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1、Unit6ElectronicCommunicationsPassageAIntroductiontoElectronicCommunicationsPassageBThePrincipleofPCMPassageCInformationSourcesPassageAIntroductiontoElectronicCommunications1.HistoricalPerspectiveThefundamentalpurposeofanelectroniccommunicationsystemistotransferinfor
2、mationfromoneplacetoanother.[1]Thus,electroniccommunicationscanbesummarizedasthetransmission,reception,andprocessingofinformationbetweentwoormorelocationsusingelectroniccircuits.Theoriginalsourceinformationcanbeinanalog(continuous)form,suchasthehumanvoiceormusic,orin
3、digital(discrete)form,suchasbinary-codednumbersoralphanumericcodes.[2]Allformsofinformation,however,mustbeconvertedtoelectromagneticenergybeforebeingpropagatedthroughanelectroniccommunicationssystem.SamuelMorsedevelopedthefirstelectroniccommunicationssystemin1837.Mor
4、seusedelectromagneticinductiontotransferinformationintheformofdots,dashes,andspacesbetweenasimpletransmitterandreceiverusingatransmissionlineconsistingofalengthofmetallicwire.Hecalledhisinventionthetelegraph.In1876AlexanderGrahamBellandThomasA.Watsonwerethefirstsucces
5、sfullytransferhumanconversationoveracrudemetallicwirecommunicationssystem,theycalledthetelephone.GuglielmoMarconisuccessfullytransmittedthefirstwirelessradiosignalsthroughEarth’satmospherein1894,andin1908LeeDeforestinventedthetriodevacuumtubewhichprovidedthefirstprac
6、ticalmeansofamplifyingelectricalsignals.Commercialradiobeganin1920whenradiostationsbeganbroadcastingamplitude-modulated(AM)signals,andin1933,MajorEdwinHowardArmstronginventedfrequencymodulation(FM).CommercialbroadcastingofFMbeganin1936.Althoughthefundamentalconceptsa
7、ndprinciplesofelectroniccommunicationshavechangedlittlesincetheirinception,themethodsandcircuitsusedtoimplementthemhaveundergoneconsiderablechange.Inrecentyears,transistorsandlinearintegratedcircuitshavesimplifiedthedesignofelectroniccommunicationscircuits,thusallowin
8、gforminiaturization,improvedperformanceandreliability,andreducedoverallcosts.Inrecentyears,therehasbeenanoverwhelmingneedfor
