通信原理(英文版).ppt

*?Whatisrandomsignal?:LetR=1,thenP=V2/R=I2R=V2=I2Signalenergy:LetSrepresentVorI,ifSvarieswithtime,thenScanberewrittenass(t), Hence,thesignalenergyE=s2(t)dtEnergysignalsatisfiesAveragepower: ,thenP=:P0,.,,,butitsenergyequalsinfinity.*:lets(t)beaperiodicpowersignal,itsperiodisT0,thenwehave where0=2/T0=2f0 ∵C(jn0)plexfunction,∴C(jn0)=|Cn|ej|-ponentwithfrequencynf0 n-ponentwithfrequencynf0Fourierseriesofsignals(t):*【】Findthespectrumofaperiodicrectangularwave. Solution:AssumetheperiodofaperiodicrectangularwaveisT,thewidthis,andtheamplitudeisV,then Itsfrequencyspectrumis*Frequencyspectrumfigure*【】Findthefrequencyspectrumofasinusoidalwaveafterfull-waverectification. Solution:Assumetheexpressionofthesignalis Itsfrequencyspectrum: TheFourierseriesofthesignalis:1f(t)t*Frequencyspectraldensityofenergysignals Letanenergysignalbes(t),thenitsfrequencyspectraldensityis TheinverseFouriertransformofS()istheoriginalsignal:【】:LettheexpressionoftherectangularpulsebeThenitsfrequencyspectraldensityisitsFouriertransform:*【】Findthewaveformandthefrequencyspectraldensityofasamplefunction. Solution:Thedefinitionofthesamplefunctionis thefrequencyspectraldensitySa(t)is: Fromtheaboveequation,weseethatSa()isagatefunction. 【】Findtheunitimpulsefunctionanditsfrequencyspectraldensity. Solution:Unitimpulsefunctionisusuallycalleddfunctiond(t).Itsdefinitionis Thefrequencyspectraldensityof(t):*d(t)anditsfrequencyspectraldensity:Physicalmeaningoffunction:Itisapulsewithinfiniteheight,i