《电子信息专业英语》-Chapter 2 Measuring Instruments and Their Usage

Text1UsingaMultimeterNavigation1.Whatdometersmeasure?2.Digitalmultimeters3.Analoguemultimeters4.Makingmeasurements1.Whatdometersmeasure?Ameterisameasuringinstrument.Anammetermeasurescurrent，avoltmetermeasuresthepotentialdifference(voltage)betweentwopoints，andanohmmetermeasuresresistance.Amultimetercombinesthesefunctions，andpossiblysomeadditionalonesaswell，intoasingleinstrument.下一页返回Text1UsingaMultimeterBeforegoingintodetailaboutmultimeters，itisimportantforyoutohaveaclearideaofhowmetersareconnectedintocircuits.DiagramsA(Fig.2-1)andB(Fig.2-2)belowshowacircuitbeforeandafterconnectinganammeter:Tomeasurecurrent，thecircuitmustbebrokentoallowtheammetertobeconnectedinseriesandammetersmusthaveaLOWresistance.Thinkaboutthechangesyouwouldhavetomaketoapracticalcircuitinordertoincludetheammeter.Tostartwith，youneedtobreakthecircuitsothattheammetercanbeconnectedinseries.Allthecurrentflowinginthecircuitmustpassthroughtheammeter.下一页返回上一页Text1UsingaMultimeterMetersarenotsupposedtoalterthebehaviourofthecircuit，oratleastnotsignificantly，anditfollowsthatanammetermusthaveaveryLOWresistance.DiagramC(Fig.2-3)showsthesamecircuitafterconnectingavoltmeter.Tomeasurepotentialdifference(voltage)，thecircuitisnotchanged:①Thevoltmeterisconnectedinparallel;②VoltmetersmusthaveaHIGHresistance.Thistime，youdonotneedtobreakthecircuit.Thevoltmeterisconnectedinparallelbetweenthetwopointswherethemeasurementistobemade.下一页返回上一页Text1UsingaMultimeterSincethevoltmeterprovidesaparallelpathway，itshouldtakeaslittlecurrentaspossible.Inotherwords，avoltmetershouldhaveaveryHIGHresistance.Whichmeasurementtechniquedoyouthinkwillhethemoreuseful？Infact，voltagemeasurementsareusedmuchmoreoftenthancurrentmeasurements.Theprocessingofelectronicsignalsisusuallythoughtofinvoltageterms.Itisanaddedadvantagethatavoltagemeasurementiseasiertomake.Theorginalcircuitdoesnotneedtochanged.Often，themeterprobesareconnectedsimplybytouchingthemtothepointsofinterest.下一页返回上一页Text1UsingaMultimeterAnohmmeterdoesnotfunctionwithacircuitconnectedtoapowersupply.Ifyouwanttomeasuretheresistanceofaparticularcomponent，youmusttakeitoutofthecircuitaltogetherandtestitseparately，asshownindiagramD(Fig.2-4).Tomeasureresistn}ce，thecomponentmustberemovedfromthecircuitaltogether.Ohmmetersworkbypassingasmallcurrentthroughthecomponentandmeasuringthevoltageproduced.Ifyoutrythiswiththecomponentconnectedintoacircuitwithapowersupply，themostlikelyresultisthatthemeterwillbedamaged.Mostmultimetershaveafusetohelpprotectagainstmisuse.下一页返回上一页Text1UsingaMultimeter2.Digitalmultimeters(Fig.2-5)Multimetersaredesignedandmassproducedforelectronicsengineers.Eventhesimplestandcheapesttypesmayincludefeatureswhichyouarenotlikelytouse.Digitalmetersgiveanoutputinnumbers，usuallyonaliquidcrystaldisplay.ThediagramFig.2-5showsaswitchedrangemultimeter.下一页返回上一页Text1UsingaMultimeterSwitchedrougemultimeterThecentralknobhaslotsofpositionsandyoumustchoosewhichoneisappropriateforthemeasurementyouwanttomake.Ifthemeterisswitchedto20VDC，forexample，then20Visthemaximumvoltagewhichcanbemeasured.Thisissometimescalled20VFSD，whereFSDisshortforfullscaledeflection.下一页返回上一页Text1UsingaMultimeterForcircuitswithpowersuppliesofupto20V，whichincludesallthecircuitsyouarelikelytobuild，the20VDCvoltagerangeisthemostuseful.DCrangesareindicatedbyonthemeter.Sometimes，youwillwanttomeasuresmallervoltages，andinthiscase，the2Vor200mVrangesareused.WhatdoesDCmean?DCmeansdirectcurrent.Inanycircuitwhichoperatesfromasteadyvoltagesource，suchasabattery，currentflowisalwaysinthesamedirection.ACmeansalternatingcurrent.Inanelectriclampconnectedtothedomesticmainselectricity，currentflowsfirstoneway，thentheother.