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1、chapter3AmplitudeModulationFundamentalsInthemodulationprocess,thebasebandvoice,video,ordigitalsignalmodifiesanother,higher-frequencysignalcalledthecarrier,whichisusuallyasinewave.CarrierAsinewavecarriercanbemodifiedbytheintelligencesignalthroughampli-tudemodulation
2、,frequencymodulation,orphasemodulation.Thefocusofthischapterisamplitudemodulation(AM).Amplitudemodulation(AM)ObjectivesAftercompletingthischapter,youwillbeableto:■CalculatethemodulationindexandpercentageofmodulationofanAMsignal,giventheamplitudesofthecarrierandmodu
3、latingsignals.■Defineovermodulationandexplainhowtoalleviateitseffects.■ExplainhowthepowerinanAMsignalisdistributedbetweenthecarrierandthesideband,andthencomputethecarrierandsidebandpowers,giventhepercentageofmodulation.■Computesidebandfrequencies,givencarrierandmod
4、ulatingsignalfrequencies.■Comparetime-domain,frequency-domain,andphasorrepresentationsofanAMsignal.■ExplainwhatismeantbythetermsDSBandSSBandstatethemainadvantagesofanSSBsignaloveraconventionalAMsignal.■Calculatepeakenvelopepower(PEP),givensignalvoltagesandloadimped
5、ances.933-1AMConceptsAsthenamesuggests,inAM,theinformationsignalvariestheamplitudeofthecarriersinewave.Theinstantaneousvalueofthecarrieramplitudechangesinaccordancewiththeamplitudeandfrequencyvariationsofthemodulatingsignal.Figure3-1showsasingle-frequencysinewavein
6、telligencesignalmodulatingahigher-frequencycarrier.Thecarrierfrequencyremainsconstantduringthemodulationprocess,butitsamplitudevariesinaccordancewiththemodulatingsignal.Anincreaseintheamplitudeofthemodulatingsignalcausestheamplitudeofthecarriertoincrease.Boththepos
7、itiveandthenegativepeaksofthecarrierwavevarywiththemodulatingsignal.Anincreaseoradecreaseintheamplitudeofthemodulatingsignalcausesacorrespondingincreaseordecreaseinboththepositiveandthenegativepeaksofthecarrieramplitude.Animaginarylineconnectingthepositivepeaksandn
8、egativepeaksofthecarrierwaveform(thedashedlineinFig.3-1)givestheexactshapeofthemodulatinginformationsignal.Thisimaginarylineonthecarrierwaveformi
