電子信息與通信工程專業(yè)英語lesson2-3

2.3RadioReceiverCircuitRF,theradiofrequencyportionoftheradiospectrumisconsideredtoextendfrom3to1,000MHz(1GHz).Attheoppositeendofthespectrum,belowthehighfrequency(HF)band(3-30MHz),theAMradiobandextendsfrom650to1,650kHz.Attheserelativelylowfrequenciestheeffectsofparasiticinductanceandcapacitanceoncircuitdesignsareminimal.OutlineThesuperheterodynereceiver-analogsystem

ReceivercharacterizationTerminologyanalogelectronics

模擬電子技術(shù)digitalsignalprocessing（DSP）

數(shù)字信號處理superheterodyne（superhet）

[電子]超外差式收音機；adj.超外差的selectivityn.選擇性sensitivityn.敏感,靈敏(度)，靈敏性preselectorn.預(yù)選器，調(diào)諧器transmittern.轉(zhuǎn)送者,傳達(dá)人發(fā)報機,話筒,發(fā)射機,讓渡者thereceivephase

接收相位harmonicsn.諧波impedancematch

阻抗匹配undesiredsignal干擾信號thelocaloscillator本機震蕩器intermediatefrequency（IF）中頻modulationsidebands調(diào)制邊帶overloadcharacteristics

過載特性demodulatorn.解調(diào)器sophisticatederror

經(jīng)驗誤差additive,white,Gaussiannoisen.加性高斯白噪聲powerspectraldensity

功率譜密度megahertzn.[物]兆赫(MHz)兆赫AGCabbr.自動增益控制Spuriousresponses

(接收機)亂真信號響應(yīng)Thesuperheterodyne,orsuperhet,receiver,isawell-knownandalmostuniversalreceiverarchitectureforradioreceivers.Wewillseethatsuchareceivercanprovidebothgoodselectivityandsensitivity,becausethenoisebandwidthcanbelimitedtothechannelbandwidthwithoutcompromisingthereceiver'sabilitytotuneacrosstheentireRFband.ItsbasiccomponentsareshowninFig.2-3-1.IntroductionTexttour1.Thesuperheterodynereceiver-analogsystem

ThepurposeofthiscomponentistofilteroutallunwantedsignalslyingoutsidetheRFbandcontainingthepossiblechannelstobedetected.Unwantedsignalscanincludesignalsfedfromthetransmitteritself,whichmightshareacommonantenna.Inthiscasethepreselectoriseitheradiplexer,afilterthatallowsonefrequencybandtopassbetweentheantennaandreceiverandanother(different)bandtopassbetweenthetransmitterandantenna(as,forexample,inCDMAmobilesystems),and/oratransmit/receiveswitchthatturnsonduringthereceivephaseandoffduringthetransmitphase(as,forexample,inGSMsystems).RFpreselectorThefunctionofthepreselectionfilterisalsotosuppresstheundesiredresponsesattheoutputthatarisefromincomingsignalslyingatspuriousfrequenciesthatthereceiverisnottunedtoreceive.Wewillseethatsuchspuriousfrequenciescanincludetheimagefrequencyaswellasharmonicsoftheincomingtunedfrequency.Ideally,thepreselectorwillalsohaveagoodimpedancematchin-bandtoavoidbandpassripple.

RFamplifierThefunctionoftheinputamplifieristolinearlyamplifytheinputsignalandminimizethenoiseaddedbythereceivertothesignalitself.Itshouldhaveagoodinputandoutputmatchtoavoidgainripple.Inaddition,theinputamplifiermustnotintroducedistortionofthesignal,becausestrongsignalsmaybesimultaneouslypresentinadjacent(unwanted)channels,andanynonlineardistortionoftheamplifiercouldswampaweakersignalinthechannelwearetryingtodetect.Ofcourse,theRFamplifiercanconsistofmultiplestagesinordertoprovidethenecessarygain.InterstageselectorTheprecedingamplifierwillprovidegaintoallthechannelswithintheRFbandwidth,anditsgainislikelytorolloffslowlybeyondit.Furthermore,theamplifierwillamplifynoiseacrosstheentireband,andpossiblyattheimagefrequencyaswell.Therefore,this(optional)componentisafiltertosuppressanygainofundesiredsignalresponsesatspuriousfrequencies,andinparticularattheimagefrequency.Itthusmaintainsthesystemnoisefigurebypreventingimagenoisefromenteringthemixer.ItalsohelpstominimizeLOreradiationfromtheRFmixerport.Thiscomponentshouldhavelowin-bandloss.Thisisastrongsignalthatisnormallygeneratedbyafrequencysynthesizer,andistypicallytunedacrossabandwidthequaltotheentireRFbandwidth,butoffsetfromit,tochooseanydesiredchannel.Itsfunctionistodrivethedeviceswithinthemixerintoanonlinearregimeforfrequencytranslation(mixing).Animportantoscillatorspecificationisitsphasenoise,sinceanyphasefluctuationontheoscillatorsignalisdirectlysuperimposedonthemixeroutputsignal.Itsbroadbandnoiseshouldalsobelowsoasnottoraisethesystemnoisefloor.Itwillalsorequireagoodtuningrangeorbandwidth,andlowspuriousandharmoniccontent.