下一页返回上一页Text1UsingaMultimeterThatis，thecurrentreverses，oralternates，indirection.WithChinamains，thecurrentreverses50timespersecond.MAINSVOLTAGEForsafetyreasons，youmustNEVERconnectamultimetertothemainssupply.YouarenotatalllikelytousetheACranges，indicatedby，onyourmultimeter.Analternativestyleofmultimeteristheautorangingmultimeter.Autorn}gi}gmultimeter(Fig.2-6)下一页返回上一页Text1UsingaMultimeterThecentralknobhasfewerpositionsandallyouneedtodoistoswitchittothequantityyouwanttomeasure.OnceswitchedtoV，themeterautomaticallyadjustsitsrangetogiveameaningfulreading，andthedisplayincludestheunitofmeasurement,VormV.Thistypeofmeterismoreexpensive，butobviouslymucheasiertouse.Wherearethetwometerprobesconnected?TheblackleadisalwaysconnectedintothesocketmarkedCOM，shortforCOMMON.TheredleadisconnectedintothesocketlabelledVΩmA.The10Asocketisveryrarelyused.下一页返回上一页Text1UsingaMultimeter3.AnaloguemultimetersAnanaloguemetermovesaneedlealongascale.Switchedrangeanaloguemultimetersareverycheapbutaredifficultforbeginnerstoreadaccurately,especiallyonresistancescales.Themetermovementisdelicateanddroppingthemeterislikelytodamageit!Eachtypeofmeterhasitsadvantages.Usedasavoltmeter,adigitalmeterisusuallybetterbecauseitsresistanceismuchhigher,1MΩor10MΩ，comparedto200kΩforananaloguemultimeteronasimilarrange.Ontheotherhand，itiseasiertofollowaslowlychangingvoltagebywatchingtheneedleonananaloguedisplay.下一页返回上一页Text1UsingaMultimeterUsedasanammeter,ananaloguemultimeterhasaverylowresistanceandisverysensitive，withscalesdownto50p,A.Moreexpensivedigitalmultimeterscanequalorbetterthisperformance.Mostmodernmultimetersaredigitalandtraditionalanaloguetypesaredestinedtobecomeobsolete.4.Makingmeasurements(1)Voltagemeasurements:Buildthecircuitshownbelowusingprototypeboardandfour10kΩresistors(Fig.2-7):Usingthemultimeterasavoltmeter,measurethepowersupplyvoltageandthenmeasurethevoltagesatpointsA，BandC.下一页返回上一页Text1UsingaMultimeterWhatdoyounoticeaboutyourresults'?Thefourresistorsareconnectedinseries，makingachainknownasapotentialdivider,orvoltagedivider.Thetotalvoltageissharedbetweenthefourresistorsand，allowingfortoleranceeachresistorreceivesanequalshare.Modifythecircuit，replacingoneormoreofthe10kΩresistorswith1kΩor100kΩvalues.Aretheresultsasyouexpect?下一页返回上一页Text1UsingaMultimeterFig.2-8showsalightsensorcircuitbuiltinasimilarway.ThecircuitusesanLDR，orlightdependentresistor.TheresistanceoftheLDRchangeswithillumination.Inthedark，theresistanceishigh，upto1MΩormore.WhenlightshinesontheLDR，thelightenergyincreasesthenumberofchargecarriersavailabletotransfercurrent，andtheresistancefalls.Inbrightlight，theresistancecanbeaslittleas100Ω.(2)Resistancemeasurements:RemovetheLDRfromthecircuitandmeasureitsresistance，asfollows(Fig.2-9).下一页返回上一页Text1UsingaMultimeterTogetthemultimetertofunctionasanohmmeter，youwillneedtoselectaresistancerange.Withaswitchedrangemeter，the200kΩpositionisusuallysuitable.Youwillseetheresistancemeasurementchangeasthelightlevelchanges.CoveringtheLDRwithyourhandincreasestheresistanceoftheLDR.Ifthemeterreads“1”thismeansthattheresistanceismorethanthemaximumwhichcanbemeasuredonthisrangeandyoumayneedtoswitchtoanewposition，2000kΩ，totakeareading.Youcancheckthevalueofanyfixedvalueresistorinthesameway，andconfirmthatyouhaveworkedoutthecolourcodecorrectly.下一页返回上一页Text1UsingaMultimeterDon'tforgetthatthecolourcodeconvertorprogramisavailabletohelpyou.(3)Currentmeasurements:Fig.2-10showsaprototypeboardsetupforthemeasurementofcurrent.

