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1、AnalogmodulationsystemresearchMostofthetobetransmittedsignalhasalowfrequencycomponent.Calledabasebandsignal,ifthedirecttransmissionofthebasebandsignal,calledabase-bandtransmission,butitisamulti-channelunsuitableforthetransmissionofthebasebandsignal,orifthebasebandsignalinwhichth
2、etransmissionwillproducealargeattenuationanddistortion.Therefore,itisnecessarytocarryoutthebasebandsignalmodulationtransformformasappropriateforchanneltransmission,modulationisthebasebandsignalm(t)togotothecontrolcarrierofa(orsome)parameters,isthesignalm(t)inaccordancewiththelaw
3、softheparameterprocessofchange.Thecarriermaybeasinewave,canalsobeasequenceofpulses,asacarriermodulatedsinusoidalsignaltosaidcontinuouswave(CW)modulation.Forcontinuouswavemodulation,themodulatedsignalcanbeexpressedasSm(t)=A(t)cos[ωt+θ(t)]ItisdeterminedbytheamplitudeA(t),theangula
4、rfrequencywandthephaseθ(t)3parametersconstitutingthecontrary.Toachievemodulationcontrolofanyofthethreeparametersacity,makingittheinformation-carryingsignal.Continuouswavemodulationintoanamplitudemodulation,frequencymodulationandphasemodulation.Frequencyadjustmentandphasemodulati
5、onofthecarrierphaseanglechanges,therefore,theyarecollectivelyreferredtoastheangleadjustment.Modulationhasaveryimportantroleinthecommunicationsystem,bymodulatingthemessagesignalspectrummove,sothatthemodulatedsignalsuitablefortransmissionchannelrequirements,butalsoconducivetothech
6、annelmultiplexing.Forexample,multi-basebandsignalmodulatedontodifferentcarrierfrequenciesonaparalleltransmissionchannelfrequencydivisionmultiplexing.Modulationmethodoftenhasagreatinfluenceontheperformanceofcommunicationsystems.Ifm(t)isacontinuoussignal,andsothatoneoftheparameter
7、scontinuouslyandm(t)corresponding,knownastheanalogmodulation.ClassificationAmplitudemodulationAmplitudemodulationisamodulationsignaltocontroltheamplitudeofthehighfrequencycarriersothatthelawofvariationoftheprocessaccordingtoamodulationsignalisoftendividedintoastandardAmplitudeMo
8、dulation(AM),suppressedcarrierdoublesidebandmod
