电子专业英语教学课件作者高明辉第13章.ppt

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1、Warming-upI.Canyoupointoutthenameofthefollowingpicture?II.Doyouknowwhatthefollowingabbreviationsorsymbolsstandfor?AMFMPMIII.TalkaboutModulationinEnglish.Andthefollowingpatternsyoucanuse:Whatisthefirstwave?Whatisthesecondwave?CouldyoutellussomethingaboutAM,FMandPM?Unit13M

2、odulation下一页返回TextComprehensionModulationInordertosendamessagebymeansofradiowaves,itisnecessarytovarysomeparameterofthesewavesaccordingtotheinformationtobetransmitted.Thiscanbeobtainedbymeansofmodulation.Modulationistheprocesswherebyoneormorecharacteristicsofawavearevari

3、edwithafunctionoftheinstantaneousvalueofanotherwave.Thefirstwave,usuallyasinglefrequencywaveofconstantamplitude,isknownasthecarrierwaveandthesecondasthemodulatingwave.Theprocessofextractingthesingleinformationfromamodulatedcarrierwaveiscalleddetectionordemodulation.Unit1

4、3Modulation上一页下一页返回Initially,amplitudemodulationwasusedexclusivelyforcommunicationsandbroadcasting.Later,withthewidespreadadoptionofhigherradiofrequencies(3000kilohertz—300megahertz)forcommunicationsandbroadcasting,frequencyandphasemodulationtechniquescameintoincreasingu

5、se.TheseareoftenreferredtoasFMandPMrespectively.Inamplitudemodulation,theinstantaneousamplitudeofthecarrierisvariedinproportiontothemodulatingsignal.Infrequencymodulation,theinstantaneousfrequencyofthecarriervariedinaccordancewiththesignaltobemodulatedonthewave.Whilethea

6、mplitudeofthecarrierfrequencywaveisconstant.Inphasemodulation,thevalueofthephaseangleofthesine—wavecarrierisvariedsothatitsmagnitudeisproportionaltotheinstantaneousamplitudeofthemodulatingsignal.Itmustbenotedthatthereisaninstantaneousfrequencychangeordeviationduringtheti

7、methatthephaseisshiftedwhenthephaseofthecurrentinacircuitischanged.Unit13Modulation上一页下一页返回Thefrequencydeviationinphasemodulationisproportionaltotheamplitudeofthemodulatingsignalanditsfrequency.Whileinfrequencymodulation,thefrequencydeviationisproportionalonlytotheamplit

8、udeofthemodulatingsignal.Inotherwords,thephasemodulationismerelyawayofobtainingafrequencymodulatedwavei