SIGNALTRANSMISSIONANDCHANNELBANDWIDTH信号传输和信道带宽

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

2、eviationpermittedinaTVsystemNeedformodulation-antennasize-interference-poorradiationatlowfrequencyAmplitudeModulationInamplitudemodulationtheintelligencetobeconveyedisusedtovarytheamplitudeofthecarrierwave.ec=Eccosωctisthecarrierwaveandem=Emcosωmtisthemodulatingsignal.Theequationofthemodulatedwaveis

3、:e=AcosωctwhereA=(Ec+kEmcosωmt)whenkisaconstantofthemodulator.OnsubstitutingthevalueofAweget:e=(Ec+kEmcosωmt)cosωct=Ec(1+mcosωmt)cosωctwherem=kEm/EcisthemodulationindexAtkEm=Ec,m=1andthecorrespondingdepthofmodulationisthentermedas100%.Thereforeifthemodulatedwaveistobetransmittedwithoutdistortionbyth

4、ismethod,thetransmissionchannelmustbeatleastofwidth2fmcentredonfc.Ifthemodulatingsignalconsistsofmorethanasinglefrequency,asitwouldbeforavideosignal,theequationcanbeextendedtoincludethesumanddifferenceofthecarrierandallfrequencycomponentsofthemodulatingsignal.ChannelBandwidthFrequencycomponentsprese

5、ntinthevideosignalextendsfrom0to5MHzThereforeBandwidthrequired=2×5MHz=10MHzAttenuationslopeof0.5MHzisprovidedattheedgeofthetwosidebands(i.e.2×0.5MHz=1MHz)EachchannelhasitsassociatedFMsoundsignalwhosecarrierfrequencylocatedat5.5MHz(intheupperlimit)Asmallguardbandof0.25MHzforeachchannelSototalChannelB

6、andwidth=10+1+0.25=11.25MHzSuchabandwidthistoolarge-limitsthenumberofchannelsinagivenhighfrequencyspectrumTwosidebandsareidentical-onlyoneisnecessary-thussaving5MHzperchannel-SSBCarrierconveysnoinformation-butitspresenceisnecessaryatthereceiverforrecoveringthemodulatingfrequencyfmfromtheUSBorfromLSB

7、.ThereforeitistransmittedThusresultsinsimplertransmittingequipmentOnlyneedsaninexpensivediodedetectoratthereceiverfordemodulationInTelevisiontransmissionweuseVSB-VestigialSideBandtransmissionChannelBa