Notethatthecurrentmustflowthroughtheammeterinordertoreachthecircuit.Astheresistanceisreduced，currentincreases.Calculatethecurrentexpectedineachcaseusingtheformula:I=V/RSmallvariations，upto±5%，canbeattributedtothetoleranceoftheresistors.下一页返回上一页ReadingDigitalMultimeters(Fig.2-11)Alldigitalmeterscontainabatterytopowerthedisplaysotheyusevirtuallynopowerfromthecircuitundertest.ThismeansthatontheirDCvoltagerangestheyhaveaveryhighresistance(usuallycalledinputimpedance)of1MΩormore，usually10MΩ，andtheyareveryunlikelytoaffectthecircuitundertest.Typicalrangesfordigitalmultimetersareliketheoneillustrated(Thevaluesgivenarethemaximumreadingoneachrange)below.DCVoltage:200mV，2000mV，20V，200V，600V.ACVoltage:200V，600V.下一页返回上一页ReadingDCCurrent:200uA，2000uA，20mA，200mA，10A.The10Arangeisusuallyunfusedandconnectedviaaspecialsocket.ACCurrent:None(Youareunlikelytoneedtomeasurethis).Resistance:200n，2000n，20kΩ，200kΩ，2000kΩ，DiodeTest.AnalogueMultimetersAnaloguemeters(Fig.2-12)takealittlepowerfromthecircuitundertesttooperatetheirpointer.Theymusthaveahighsensitmtyofatleast20kΩ/Vortheymayupsetthecircuitundertestandgiveanincorrectreading.Seethesectionbelowonsensitivityformoredetails.下一页返回上一页ReadingBatteriesinsidethemeterprovidepowerfortheresistanceranges，theywilllastseveralyearsbutyoushouldavoidleavingthemetersettoaresistancerangeincasetheleadstouchaccidentallyandrunthebatteryflat.Typicalrangesforanaloguemultimetersareliketheoneillustratedbelow(Thevoltageandcurrentvaluesgivenarethemaximumreadingoneachrange).DCVoltage:0.5V，2.5V，10V，50V，250V，1000V.ACVoltage:10V，50V，250V，1000V.DCCurrent:50uA，2.5mA，25mA，250mA.下一页返回上一页ReadingAhighcurrentrangeisoftenmissingfromthistypeofmeter.ACCurrent:None(Youareunlikelytoneedtomeasurethis).Resistance:20Ω，200Ω，2kΩ，20kΩ，200kΩ.Theseresistancevaluesareinthemiddleofthescaleforeachrange.ItisagoodideatoleaveananaloguemultimetersettoaDCvoltagerangesuchas10Vwhennotinuse.Itislesslikelytobedamagedbycarelessuseonthisrange，andthereisagoodchancethatitwillbetherangeyouneedtousenextanyway!返回上一页Text2OscilloscopeAnoscilloscope(Fig.2-13)(commonlyabbreviatedtoscopeorO-scope)isatypeofelectronictestequipmentthatallowssignalvoltagestobeviewed，usuallyasatwo-dimensionalgraphofoneormoreelectricalpotentialdifferences(verticalaxis)plottedasafunctionoftimeorofsomeothervoltage(horizontalaxis).Theoscilloscopeisoneofthemostversatileandwidely-usedelectronicinstruments.Oscilloscopesarewidelyusedwhenitisdesiredtoobservetheexactwaveshapeofanelectricalsignal.Inadditiontotheamplitudeofthesignal，anoscilloscopecanmeasurethefrequency，showdistortion，andshowtherelativetimingoftworelatedsignals.下一页返回Text2OscilloscopeOscilloscopesareusedinthesciences，medicine，engineering，telecommunications，andindustry.General-purposeinstrumentsareusedformaintenanceofelectronicequipmentandlaboratorywork.Special-purposeoscilloscopesmaybeusedforsuchpurposesasadjustinganautomotiveignitionsystem，ortodisplaythewaveformoftheheartbeat.Originallyalloscilloscopesusedcathoderaytubesastheirdisplayelement，butmoderndigitaloscilloscopesusehigh-speedanalogue-to-digitalconvertersandcomputer-likedisplayscreenstoprocesssignals.Oscilloscopeperipheralmodulesforgeneralpurposelaptopordesktoppersonalcomputerscanturnthemintousefulandflexibletestinstruments.