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1、AnalogmodulationsystemresearchMostofthetobetransmittedsignalhasalowfrequencycomponent.Calledabasebandsignal,ifthedirecttransmissionofthebasebandsignal,calledabase-bandtransmission,butitisamulti-channelunsuitableforthetransmissionofthebasebandsignal,orifthebaseban
2、dsignalinwhichthetransmissionwillproducealargeattenuationanddistortion.Therefore,itisnecessarytocarryoutthebasebandsignalmodulationtransformformasappropriateforchanneltransmission,modulationisthebasebandsignalm(t)togotothecontrolcarrierofa(orsome)parameters,isthe
3、signalm(t)inaccordancewiththelawsoftheparameterprocessofchange.Thecarriermaybeasinewave,canalsobeasequenceofpulses,asacarriermodulatedsinusoidalsignaltosaidcontinuouswave(CW)modulation.Forcontinuouswavemodulation,themodulatedsignalcanbeexpressedasSm(t)=A(t)cos[ωt
4、+θ(t)]ItisdeterminedbytheamplitudeA(t),theangularfrequencywandthephaseθ(t)3parametersconstitutingthecontrary.Toachievemodulationcontrolofanyofthethreeparametersacity,makingittheinformation-carryingsignal.Continuouswavemodulationintoanamplitudemodulation,frequency
5、modulationandphasemodulation.Frequencyadjustmentandphasemodulationofthecarrierphaseanglechanges,therefore,theyarecollectivelyreferredtoastheangleadjustment.Modulationhasaveryimportantroleinthecommunicationsystem,bymodulatingthemessagesignalspectrummove,sothatthem
6、odulatedsignalsuitablefortransmissionchannelrequirements,butalsoconducivetothechannelmultiplexing.Forexample,multi-basebandsignalmodulatedontodifferentcarrierfrequenciesonaparalleltransmissionchannelfrequencydivisionmultiplexing.Modulationmethodoftenhasagreatinfl
7、uenceontheperformanceofcommunicationsystems.Ifm(t)isacontinuoussignal,andsothatoneoftheparameterscontinuouslyandm(t)corresponding,knownastheanalogmodulation.ClassificationAmplitudemodulationAmplitudemodulationisamodulationsignaltocontroltheamplitudeofthehighfrequ
8、encycarriersothatthelawofvariationoftheprocessaccordingtoamodulationsignalisoftendividedintoastandardAmplitudeModulation(AM),suppressedcarrierdoublesidebandmod