1、 1. 引言 调制的定义 调制的分类 线性调制原理 非线性调制 -角度调制 调制系统的比较（抗噪声性能分析和比较） FDM原理 总结 重点：调制系统的抗噪声性能第四章 模拟调制系统 Definition:A baseband waveform has a spectral magnitude that is nonzero for frequencies in the vicinity of the origin and negligible elsewhere. Definition:A bandpass waveform has a spectral magnitude that is n
2、onzero for frequencies in some band concentrated about a frequency f= fc ,where fc0.The spectral magnitude is negligible elsewhere. fc is called the carrier frequency.fc may be arbitrarily assigned. Definition:Modulation is the process of imparting the source information onto a bandpass signal with
3、a carrier frequency fc by the introduction of amplitude and/or phase perturbation.This bandpass signal is called the modulated signal s(t),and the baseband source signal is called the modulating signal m(t).1.调制的定义 modulationDiagram of a typical modulation systemm(t)Baseband signalModulating signalM
4、odulators(t)Bandpass signalModulated signalLocal oscillatorcosctCarrier Bandpass communication systemm(t)Baseband signalModulating signalModulators(t)Bandpass signalLocal oscillatorcosctCarrier Modulated signalchannelDemodulator m(t)Corrupted baseband signalCorrupted bandpass signalnoise 调制系统的分类：幅度调
5、制（线性调制），非线性调制（角度调制）和数字调制（ PCM） 线性调制： AM,DSB-SC,SSB,VSB2.线性调制系统 All banpass waveforms can be represented by their complex envelope forms. Theorem:Any physical banpass waveform can be represented by:v(t)=Reg(t)ejctRe.:real part of .g(t) is called the complex envelope of v(t),and fc is the associated c
6、arrier frequency.Two other equivalent representations are:v(t)=R(t)cosct+(t)andv(t)=x(t)cos ct-y(t)sin ctwhere g(t)=x(t)+jy(t)=R(t) ej(t)Complex envelope representation Representation of modulated signals The modulated signals a special type of bandpass waveform So we have s(t)=Reg(t)ejctthe complex
7、 envelope is function of the modulating signal m(t): g(t)=gm(t)g.: mapping functionAll type of modulations can be represented by a special mapping function g. Complex envelope functions for various types of modulation Type of modulation mapping functions g(m)AM Ac1+m(t) linear(?)DSB-SC Acm(t) linear
8、SSB Acm(t)jm(t) linearPM AcejDpm(t) non-linearFM non-linear Bandpass signals spectrum complex envelopes spectrum Theorem:If a bandpass waveform is represented by:v(t)=Reg(t)ejctthen the spectrum of the bandpass waveform isV(f)=1/2G(f-fc)+G*(-f-fc)and the PSD of the waveform isPv(f)=1/4Pg(f-fc)+Pg(-f
9、-fc)where G(f)=Fg(t), Pg(f) is the PSD of g(t).Proof: v(t)=Reg(t)ejct=1/2g(t)ejct+g*(t)e-jctV(f)=1/2Fg(t)ejct+1/2Fg*(t)e-jctSpectrum of bandpass signalsWe have Fg*(t)=G*(-f)Then V(f)=1/2G(f-fc)+G*-(f+fc) The PSD for v(t) is obtained by first evaluating the autocorrelation for v(t).Rv()=+ 1/2Re negligible?But Rg()= Rv()=1/2Re=1/2 ReRg() ejcPv(f)=FRv()=1/4Pg(f-fc)+ Pg*(-f-fc)But Pg*(f)= Pg(f),so Pv(f) is real.
