通信原理(英文版).ppt

《通信原理(英文版).ppt》由会员上传分享，免费在线阅读，更多相关内容在行业资料-天天文库。

1、Chapter2Signals2.1ClassificationofSignals2.1.1DeterministicsignalsandrandomsignalsWhatisdeterministicsignal?Whatisrandomsignal?2.1.2EnergysignalsandpowersignalsSignalpower:LetR=1,thenP=V2/R=I2R=V2=I2Signalenergy：LetSrepresentVorI，ifSvarieswithtime，thenScan

2、berewrittenass(t),Hence,thesignalenergyE=s2(t)dtEnergysignalsatisfiesAveragepower:,thenP=0forenergysignal.Forpowersignal:P0,i.e.,powersignalhasinfiniteduration.Energysignalhasfiniteenergy,butitsaveragepowerequals0.Powersignalhasfiniteaveragepower,butitse

3、nergyequalsinfinity.12.2Characteristicsofdeterministicsignals2.2.1CharacteristicsinfrequencydomainFrequencyspectrumofpowersignal:lets(t)beaperiodicpowersignal,itsperiodisT0,thenwehavewhere0=2/T0=2f0∵C(jn0)isacomplexfunction,∴C(jn0)=

4、Cn

5、ejnwhere

6、Cn

7、－a

8、mplitudeofthecomponentwithfrequencynf0n－phaseofthecomponentwithfrequencynf0Fourierseriesofsignals(t):2【Example2.1】Findthespectrumofaperiodicrectangularwave.Solution:AssumetheperiodofaperiodicrectangularwaveisT,thewidthis,andtheamplitudeisV,thenItsfrequen

9、cyspectrumis3Frequencyspectrumfigure4【Example2.2】Findthefrequencyspectrumofasinusoidalwaveafterfull-waverectification.Solution：AssumetheexpressionofthesignalisItsfrequencyspectrum:TheFourierseriesofthesignalis:1f(t)t5Frequencyspectraldensityofenergysignals

10、Letanenergysignalbes(t),thenitsfrequencyspectraldensityisTheinverseFouriertransformofS()istheoriginalsignal:【Example2.3】Findthefrequencyspectraldensityofarectangularpulse.Solution:LettheexpressionoftherectangularpulsebeThenitsfrequencyspectraldensityisits

11、Fouriertransform:6【Example2.4】Findthewaveformandthefrequencyspectraldensityofasamplefunction.Solution:ThedefinitionofthesamplefunctionisthefrequencyspectraldensitySa(t)is:Fromtheaboveequation,weseethatSa()isagatefunction.【Example2.5】Findtheunitimpulsefunc

12、tionanditsfrequencyspectraldensity.Solution:Unitimpulsefunctionisusuallycalleddfunctiond(t).ItsdefinitionisThefrequencyspectraldensityof(t):7d(t)anditsfrequencyspectraldensity:Physicalmeaningoffunction:Itis