电子通信英语第二章.ppt

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1、Chapter 2 measuring instrument and useUnit9theuseofammeterandvoltmeter Unit10UsingamultimeterUnit11SignalGeneratorUnit12PowerSupplyUnit13UsingAnOscilloscope Unit 9 the use of ammeter and voltmeter AnaloguedisplayAnaloguedisplayshaveapointerwhichmovesoveragraduatedscale.Theycanbedifficulttoreadbecaus

2、eoftheneedtoworkoutthevalueofthesmallestscaledivision.Forexamplethescaleinthepicturehas10smalldivisionsbetween0and1soeachsmalldivisionrepresents0.1.Thereadingistherefore1.25V(thepointerisestimatedtobehalfwaybetween1.2and1.3).下一页返回Unit 9 the use of ammeter and voltmeterThemaximumreadingofananalogueme

3、teriscalledfull-scaledeflectionorFSD.Analoguemetersmustbeconnectedthecorrectwayroundtopreventthembeingdamagedwhenthepointertriestomoveinthewrongdirection.Theyareusefulformonitoringcontinuouslychangingvalues(suchasthevoltageacrossacapacitordischarging)andtheycanbegoodforquickroughreadingsbecausethemo

4、vementofthepointercanbeseenwithoutlookingawayfromthecircuitundertest.上一页 下一页返回Unit 9 the use of ammeter and voltmeterTakingaccuratereadingsTotakeanaccuratereadingfromananaloguescaleyoumusthaveyoureyeinlinewiththepointer.Avoidlookingatananglefromtheleftorrightbecauseyouwillseeareadingwhichisalittleto

5、ohighortoolow.Manyanaloguemetershaveasmallstripofmirroralongthescaletohelpyou.Whenyoureyeisinthecorrectpositionthereflectionofthepointerishiddenbehindthepointeritself.Ifyoucanseethereflectionyouarelookingatanangle.上一页 下一页返回Unit 9 the use of ammeter and voltmeterDigitaldisplayValuescanbereaddirectlyf

6、romdigitaldisplayssotheyareeasytoreadaccurately.Itisnormalfortheleastsignificantdigit(ontheright)tocontinuallychangebetweentwoorthreevalues,thisisafeatureofthewaydigitalmeterswork,notanerror!Normallyyouwillnotneedgreatprecisionandtheleastsignificantdigitcanbeignoredorroundedup.Digitalmetersmaybeconn

7、ectedeitherwayroundwithoutdamage,theywillshowaminussign(-)whenconnectedinreverse.Ifyouexceedthemaximumreadingmostdigitalmetersshowanalmostblankdisplaywithjusta1ontheleft-handside.上一页 下一页返回Unit 9 the use of ammeter and voltmeterAlldigitalmeterscontainabatterytopowerthedisplaysotheyusevirtuallynopower

8、fromthecircuitundertest.Thismeansthatdigitalvoltmetershaveaveryhighresistance(usuallycalledinputimpedance)of1Mormore,usually10M,andtheyareveryunlikelytoaffectthecircuitundertest.Forgeneralusedigitalmetersarethebesttype.Theyareeasytoread,theymaybeconnectedinreverseandtheyareunlikelytoaffectthecircuit

9、undertest.上一页 下一页返回Unit 9 the use of ammeter and voltmeterConnectingmetersItisimportanttoconnectmetersthecorrectwayround:Thepositiveterminalofthemeter,marked+orcolouredredshouldbeconnectednearestto+onthebatteryorpowersupply.Thenegativeterminalofthemeter,marked-orcolouredblackshouldbeconnectednearest

10、to-onthebatteryorpowersupply.上一页 下一页返回Unit 9 the use of ammeter and voltmeterVoltmetersVoltmetersmeasurevoltage.Voltageismeasuredinvolts,V.Voltmetersareconnectedinparallelacrosscomponents.Voltmetershaveaveryhighresistance.MeasuringvoltageatapointWhentestingcircuitsyouoftenneedtofindthevoltagesatvari

