通信原理(英文版)3.ppt

《通信原理(英文版)3.ppt》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

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

2、gnals(t)－signalafterbeingmodulatedModulator－thedeviceformodulationFigure3.1.1ModulatorModulatorModulatedsignals(t)Modulatingsignalm(t)Chapter3Analogmodulationsystem1PurposeofmodulationFrequencyspectrummovement---foraccommodatingtherequirementofchanneltransmiss

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

4、(b)Frequencyspectraldensityofoutputsignal3.2Linearmodulation3.2.0BasicconceptAssume:thecarrieris:c(t)=Acos0t=Acos2f0tmodulatingsignalisaenergysignalm(t),itsspectrumisM(f)carrier:c(t)multiplicationresult:s(t)filteroutput:s(t)“”isusedtoexpressFouriertransfor

5、m:where,H(f)s(t)Modulatedsignals(t)Modulatingsignalm(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(

9、t)101+m(t)4FrequencydensityContaindiscretecarriercomponentWhenm(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

10、:envelopedetectonPrinciple:Characteristics:assumetheinputvoltageiswhereistheinputnoisevoltageofthedetectorTheenvelopeofy(t):ForlargeS/N:RectifierLow-passfilterFigure3.2.4Envelopedetector6Afterdetection(D.C.componenthasbeenfiltered):Signaltonoisepowerratioofout

11、putsignal:∵signaltonoisepowerratiobeforedetectionequals∴RatioofS/Nbeforeandafterdetection:Sincem(t)1，obviouslytheaboveratior0/riislessthan1,i.e.,theS/Ndecreasedafterdetection.73.