SIGNAL TRANSMISSION AND CHANNEL BANDWIDTH信号传输和信道带宽.ppt

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1、4.SIGNALTRANSMISSIONANDCHANNELBANDWIDTHPreparedbySamKollannoreU.Lecturer,DepartmentofElectronicsM.E.S.College,Marampally,Aluva-7Picture–amplitudemodulatedSound–frequencymodulatedChannelbandwidth–determinedbythehighestvideofrequencyrequiredforproperpicturereceptionandthem

2、aximumsoundcarrierfrequencydeviationpermittedinaTVsystemNeedformodulation-antennasize-interference-poorradiationatlowfrequencyAmplitudeModulationInamplitudemodulationtheintelligencetobeconveyedisusedtovarytheamplitudeofthecarrierwave.ec=Eccosωctisthecarrierwaveandem=Emcosω

3、mtisthemodulatingsignal.Theequationofthemodulatedwaveis:e=AcosωctwhereA=(Ec+kEmcosωmt)whenkisaconstantofthemodulator.OnsubstitutingthevalueofAweget:e=(Ec+kEmcosωmt)cosωct=Ec(1+mcosωmt)cosωctwherem=kEm/EcisthemodulationindexAtkEm=Ec,m=1andthecorrespondingdepthofmodulationis

4、thentermedas100%.Thereforeifthemodulatedwaveistobetransmittedwithoutdistortionbythismethod,thetransmissionchannelmustbeatleastofwidth2fmcentredonfc.Ifthemodulatingsignalconsistsofmorethanasinglefrequency,asitwouldbeforavideosignal,theequationcanbeextendedtoincludethesumand

5、differenceofthecarrierandallfrequencycomponentsofthemodulatingsignal.ChannelBandwidthFrequencycomponentspresentinthevideosignalextendsfrom0to5MHzThereforeBandwidthrequired=2×5MHz=10MHzAttenuationslopeof0.5MHzisprovidedattheedgeofthetwosidebands(i.e.2×0.5MHz=1MHz)Eachchanne

6、lhasitsassociatedFMsoundsignalwhosecarrierfrequencylocatedat5.5MHz(intheupperlimit)Asmallguardbandof0.25MHzforeachchannelSototalChannelBandwidth=10+1+0.25=11.25MHzSuchabandwidthistoolarge-limitsthenumberofchannelsinagivenhighfrequencyspectrumTwosidebandsareidentical-onlyon

7、eisnecessary-thussaving5MHzperchannel-SSBCarrierconveysnoinformation-butitspresenceisnecessaryatthereceiverforrecoveringthemodulatingfrequencyfmfromtheUSBorfromLSB.ThereforeitistransmittedThusresultsinsimplertransmittingequipmentOnlyneedsaninexpensivediodedetectorattherece

8、iverfordemodulationInTelevisiontransmissionweuseVSB-VestigialSideBandtransmissionChannelB