11、ouspoints,forexamplethevoltageatpin2ofa555timerchip.Thiscanseemconfusing-whereshouldyouconnectthesecondvoltmeterlead?上一页 下一页返回Unit 9 the use of ammeter and voltmeterConnecttheblack(negative-)voltmeterleadto0V,normallythenegativeterminalofthebatteryorpowersupply.Connectthered(positive+)voltmeterleadt

12、othepointyouwhereyouneedtomeasurethevoltage.Theblackleadcanbeleftpermanentlyconnectedto0Vwhileyouusetheredleadasaprobetomeasurevoltagesatvariouspoints.Youmaywishtouseacrocodileclipontheblackleadtoholditinplace.上一页 下一页返回Unit 9 the use of ammeter and voltmeterVoltageatapointreallymeansthevoltagediffer

13、encebetweenthatpointand0V(zerovolts)whichisnormallythenegativeterminalofthebatteryorpowersupply.Usually0Vwillbelabelledonthecircuitdiagramasareminder.Analoguemeterstakealittlepowerfromthecircuitundertesttooperatetheirpointer.Thismayupsetthecircuitandgiveanincorrectreading.Toavoidthisvoltmetersshould

14、havearesistanceofatleast10timesthecircuitresistance.上一页 下一页返回Unit 9 the use of ammeter and voltmeterMostanaloguevoltmetersusedinschoolsciencearenotsuitableforelectronicsbecausetheirresistanceistoolow,typicallyafewk.100kormoreisrequiredformostelectroniccircuits.AmmetersAmmetersmeasurecurrent.Currenti

15、smeasuredinamps(amperes),A.1Aisquitelarge,somA(milliamps)andA(microamps)areoftenused.1000mA=1A,1000A=1mA,1000000A=1A.Ammetersareconnectedinseries.上一页 下一页返回Unit 9 the use of ammeter and voltmeterToconnectinseriesyoumustbreakthecircuitandputtheammeteracrossthegap.Ammetershaveaverylowresistance.Theneed

16、tobreakthecircuittoconnectinseriesmeansthatammetersaredifficulttouseonsolderedcircuits.Mosttestinginelectronicsisdonewithvoltmeterswhichcanbeeasilyconnectedwithoutdisturbingcircuits.上一页 返回Unit 10 Using a multimeterMultimetersareveryusefultestinstruments.Byoperatingamulti-positionswitchonthemeterthey

17、canbequicklyandeasilysettobeavoltmeter,anammeteroranohmmeter.Theyhaveseveralsettings(calledranges)foreachtypeofmeterandthechoiceofACorDC.Somemultimetershaveadditionalfeaturessuchastransistortestingandrangesformeasuringcapacitanceandfrequency.下一页返回Unit 10 Using a multimeterChoosingamultimeterAdigital

18、multimeteristhebestchoiceforyourfirstmultimeter,eventhecheapestwillbesuitablefortestingsimpleprojects.Ifyouarebuyingananaloguemultimetermakesureithasahighsensitivityof20k/VorgreateronDCvoltageranges,anythinglessisnotsuitableforelectronics.Thesensitivityisnormallymarkedinacornerofthescale,ignorethelo

19、werACvalue(sensitivityonACrangesislessimportant),thehigherDCvalueisthecriticalone.Bewareofcheapanaloguemultimeterssoldforelectricalworkoncarsbecausetheirsensitivityislikelytobetoolow.上一页 下一页返回Unit 10 Using a multimeterMeasuringvoltageandcurrentwithamultimeter1.Selectarangewithamaximumgreaterthanyoue

20、xpectthereadingtobe.2.Connectthemeter,makingsuretheleadsarethecorrectwayround.3.Digitalmeterscanbesafelyconnectedinreverse,butananaloguemetermaybedamaged.4.Ifthereadinggoesoffthescale:immediatelydisconnectandselectahigherrange.Multimetersareeasilydamagedbycarelessuse，sopleasetaketheseprecautions:上一页