下一页返回上一页Text2OscilloscopeNavigation1.Inputs2.Thetrace3.Trigger4.Bandwidth5.X-Ymode6.Otherfeatures7.Software1.InputsThesignaltobemeasuredisfedtooneoftheinputconnectors,下一页返回上一页Text2OscilloscopewhichisusuallyacoaxialconnectorsuchasaBNCorNtype.Bindingpostsorbananaplugsmaybeusedforlowerfrequencies.Ifthesignalsourcehasitsowncoaxialconnector,thenasimplecoaxialcableisused;otherwise，aspecialisedcablecalleda“scopeprobe，”suppliedwiththeoscilloscope，isused.General-purposeoscilloscopeshaveastandardizedinputresistanceof1megohminparallelwithacapacitanceofaround20picofarads.Thisallowstheuseofstandardoscilloscopeprobes.Scopesforusewithveryhighfrequenciesmayhave50-ohminputs，whichmustbeeitherconnecteddirectlytoa50-ohmsignalsourceorusedwith

Z0oractiveprobes.下一页返回上一页Text2Oscilloscope2.ThetraceInitssimplestmode，theoscilloscoperepeatedlydrawsahorizontallinecalledthetraceacrossthemiddleofthescreenfromlefttoright.Oneofthecontrols，thetimebasecontrol，setsthespeedatwhichthelineisdrawn，andiscalibratedinsecondsperdivision.Iftheinputvoltagedepartsfromzero，thetraceisdeflectedeitherupwardsordownwards.Anothercontrol，theverticalcontrol，setsthescaleoftheverticaldeflection，andiscalibratedinvoltsperdivision.Theresultingtraceisaplotofvoltageagainsttime，withthemoredistantpastontheleftandthemorerecentpastontheright.下一页返回上一页Text2OscilloscopeIftheinputsignalisperiodic，thenanearlystabletracecanbeobtainedjustbysettingthetimebasetomatchthefrequencyoftheinputsignal.Forexample，iftheinputsignalisa50Hzsinewave，thenitsperiodis20ms，sothetimebaseshouldbeadjustedsothatthetimebetweensuccessivehorizontalsweepsis20ms.Thismodeiscalledcontinualsweep.Sincethecalibratedoscilloscopetimebasemaynotexactlymatchtheperiodoftheinputsignal，thetracewilldriftacrossthescreenmakingmeasurementsdifficult.Ifthetimebaseisadjustedtostabilizethetrace，thetimeperhorizontaldivisionisaltered，andusuallyuncalibrated.下一页返回上一页Text2Oscilloscope3.TriggerToprovideamorestabletrace，modernoscilloscopeshaveafunctioncalledthetrigger.Whenusingtriggering，thescopewillpauseeachtimethesweepreachestheextremerightsideofthescreen.Thescopethenwaitsforaspecifiedeventbeforedrawingthenexttrace.Thetriggereventisusuallytheinputwaveformreachingsomeuser-specifiedthresholdvoltageinthespecifieddirection(goingpositiveorgoingnegative).Theeffectistoresynchronizethetimebasetotheinputsignal，preventinghorizontaldriftofthetrace.下一页返回上一页Text2OscilloscopeInthisway，triggeringallowsthedisplayofperiodicsignalssuchassinewavesandsquarewaves.Triggercircuitsalsoallowthedisplayofnonperiodicsignalssuchassinglepulsesorpulsesthatdon'trecuratafixedrate.4.BandwidthThebandwidthofthescopeislimitedbytheverticalamplifiersandCRT(inanalogueinstruments)orbythesamplingrateoftheanaloguetodigitalconverterindigitalinstruments.Foradigitaloscilloscope，aruleofthumbisthatthecontinuoussamplingrateshouldbetentimesthehighestfrequencydesiredtoresolve.