通信原理英文PPT学习教案

《通信原理英文PPT学习教案》由会员分享，可在线阅读，更多相关《通信原理英文PPT学习教案（24页珍藏版）》请在装配图网上搜索。

1、会计学1通信原理英文通信原理英文2第1页/共24页3(a) Frequency spectral density of input signalS (f)f0f-f00M(f)f0(b) Frequency spectral density of output signal( )( )m tM f)(cos)(0fStAtm)()(2)(00ffMffMAfS H(f)s (t)Modulated signals(t)Modulatingsignalm(t)Acos 0t第2页/共24页4=m (t)101+m (t)101+m (t)第3页/共24页5Figure 3.2.3 Wavefor

2、m and spectrum of modulated signalm(t)s(t)M(f)C(f)c(t)A-Atfm-fmf0-f02fmS (f)2fm-f0f0ffftt第4页/共24页6The envelope of y(t):For large S/N:ttnttnAtmtysc00sin)(cos)()( 1)(ttnttnsc00sin)(cos)()()()( 1)(22tntnAtmtVscy)()( 1 )(tnAtmtVcyRectifierLow-pass filterFigure 3.2.4 Envelope detector第5页/共24页7Since m(t)

3、1，obviously the above ratio r0/ri is less than 1, i.e., the S/N decreased after detection. )()( )(tnAtmtvc)(/)(2220tnAtmErc)(/)( 121222tnAtmEri222222220)( 1 )(2)(/)( 121)(/)(tmtmEtnAtmtnAtmErrci第6页/共24页8M(f)f0f00-f0fS(f)(a) Frequency spectral density of modulating signal Figure 3.2.5 Spectrum of dou

4、ble-sideband modulation signalUpper-sideband(b) Frequency spectral density of modulated signalUpper-sidebandLower-sideband第7页/共24页9phase error.nAdvantage & disadvantage: DSB signal can save transmitting power, but the receiving circuit will be rather complicated.ttm0cos)()cos(0t)2cos()cos(21)cos(cos

5、)()(000tttmtttmtr)cos()(21ttmFigure 3.2.6 Block diagram of DSB signal demodulatorBaseband signalm(t)Received signals(t)cos0tr ( t) H(f)第8页/共24页10cos0t is multiplied with the received signal; it is equivalent to convolution of carrier and signal spectra in frequency domain.Figure 3.2.7 Frequency spec

6、trum of single-sideband signal(b)Upper-sideband filter characteristic and signal frequency spectrumf00f-f00ff0-f0HH(f)HH(f)S(f)U p p e r -sidebandU p p e r -sideband(c)Lower-sideband filter characteristic and signal frequency spectrum-f0f0fHL(f)S(f)L o w e r -sideband(a)Signal frequency spectrum bef

7、ore filteringU p p e r -sidebandS(f)U p p e r -sidebandL o w e r -sideband第9页/共24页11nAdvantages of SSB: smaller transmitting power and bandwidth than DSB signal.Figure 3.2.7 Frequency spectrum of single-sideband signal(b) Upper-sideband filter characteristic and signal frequency spectrumf00f-f00ff0-

8、f0HH(f)HH(f)S(f)Upper-sidebandUpper-sideband(c) Lower-sideband filter characteristic and signal frequency spectrum-f0f0fHL(f)S(f)Lower-sideband(a) Signal frequency spectrum before filteringUpper-sidebandS(f)Upper-sidebandLower-sideband第10页/共24页12Assume the transfer function of the filter of modulato

9、r is H( f )，then the frequency spectrum of the modulated signal after filtering is)()(2)(00ffMffMAfSs (t)A Acoscos 0 0t tH H( (f f) )DemoduDemodulated lated signalsignals s( (t t) )Modulating Modulating signalsignalm m( (t t) )()()(2)(00fHffMffMAfS第11页/共24页1300by the low-pass filter, hence the spect

10、ral density of the demodulated signal from filtering is:Baseband Baseband signalsignalm m( (t t) )ReceiveReceived d signalsignals s( (t t) )coscos 0 0t tr (t) H H( (f f) )()(2100ffSffS)()()(2)(00fHffMffMAfS)()()2()()()2(40000ffHfMffMffHfMffMA)()()(400ffHffHfMA第12页/共24页14)()()(400ffHffHfMACffHffH)()(

11、00mfffM当, 0)(mffCffHffH,)()(00第13页/共24页15H(f + f0)-(f0+fm)0000ffff0-f0f0+fm-2f02f0-2f02f0fm-fmfmfH(f)H(f - f0)H(f + f0) + H(f f0)mffCffHffH,)()(00第14页/共24页16The above equation can be written as:)cos()(cos)(00tAtAtcdttdti)()(0)()(dttti第15页/共24页17modulated carrier is: The above equation shows that the

12、 instantaneous frequency in phase modulation linearly varies with the derivative of the modulating signal. )cos()(cos)(00tAtAtc00( )( )pttk m t00( )cos( )pps tAtk m t)(t)0tmdtdkpi（第16页/共24页18integration of modulating signal.)()(0tmktfi000)()()(dttmktdtttfi)(cos)(00dttmktAtsff第17页/共24页19第18页/共24页20 i

13、第19页/共24页21 where, mf / fm - ratio of max. frequency deviation to baseband signal frequency, it is called modulation index mf , i.e.ttmmcos)(tktmktmfficos)()(00sin)/(coscoscos)(00ttAtdtktAtsmmmffmfmmfkffm第20页/共24页22sin)/(coscoscos)(00ttAtdtktAtsmmmff)3cos()3)cos()2cos()2)cos()cos()cos(cos)()(00300200100ttmJttmJttmJtmJAtsmmfmmfmmfff()() whenis even()() whenis oddnfnfnfnfJ mJmnJ mJmnnmfnftnmJAts)cos()()(0 x第21页/共24页23mmkHzkHzkHzk H z作kHzkHzkHzkHz)( 2mB)(2mff 第22页/共24页24第23页/共24页