21、 下一页返回Unit 10 Using a multimeterAlwaysdisconnectthemultimeterbeforeadjustingtherangeswitch.Alwayscheckthesettingoftherangeswitchbeforeyouconnecttoacircuit.Neverleaveamultimetersettoacurrentrange(exceptwhenactuallytakingareading).Thegreatestriskofdamageisonthecurrentrangesbecausethemeterhasalowresist

22、ance.上一页 下一页返回Unit 10 Using a multimeterReadinganaloguescalesCheckthesettingoftherangeswitchandchooseanappropriatescale.Forsomerangesyoumayneedtomultiplyordivideby10or100asshowninthesamplereadingsbelow.ForACvoltagerangesusetheredmarkingsbecausethecalibrationofthescaleisslightlydifferent.上一页 下一页返回Uni

23、t 10 Using a multimeterSamplereadingsonthescalesshown:(Fig.2.10)DC10Vrange:4.4V(read0-10scaledirectly)DC50Vrange:22V(read0-50scaledirectly)DC25mArange:11mA(read0-250anddivideby10)AC10Vrange:4.45V(usetheredscale,reading0-10)MeasuringresistancewithamultimeterTomeasuretheresistanceofacomponentitmustnot

24、beconnectedinacircuit.Ifyoutrytomeasureresistanceofcomponentsinacircuityouwillobtainfalsereadings(evenifthesupplyisdisconnected)andyoumaydamagethemultimeter.上一页 下一页返回Unit 10 Using a multimeterReadinganalogueresistancescalesForresistanceusetheupperscale,notingthatitreadsbackwardsandisnotlinear(evenly

25、spaced).Checkthesettingoftherangeswitchsothatyouknowbyhowmuchtomultiplythereading.Samplereadingsonthescalesshown:(Fig.2.11)10range:2601krange:26k 上一页返回Unit 11 Signal GeneratorIntroductionToSignalGeneratorThegenerationofsignalsisanimportantdevelopmentinthetroubleshootinganddevelopmentofelectronicdesi

26、gn.Thegeneratorisusedtoprovideknowntestconditionsforthetroubleshootingofelectronicsystemdesignandforreplacingsignalsthataremissinginsystemsduringrepairingwork.Mostcommonlytheygeneratesinewaves.Therearemanytypesofgeneratorsbuttheyallhavethefollowingcommoncharacteristics.下一页返回Unit 11 Signal Generatora

27、)Thefrequencyofthesignalmustbeknownandstable.b)Thesignalmustbefreeofdistortion.c)Theamplitudemustbecontrollablefromsmalltorelativelylargevalues.Someexamplesofthesegeneratorsarefunctiongenerator,pulseandsweepgenerator,sinewavegenerator,pulseandsquarewavegeneratorandaudiofrequencysignalgenerator.上一页 下

28、一页返回Unit 11 Signal GeneratorAsimplesinewavegeneratorconsistsofanoscillatorandanattenuator.Thefrequencystabilityandfreedomfromdistortiondependsonthedesignoftheoscillatorwhereastheamplitudeaccuracydependsonthedesignoftheattenuator.Functiongeneratorbasicallyproducessquarewaves,sawtoothsandtriangularwav

29、esbesidessinewave.Forabeginnertoelectronicsdesignwhodesignsimpletomediumcomplexityofcircuitry,itisgenerallyagoodideatohaveabasicfunctiongeneratorasastart.上一页 下一页返回Unit 11 Signal GeneratorAudiosignalgeneratorsproducesignalsinarangefromafewHertzuptoseveralKilohertz.Signalscanbeinjectedintoaudioamplifi

30、erstoseehowtheybehaveatvariousaudiofrequencies.Amplificationandfrequencyresponsecanbemeasuredanddistortionofthesignalcanbeobserved.Radiofrequencygeneratorscanprovidefrequenciesfromabout100KilohertzuptoseveralhundredMegahertz.WithradiofrequencygeneratorsitisusuallypossibletomodulatetheR.F.withanaudio

31、signaltosimulatearadiostation.Amplitudeandfrequencymodulationareavailable.UsinganR.F.generatorthevarioustunedcircuitsinaradiocanbeadjustedforpeakperformance.上一页 下一页返回Unit 11 Signal GeneratorFunctionGeneratorSpecificationsAtypicalfunctiongeneratorspecificationfromaTektronixmodelisaslistedbelow.a)Buil

