensitivityby2timesfor>10mv/divto5V/divSameastheDC-coupledlimitfromDCto7kHz,attenuatessignalsabove80kHzSameastheDC-coupledlimitsforfrequenciesabove300kHz,attenuatessignalsbelow300kHz±8divisionsfromcenterofscreenAccuraciesareforsignalshavingriseandfalltimes≥20nsceAccuracycenterscreen±(6%ofsetting+40mV)±(6%ofsetting+200mV)Operateswithinputsignals≥50HzCouplingisACandAutoexceptforasinglesequenceacquisitionCompositevideosignalceSupportsNTSC,PAL,andSECAMbroadcastsystemsforanyfieldoranyline500nsto10stypicalTriggerLevelRangeTriggerLevelAccuracy,typicalSETLEVELTO50%,typicalDefaultSettings,VideoTriggerSensitivity,VideoTriggerType,typicalSignalFormatsandFieldRates,VideopeHoldoffRangePk-pkamplitudeof2divisions400mV2V1Bandwidthreducedto6MHzwitha1Xprobe.2TheoscilloscoperevertstoSamplemodewhentheSEC/DIV(horizontalscale)issetfrom2.5_s/divto5ns/divon1GS/smodels,orfrom2.5_s/divto2.5ns/divon2GS/smodels.TheSamplemodecanstillcapture10nsglitches.Inadditiontodisplayingwaveforms,thedisplayisfilledwithmanydetailsaboutthewaveformandtheoscilloscopecontrolsettings.1.Icondisplayshowsacquisitionmode.SamplemodePeakdetectmodeAveragemode2.Triggerstatusindicatesthefollowing:Armed.Theoscilloscopeisacquiringpretriggerdata.Alltriggersareignoredinthisstate.ReadyReady.Allpretriggerdatahasbeenacquiredandtheoscilloscopeisreadytoacceptatrigger.TrigTrig’d.Theoscilloscopehasseenatriggerandisacquiringtheposttriggerdata.Stop.Theoscilloscopehasstoppedacquiringwaveformdata.Acq.Complete.TheoscilloscopehascompletedaSingleSequenceacquisition.Auto.Theoscilloscopeisinautomodeandisacquiringwaveformsintheabsenceoftriggers.ScanScan.Theoscilloscopeisacquiringanddisplayingwaveformdatacontinuouslyinscanmode.3.Markershowshorizontaltriggerposition.TurntheHORIZONTALPOSITIONknobtoadjustthepositionofthemarker.4.Readoutshowsthetimeatthecentergraticule.Thetriggertimeiszero.5.MarkershowsEdgeorPulseWidthtriggerlevel.6.On-screenmarkersshowthegroundreferencepointsofthedisplayedwaveforms.Ifthereisnomarker,thechannelisnotdisplayed.7.Anarrowiconindicatesthatthewaveformisinverted.8.Readoutsshowtheverticalscalefactorsofthechannels.9.ABWiconindicatesthatthechannelisbandwidthlimited.10.Readoutshowsmaintimebasesetting.11.Readoutshowswindowtimebasesettingifitisinuse.12.Readoutshowstriggersourceusedfortriggering.13.Iconshowsselectedtriggertypeasfollows:Edgetriggerfortherisingedge.Edgetriggerforthefallingedge.--Videotriggerforlinesync.--Videotriggerforfieldsync.PulseWidthtrigger,positivepolarity.PulseWidthtrigger,negativepolarity.14.ReadoutshowsEdgeorPulseWidthtriggerlevel.15.Displayareashowshelpfulmessages;somemessagesdisplayforonlythreeseconds.Ifyourecallasavedwaveform,readoutshowsinformationaboutthereferencewaveform,suchasRefA1.00V500μs.Theoscilloscopedisplaysamessageareaatthebottomofthescreenthatconveysthefollowingtypesofhelpfulinformation:＊Directionstoaccessanothermenu,suchaswhenyoupushtheTRIGMENUbutton:ForTRIGGERHOLDOFF,gotoHORIZONTALMenu＊Suggestionofwhatyoumightwanttodonext,suchaswhenyoupushtheMEASUREbutton:Pushanoptionbuttontochangeitsmeasurement＊Informationabouttheactiontheoscilloscopeperformed,suchaswhenyoupushtheDEFAULTSETUPbutton:Defaultsetuprecalled＊Informationaboutthewaveform,suchaswhenyoupushtheAUTOSETbutton:SquarewaveorpulsedetectedonCH116.Readoutshowstriggerfrequency.TheuserinterfaceofTDS1000-andTDS2000-seriesoscilloscopeswasdesignedforeasyaccesstospecializedfunctionsthroughthemenustructure.Whenyoupushafront-panelbutton,theoscilloscopedisplaysthecorrespondingmenuontherightsideofthescreen.Themenushowstheoptionsthatareavailablewhenyoupushtheunlabeledoptionbuttonsdirectlytotherightofthescreen.