电子信息与通信工程专业英语lesson2-3

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

ReceivercharacterizationTerminologyanalogelectronics

模拟电子技术digitalsignalprocessing（DSP）

数字信号处理superheterodyne（superhet）

[电子]超外差式收音机；adj.超外差的selectivityn.选择性sensitivityn.敏感,灵敏(度)，灵敏性preselectorn.预选器，调谐器transmittern.转送者,传达人发报机,话筒,发射机,让渡者thereceivephase

接收相位harmonicsn.谐波impedancematch

阻抗匹配undesiredsignal干扰信号thelocaloscillator本机震荡器intermediatefrequency（IF）中频modulationsidebands调制边带overloadcharacteristics

过载特性demodulatorn.解调器sophisticatederror

经验误差additive,white,Gaussiannoisen.加性高斯白噪声powerspectraldensity

功率谱密度megahertzn.[物]兆赫(MHz)兆赫AGCabbr.自动增益控制Spuriousresponses

(接收机)乱真信号响应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