32、t-InHigh-ResolutionMonitorWithGraphicalUserInterfaceb)12-Bit(1/4096)VerticalResolutionc)250MHz,1ppmAccuracy,0.5Hz(10digits)ResolutionClock 上一页 下一页返回Unit 11 Signal Generatord)16SeparateArbitraryWaveformsWith1K(1024points)RecordLengthse)StandardWaveforms:Sinewave-0.5Hzto100MHzSquarewave-0.5Hzto50MHzTr

33、iangle,ramp,andpulse-0.5Hzto31.2MHzf)InternalAM,FM,PSK,andFSKModulationg)ExternalAMModulationh)Built-InLinear/LogSweepi)Built-InTriggerDelay(700nsto100s)上一页 返回Unit 12 Power SupplyTypesofPowerSupplyTherearemanytypesofpowersupply.MostaredesignedtoconverthighvoltageACmainselectricitytoasuitablelowvolta

34、gesupplyforelectronicscircuitsandotherdevices.Apowersupplycanbybrokendownintoaseriesofblocks,eachofwhichperformsaparticularfunction.Forexamplea5Vregulatedsupply:下一页返回Unit 12 Power SupplyEachoftheblocksisdescribedinmoredetailbelow:Transformer-stepsdownhighvoltageACmainstolowvoltageAC.Rectifier-conver

35、tsACtoDC,buttheDCoutputisvarying.Smoothing-smoothestheDCfromvaryinggreatlytoasmallripple.Regulator-eliminatesripplebysettingDCoutputtoafixedvoltage.上一页 下一页返回Unit 12 Power SupplyDualSuppliesSomeelectroniccircuitsrequireapowersupplywithpositiveandnegativeoutputsaswellaszerovolts(0V).Thisiscalledaduals

36、upplybecauseitisliketwoordinarysuppliesconnectedtogether.Dualsupplieshavethreeoutputs,forexamplea9Vsupplyhas+9V,0Vand-9Voutputs.上一页 下一页返回Unit 12 Power SupplyRectifierThereareseveralwaysofconnectingdiodestomakearectifiertoconvertACtoDC.Thebridgerectifieristhemostimportantanditproducesfull-wavevarying

37、DC.Asinglediodecanbeusedasarectifierbutitonlyusesthepositive(+)partsoftheACwavetoproducehalf-wavevaryingDC.上一页 下一页返回Unit 12 Power SupplySmoothingSmoothingisperformedbyalargevalueelectrolyticcapacitorconnectedacrosstheDCsupplytoactasareservoir,supplyingcurrenttotheoutputwhenthevaryingDCvoltagefromthe

38、rectifierisfalling.ThediagramshowstheunsmoothedvaryingDC(dottedline)andthesmoothedDC(solidline).ThecapacitorchargesquicklynearthepeakofthevaryingDC,andthendischargesasitsuppliescurrenttotheoutput.上一页 下一页返回Unit 12 Power SupplyNotethatsmoothingsignificantlyincreasestheaverageDCvoltagetoalmostthepeakva

39、lue.Smoothingisnotperfectduetothecapacitorvoltagefallingalittleasitdischarges,givingasmallripplevoltage.上一页 下一页返回Unit 12 Power SupplyRegulatorVoltageregulatorICsareavailablewithfixed(typically5,12and15V)orvariableoutputvoltages.Theyarealsoratedbythemaximumcurrenttheycanpass.Negativevoltageregulators

40、areavailable,mainlyforuseindualsupplies.Mostregulatorsincludesomeautomaticprotectionfromexcessivecurrent(overloadprotection)andoverheating(thermalprotection).上一页返回Unit 13 Using An Oscilloscope Whatisanoscilloscopeusedfor?Measuringtime-varyingsignals-byshowingdetailsofthewaveshape.Measuringaspectsoft