下一页返回上一页Text2Oscilloscope5.X-YmodeMostmodernoscilloscopeshaveseveralinputsforvoltages，andthuscanbeusedtoplotonevaryingvoltageversusanother.ThisisespeciallyusefulforgraphingI-Vcurves(currentversusvoltagecharacteristics)forcomponentssuchasdiodes，aswellasLissajouspatterns.Lissajousfiguresareanexampleofhowanoscilloscopecanbeusedtotrackphasedifferencesbetweenmultipleinputsignals.Thisisveryfrequentlyusedinbroadcastengineeringtoplottheleftandrightstereophonicchannels，toensurethatthestereogeneratoriscalibratedproperly.下一页返回上一页Text2Oscilloscope6.OtherfeaturesSomeoscilloscopeshavecursors，whicharelinesthatcanbemovedaboutthescreentomeasurethetimeintervalbetweentwopoints，orthedifferencebetweentwovoltages.Oscilloscopesmayhavetwoormoreinputchannels，allowingthemtodisplaymorethanoneinputsignalonthescreen.Usuallytheoscilloscopehasaseparatesetofverticalcontrolsforeachchannel，butonlyonetriggeringsystemandtimebase.Betterqualitygeneralpurposeoscilloscopesincludeacalibrationsignalforsettingupthecompensationoftestprobes;thisis(often)a1kHzsquare-wavesignalavailableatatestterminalonthefrontpanel.下一页返回上一页Text2OscilloscopeSometimestheeventthattheuserwantstoseemayonlyhappenoccasionally.Tocatchtheseevents，someoscilloscopes，knownas“storagescopes，”preservethemostrecentsweeponthescreen.ThiswasoriginallyachievedbyusingaspecialCRT，a“storagetube，”whichwouldretaintheimageofevenaverybriefeventforalongtime.Somedigitaloscilloscopescansweepatspeedsasslowasonceperhour,emulatingastripchartrecorder.Thatis，thesignalscrollsacrossthescreenfromrighttoleft.Mostoscilloscopeswiththisfacilityswitchfromasweeptoastrip-chartmodeataboutonesweeppertenseconds.下一页返回上一页Text2OscilloscopeThisisbecauseotherwise，thescopelooksbroken:It'scollectingdata，butthedotcannotbeseen.Oscilloscopeswereoriginallyanaloguedevices.Inmorerecenttimesdigitalsignalsamplingismoreoftenusedforallbutthesimplestmodels.Manyoscilloscopeshavedifferentplug-inmodulesfordifferentpurposes，e.g.，high-sensitivityamplifiersofrelativelynarrowbandwidth，differentialamplifiers，amplifierswith4ormorechannels，samplingpluginsforrepetitivesignalsofveryhighfrequency，andspecial-purposeplugins.下一页返回上一页Reading7.SoftwareManyoscilloscopestodayprovideoneormoreexternalinterfacestoallowremoteinstrumentcontrolbyexternalsoftware.Theseinterfaces(orbuses)includeGPIB，Ethernet，serialport，andUSB.DigitalStorageOscilloscope(Fig.2-14)Thedigitalstorageoscilloscope，orDSOforshort，isnowthepreferredtypeformostindustrialapplications，althoughsimpleanalogueCROsarestillusedbyhobbyists.Itreplacestheunreliablestoragemethodusedinanaloguestoragescopeswithdigitalmemory，whichcanstoredataaslongasrequiredwithoutdegradation.下一页返回上一页ReadingItalsoallowscomplexprocessingofthesignalbyhigh-speeddigitalsignalprocessingcircuits.Theverticalinput，insteadofdrivingtheverticalamplifier,isdigitizedbyananaloguetodigitalconvertertocreateadatasetthatisstoredinthememoryofamicroprocessor.Thedatasetisprocessedandthensenttothedisplay，whichinearlyDSOswasacathoderaytube，butisnowmorelikelytobeanLCDflatpanel.DSOswithcolorLCDdisplaysarecommon.ThedatasetcanbesentoveraLANoraWANforprocessingorarchiving.Thescreenimagecanbedirectlyrecordedonpaperbymeansofanattachedprinterorplotter，withouttheneedforanoscilloscopecamera.