(Somedocumentationmayalsorefertotheoptionbuttonsasscreenbuttons,side-menubuttons,bezelbuttons,orsoftkeys.)Theoscilloscopeusesfourmethodstodisplaymenuoptions:＊Page(Submenu)Selection:Forsomemenus,youcanusethetopoptionbuttontochoosetwoorthreesubmenus.Eachtimeyoupushthetopbutton,theoptionschange.Forexample,whenyoupushthetopbuttonintheSAVE/RECMenu,theoscilloscopecyclesthroughtheSetupsandWaveformssubmenus.＊CircularList:Theoscilloscopesetstheparametertoadifferentvalueeachtimeyoupushtheoptionbutton.Forexample,youcanpushtheCH1MENUbuttonandthenpushthetopoptionbuttontocyclethroughtheVertical(channel)Couplingoptions.＊Action:TheoscilloscopedisplaysthetypeofactionthatwillimmediatelyoccurwhenyoupushanActionoptionbutton.Forexample,whenyoupushtheDISPLAYMenubuttonandthenpushtheContrastIncreaseoptionbutton,theoscilloscopechangesthecontrastimmediately.＊Radio:Theoscilloscopeusesadifferentbuttonforeachoption.Thecurrently-selectedoptionishighlighted.Forexample,theoscilloscopedisplaysvariousacquisitionmodeoptionswhenyoupushtheACQUIREMenubutton.Toselectanoption,pushthecorrespondingbutton.CH1,CHCH1,CH2,CURSOR1andCURSOR2POSITION.Positionsthewaveformvertically.Whenyoudisplayandusecursors,anLEDlightstoindicatethealternativefunctionoftheknobstomovethecursors.CH1,CH2MENU.Displaystheverticalmenuselectionsandtogglesthedisplayofthechannelwaveformonandoff.VOLTS/DIV(CH1&CH2).Selectscalibratedscalefactors.MATHMENU.Displayswaveformmathoperationsmenuandcanalsobeusedtotogglethemathwaveformonandoff.POSITION.Adjuststhehorizontalpositionofallchannelandmathwaveforms.Theresolutionofthiscontrolvarieswiththetimebasesetting.NOTE.Tomakealargeadjustmenttothehorizontalposition,turntheSEC/DIVknobtoalargervalue,changethehorizontalposition,andthenturntheSEC/DIVknobbacktothepreviousvalue.Whenyouviewhelptopics,youcanusethisknobtoscrollthroughlinksorindexentries.HORIZMENU.DisplaystheHorizontalMenu.SETTOZERO.Setsthehorizontalpositiontozero.SEC/DIV.Selectsthehorizontaltime/div(scalefactor)forthemainorthewindowtimebase.WhenWindowZoneisenabled,itchangesthewidthofthewindowzonebychangingthewindowtimebase.TriggerControlsLEVELandUSERSELECT.WhenyouuseanEdgetrigger,theprimaryfunctionoftheLEVELknobistosettheamplitudelevelthesignalmustcrosstocauseanacquisition.YoucanalsousetheknobtoperformUSERSELECTalternativefunctions.TheLEDlightsbelowtheknobtoindicateanalternativefunction.USERUSERSELECTescriptionSetstheamountoftimebeforeanothertriggereventcanbeaccepted;VideolineSetstheoscilloscopetoaspecificlinenumberwhentheTriggerTypeberoptionissettoVideoandtheSyncoptionissettoLineNumberdthSetsthewidthofthepulsewhentheTriggerTypeoptionissettoPulseandyouselecttheSetPulseWidthoptionTRIGMENU.DisplaystheTriggerMenu.SETTO50%.Thetriggerlevelissettotheverticalmidpoint