41、ime-varyingsignals:FrequencyofasignalPeakvalueofasignal 下一页返回Unit 13 Using An OscilloscopeTheoscilloscopeisthemostpowerfulinstrumentinourarsenalofelectronicinstruments.Itiswidelyusedformeasurementoftime-varyingsignals.Anytimeyouhaveasignalthatvarieswithtime-slowlyorquickly-youcanuseanoscilloscopetom

42、easureit-tolookatit,andtofindanyunexpectedfeaturesinit.Thefeaturesyouseeinasignalwhenyouuseanoscilloscopetolookatasignalarefeaturesyoucannotseeotherwise.Inthislessonyouwilllearnaboutoscilloscopesandyoushouldkeepthisgoalinmindasyouproceedthroughthelesson.上一页 下一页返回Unit 13 Using An OscilloscopeGivenati

43、mevaryingsignalthatyouneedinformationabout,Beabletouseanoscilloscopetoportraythesignalasafunctionoftime.Beabletomeasuresignalparameterswithanoscilloscope.Whatdoesanoscilloscopelooklike?HeresaphotoofaHewlett-Packard(HP)54601A(Fig.2.16)上一页 下一页返回Unit 13 Using An OscilloscopeNotethefollowingfeaturesofth

44、eoscilloscope.ThereisaCRT(CathodeRayTube)screenonwhichthesignalswillbepresented.Thatsattheleft.Therearenumerouscontrolstocontrolthingslike:ThetimescaleofthepresentationAverticalscaleAcable(IEEE-488)toconnecttheoscilloscopetoacomputer.Thatletsyou:Takemeasurementswiththescope 上一页 下一页返回Unit 13 Using An

45、 OscilloscopePutthemeasurementsinacomputerfileAnalysethedatawithMathcad,Matlab,Excel,etc.Noticethatthisoscilloscopehastwoinputchannels.Thecontrolsforthetwochannelsarejusttotherightofthescreen.Howdoyouuseanoscilloscope?Plugitin.Thatsnotfacetious.Turniton.Thereisapushbuttonatthelowerrightedgeofthescre

46、en.ItsaysLineandindicatesa0anda1setting.Depressthatbutton.Applyasignaltotheinputterminals.上一页 下一页返回Unit 13 Using An OscilloscopeYouroscilloscopemayhaveprovisionformorethanonesignalinput.ChooseChannel1ifthatisthecase.Makesurethatthesettingsmatchthesignal.Forexample:lIfyouhaveasignalat1000Hz,thenthepe

47、riodofthesignalis1millisecond(.001sec)andyouwouldnotwantthetimescalesetsothatyouonlydisplayamicrosecondofdata,andyoualsoprobablywontseemuchifyoudisplay10secondsworthofdata.lIfyouhaveasignalthatis10millivoltshigh,youwontseemuchifyousettheoscilloscopetoshownyouasignalat20voltsfull-scale.Conversely,you

48、wontseemuchofa20voltsignalifthescopeissetfor10millivoltsfull-scale.上一页 下一页返回Unit 13 Using An OscilloscopeShowingaSimpleSignalontheScopeTogetfamiliarwiththescope,youcanshowasinesignalonthescope.Weregoingtoaskthatyoushowasignalwiththefollowingcharacteristics.2vpeak-to-peaksignal.Inotherwords,ithasapea

49、kof1voltandanegativepeakat-1volt.Afrequencyof1000Hz(i.e.1KHz).Asinusoidalsignal.Inotherwords,itlookslikeafamiliarsinewave.上一页 下一页返回Unit 13 Using An OscilloscopeWhatwillthesignallooklike?Ifyouhaveasinusoidalsignalthatrepeatseveryhalfmillisecond-afrequencyof2kHz-youwouldgetapicturelikethisone.Itwoulda

50、ppeartobestationaryontheoscilloscopescreen,butitreallyisnt.Itsjustthatitrepeatssofrequentlythatyouseeitasaconstantimage.上一页 下一页返回Unit 13 Using An OscilloscopeMeasuringASimpleSignalHerestheconnectionsforasimplesignalmeasurement.Theoscilloscopeisbeingusedtodisplaythevoltageoutputfromasignalgenerator.Y