下一页返回上一页ReadingThescope'sownsignalanalysissoftwarecanextractmanyusefultime-domainfeatures(e.g.risetime，pulsewidth，amplitude)，frequencyspectra，histogramsandstatistics，persistencemaps，andalargenumberofparametersmeaningfultoengineersinspecializedfieldssuchastelecommunications，diskdriveanalysisandpowerelectronics.Digitaloscilloscopesarelimitedprincipallybytheperformanceoftheanalogueinputcircuitryandthesamplingfrequency.Ingeneral，thesamplingfrequencyshouldbeatleasttheNyquistrate，doublethefrequencyofthehighest-frequencycomponentoftheobservedsignal，otherwiseabasingmayoccur.下一页返回上一页ReadingDigitalstoragealsomakespossibleanotheruniquetypeofoscilloscope，theequivalent-timesamplescope.Insteadoftakingconsecutivesamplesafterthetriggerevent，onlyonesampleistaken.However,theoscilloscopeisabletovaryitstimebasetopreciselytimeitssample，thusbuildingupthepictureofthesignaloverthesubsequentrepeatsofthesignal.Thisrequiresthateitheraclockorrepeatingpatternbeprovided.Thistypeofscopeisfrequentlyusedforveryhighspeedcommunicationbecauseitallowsforaveryhigh“samplerate”andlowamplitudenoisecomparedtotraditionalreal-timescopes.下一页返回上一页ReadingTosumthisup，advantagesovertheanalogueoscilloscopeareasfollows:BrighterandbiggerdisplaywithcolortodistinguishmultipletracesEquivalenttimesamplingandaverageacrossconsecutivesamplesorscansleadtohigherresolutiondowntouVPeakdetectionPre-trigger(Eventsbeforethetriggeroccurscanbedisplayed)Easypanandzoomacrossmultiplestoredtracesallowsbeginnerstoworkwithoutatrigger▲Thisneedsafastreactionofthedisplay(somescopeshave1msdelay)下一页返回上一页Reading▲TheknobshavetobelargeandturnsmoothlyAlsoslowtraceslikethetemperaturevariationacrossadaycanberecordedThememoryoftheoscilloscopecanbearrangednotonlyasaone-dimensionallistbutalsoasatwo-dimensionalarraytosimulateaphosphorusscreen.Thedigitaltechniqueallowsaquantitativeanalysis.Allowsforautomation，thoughmostmodelslocktheaccesstotheirsoftware.Adisadvantageofdigitaloscilloscopesisthelimitedrefreshrateofthescreen.Onananalogueoscilloscope，theusercangetanintuitivesenseofthetriggerratesimplybylookingatthesteadinessoftheCRTtrace.下一页返回上一页ReadingForadigitaloscilloscope，thescreenlooksexactlythesameforanysignalratewhichexceedsthescreen'srefreshrate.Additionally，itissometimeshardtospot“glitches”orotherrarephenomenaontheblack-and-whitescreensofstandarddigitaloscilloscopes;theslightpersistenceofCRTphosphorsonanaloguescopesmakesglitchesvisibleevenifmanysubsequenttriggersoverwritethem.Bothofthesedifficultieshavebeenovercomerecentlyby“digitalphosphoroscilloscopes，”whichstoredataataveryhighrefreshrateanddisplayitwithvariableintensity，tosimulatethetracepersistenceofaCRTscope.