通信原理（英文版）.ppt

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1、3.1IntroductionAnalogmodulation:modulationofacarrierbyasourcebasebandanalogsignalCarrier:adeterministicperiodicwaveform－cosinusoidalwhere,A---amplitude0---angularfrequencyofcarrier0---initialphaseDefinition:Modulatingsignalm(t)－signalfromthesourceModulatedsignals(t)－sig

2、nalafterbeingmodulatedModulator－thedeviceformodulationFigure3.1.1ModulatorModulatorModulatedsignals(t)Modulatingsignalm(t)Chapter3Analogmodulationsystem1PurposeofmodulationFrequencyspectrummovement---foraccommodatingtherequirementofchanneltransmissionorforcombiningseveral

3、signalsformulti-channeltransmissionImprovementofanti-jammingabilityClassificationofanalogmodulation:Linearmodulation:AM,SSB,DSB,VSB,…Nonlinearmodulation(angularmodulation):FM,PM2(a)FrequencyspectraldensityofinputsignalS(f)f0f-f00M(f)f0(b)Frequencyspectraldensityofoutputs

4、ignal3.2Linearmodulation3.2.0BasicconceptAssume:thecarrieris:c(t)=Acos0t=Acos2f0tmodulatingsignalisaenergysignalm(t),itsspectrumisM(f)carrier:c(t)multiplicationresult:s(t)filteroutput:s(t)“”isusedtoexpressFouriertransform:where,H(f)s(t)Modulatedsignals(t)Modulatings

5、ignalm(t)Acos0t33.2.1AMBasicprinciplesLet:m(t)=[1+m(t)]，

6、m(t)

7、1，m(t)

8、max=m－modulationindex,thenwehavetheAMsignals(t)=[1+m(t)]Acos0t，where,[1+m(t)]0,i.e.,theenvelopeofs(t)isnon-negative+1==m(t)101+m(t)101+m(t)4FrequencydensityContaindiscretecarriercomponentW

9、henm(t)iscosinusoidal,andm＝100％,sumofthetwosidebandpower＝halfofcarrierpowerFigure3.2.3Waveformandspectrumofmodulatedsignalm(t)s(t)M(f)C(f)c(t)A-Atfm-fmf0-f02fmS(f)2fm-f0f0ffftt5ReceptionofAMsignal:envelopedetectonPrinciple:Characteristics:assumetheinputvoltageiswherei

10、stheinputnoisevoltageofthedetectorTheenvelopeofy(t):ForlargeS/N:RectifierLow-passfilterFigure3.2.4Envelopedetector6Afterdetection(D.C.componenthasbeenfiltered):Signaltonoisepowerratioofoutputsignal:∵signaltonoisepowerratiobeforedetectionequals∴RatioofS/Nbeforeandafterdete

11、ction:Sincem(t)1，obviouslytheaboveratior0/riislessthan1,i.e.,theS/Ndecreasedafterdetection.73.