LocaloscillatorFirstmixerThiscomponenttranslatesallincomingsignalsintheRFfrequencyrangeintosignalsinsomeintermediatefrequencyrange,dependingonthelocaloscillatorsignalfrequency.Themixertranslatesallfrequencieslinearly,preservingphaseinformationwithinthenewrangeoffrequencies.WithinsomerangeofRFsignalamplitudes,theamplitudeoftheoutputsignalisalsopreservedontheIF.ItrequireslowLOfeedthroughtotheRFandIF,andalargespurious-freedynamicrangefortheincomingRFsignal.TheselectionoftheIFfrequencyisimportantinensuringthereceiverresponsetounwantedspuriousresponsesisminimized.IFfrequenciesfrom45to82MHzarecommonformobileradioreceiversinthe800-MHzband,andfrom110to300MHzforradiosinthe1,800-MHzbandorforbothbands.IFfilter

Thiscomponentrejectstheunwantedsignalcomponentsgeneratedbythemixerandothercomponents.Itsbandwidthmustbesufficientlywidetopassthemodulationsidebandsinthedesiredchannelwithoutdistortion.ItcanbehighQbecauseitisoffixedfrequency.Theblockingandoverloadcharacteristicsofthereceiverareoftendeterminedhereincombinationwiththefollowing.IFamplifier

ThiscomponentshouldprovideadequategaintotheIFsignaltodrivethefollowingstages.Becauseitisatafixedfrequencyitcanprovidehighgainandbewellstabilized.Secondmixer,localoscillator,andsecondfilter

SometimesknownasthesecondIFstrip,thesecomponentsmirrorthefunctionsofthefirstIFstrip,butacrossadifferentfrequencybandandwithdifferentpassbandcharacteristics.Thepowerhandlingcapability(interceptpoint)isgenerallyhigher.ThepurposeofasecondIFstripwillunfoldlaterinthischapter,butasageneralrule,thefirstIFstripisdesignedtoreceivetheentireRFpassbandandtorejectspuriousfrequenciesandtheimagefrequencyinparticular,whilethefunctionofthesecondIFstripistonarrowinandselectthedesiredchannelfromtheentirepassband,therebyprovidingadditionalselectivitytothereceiver.ForcommercialAMradios,455kHzisafairlystandardsecondIFfrequency,whilemobilephonereceiverstypicallyusefrequenciesaround10MHz.Demodulator

ThiscomponentextractsthemodulatedsignalfromtheIFsignalandconvertsittobaseband.Inthecaseofananalogsystem,thisinformationwillbeeitherAMorFM;foradigitalsystem,itwilltypicallybesymbolshavingmultiamplitudelevelsthatarelaterdecoded.(Asymbolisthewayabitorcombinationsofbitsarecodedinthewaveform.)

Basebandamplifier

Thisprovidesoutputpowertoderivedrivetherelevantoutputdevice,whichcouldtypicallybeaspeaker,faxoutput,orvideoscreen.2.ReceivercharacterizationCommunicationschannelAresultthatcanbederivedfromtheaboveisthatforagivenchannelwithsomemaximuminformationthroughput,thepower(ormorespecificallv,thesignal-to-noiseratioatthedetector)neededtotransmitthatinformationthroughitcanbetradedoffforspectraloccupancyorbandwidthaccordingtoThusifthespectraloccupancycanbeincreased,thesignal-to-noiseratioSNRcanbelowerforthesameinformationtransferrate.ReceivernoiseAlthoughwesometimesrefertonoisevoltageandnoisecurrentperhertz,inRFsystemswetypicallymeasurepowerintoafixedreferenceimpedancelevel(usually).Inthiscase,thefundamentalresultisnoisepowerwhereKisBoltzmann'sconstantandTisthetemperatureinKelvin.Clearly,thelargerthebandwidth,thegreaterthenoisepower.ItisforthisreasonthatthefinalIFfilterneedstobeasnarrowaspossible,inordertominimizethenoisepowerjustpriortodemodulationanddetection.ThisfinalIFfilterdeterminestheoverallnoisebandwidthoftheentirereceiversinceitwillbethemostnarrowbandcomponentintheentirechainpriortodetection.ReceiversensitivityThisminimuminputsignalstrengthneededtoproduceagoodqualityoutputsignalisreferredtoasthereceiversensitivity.However,justastherearemanydefinitionsofwhatconstitutesacceptablequality,thereareequallynumerousdefinitionsofsensitivity.Mostaudiophilesbelievethatasignal-to-noiseratioofatleast10dBisnecessaryforacceptablesoundquality,sothatanoutputsignal-to-noiseratioof10dBisusedtomeasurethesystemsensitivity.SystemnonlinearityThecomponentsinanysystemultimatelyreachapointatwhichtheybecomenonlinear，essentiallyacomponentisnonlinearwhenitsoutputamplitudeorphaseisnolongerlinearlyproportionaltoitsinputamplitudeorphase.Inmostchangesactivedevices,thisresultspredominantlyfromamplitude-inducedinthedevicetransconductance,butthevariationofdeviceresistancesandcapacitanceswithvoltageamplitudealsocontributestononlinearbehavior.Grossnonlinearityarisesfromcutoffandsaturationeffectsthatoccurasthedeviceexceedsthelimitsofitsnormalactivemomentarily.Asaresult,weobserveeffectssuchasgaindistortionandcrossmodulationsuchasAM-to-PMconversion.ReceiverdynamicrangeInordertoincreasetheusefulrangeofareceiver,thegainofasystemcanbeautomaticallycontrolledinordertodecreasethegainwhenstrongsignalscauseoverloadordistortion.However,theimpactofdoingthisisnotalwaysobviousandwillrequiresomecareinconsideringthesignal-to-noiseratioandmakinganumberoftrade-offs.Forinstance,considerAGCappliedtoanLNA.AGCistypicallyappliedinananalogreceiver.Asthedesiredsignalincreasesininputpower,thegainofthesystemisreducedinordertominimizedistortion.Thegainoftheoutput