返回上一页Text3SignalGeneratorAsignalgenerator，alsoknownvariouslyasatestsignalgenerator,functiongenerator,tonegenerator,arbitrarywaveformgenerator,digitalpatterngeneratororfrequencygeneratorisanelectronicdevicethatgeneratesrepeatingornon-repeatingelectronicsignals(ineithertheanalogueordigitaldomains).Theyaregenerallyusedindesigning，testing，troubleshooting，andrepairingelectronicorelectroacousticdevices.Therearemanydifferenttypesofsignalgenerators，withdifferentpurposesandapplications.Ingeneral，nodeviceissuitableforallpossibleapplications.下一页返回Text3SignalGeneratorNavigation1.Generalpurposesignalgenerators1.1Functiongenerators1.2Arbitrarywaveformgenerators2.SpecialpurposesignalgeneratorsVideosignalgenerators1.Generalpurposesignalgenerators1.1Functiongenerators(Fig.2-15)Afunctiongeneratorisadevicewhichproducessimplerepetitivewaveforms.Suchdevicescontainanelectronicoscillator,acircuitthatiscapableofcreatingarepetitivewaveform.下一页返回上一页Text3SignalGenerator(Moderndevicesmayusedigitalsignalprocessingtosynthesizewaveforms，followedbyadigitaltoanalogueconverter,orDAC，toproduceananalogueoutput).Themostcommonwaveformisasinewave，butsawtooth，step(pulse)，square，andtriangularwaveformoscillatorsarecommonlyavailableasarearbitrarywaveformgenerators(AWGs).Iftheoscillatoroperatesabovetheaudiofrequencyrange(>20kHz)，thegeneratorwilloftenincludesomesortofmodulationfunctionsuchasamplitudemodulation(AM)，frequencymodulation(FM)，orphasemodulation(PM)aswellasasecondoscillatorthatprovidesanaudiofrequencymodulationwaveform.下一页返回上一页Text3SignalGeneratorAfunctiongeneratoraretypicallyusedinsimpleelectronicsrepairanddesignwheretheyareusedtostimulateacircuitundertest.Adevicesuchasanoscilloscopeisthenusedtomeasurethecircuit'soutput.Afunctiongeneratorvaryinthenumberofoutputstheyfeature，suchasfrequencyrange，frequencyaccuracyandstability，andseveralotherparameters.下一页返回上一页Text3SignalGenerator1.2Arbitrarywaveformgenerators(Fig.2-16)Arbitrarywaveformgenerators，orAWGs，aresophisticatedsignalgeneratorswhichallowtheusertogeneratearbitrarywaveforms.Unlikefunctiongenerators，whicharelimitedtoasimplesetofwaveforms，anAWGallowstheusertospecifyasourcewaveforminavarietyofdifferentways.AWGsaregenerallymoreexpensivethanfunctiongenerators，andareoftenmorehighlylimitedinavailablebandwidth;asaresult，theyaregenerallylimitedtohigher-enddesignandtestapplications.下一页返回上一页Text3SignalGenerator2.SpecialpurposesignalgeneratorsInadditiontotheabovegeneral-purposedevices，thereareseveralclassesofsignalgeneratorsdesignedforspecificapplications.Videosignalgenerators(Fig.2-17)Avideosignalgeneratorisadevicewhichoutputspredeterminedvideoand/ortelevisionwaveforms，andothersignalsusedtostimulatefaultsin，oraidinparametricmeasurementsoftelevisionandvideosystems.Thereareseveraldifferenttypesofvideosignalgeneratorsinwidespreaduse.下一页返回上一页Text3SignalGeneratorRegardlessofthespecifictype，theoutputofavideogeneratorwillgenerallycontainsynchronizationsignalsappropriatefortelevision，includinghorizontalandverticalsyncpulses(inanalogue)orsyncwords(indigital).Generatorsofcompositevideosignals(suchasNTSCandPAL)willalsoincludeacolorburstsignalaspartoftheoutput.Videosignalgeneratorsareavailableforawidevarietyofap