模拟调制系统

1、1图3.1.1 调制器调制器已调信号s(t)调制信号m(t)cos()(00tAtc23s(t)调制信号m(t)Acos0tH H( (f f) )已调信号s(t)()(fMtm)(cos)(0fStAtm)()(2)(00ffMffMAfSM(f)f0S(f)f0f-f00(a) 输入信号频谱密度(b) 输出信号频谱密度4m(t)101+m(t)101+m(t)5-fmm(t)s(t)M(f)C(f)c(t)A-Atfmf0-f02fmS (f)2fm-f0f0ffftt101+m(t)6整流器低通滤波器图3.2.4 包络检波器解调调幅信号ttnttnAtmtysc00sin)(cos)()

2、( 1)(ttnttnsc00sin)(cos)()()()( 1)(22tntnAtmtVscy)()( 1 )(tnAtmtVcy7)()( )(tnAtmtvc)(/)(2220tnAtmErc)(/)( 121222tnAtmEri222222220)( 1 )(2)(/)( 121)(/)(tmtmEtnAtmtnAtmErrci8图3.2.5 双边带调制信号的频谱(a) 调制信号频谱密度M(f)f0(b) 已调信号频谱密度f00-f0fS(f)上边带上边带下边带9图3.2.6 双边带信号解调器原理方框图基带信号m(t)接收信号s(t)cos0tr(t)H(f)ttm0cos)()c

3、os(0t)2cos()cos(21)cos(cos)()(000tttmtttmtr)cos()(21ttm10-f0HL(f)特性上边带(b) 上边带滤波器特性和信号频谱上边带f00f图3.2.7 单边带信号的频谱上边带S(f)上边带下边带HH(f)特性HH(f)特性(a) 滤波前信号频谱(c) 下边带滤波器特性和信号频谱S(f)S(f)-f00f-f0f0f下边带f011图3.2.8 单边带信号的解调S(f)(b) 上边带信号频谱上边带上边带f00-f0f2f0-2f0(a) 载波频谱f00-f0fC(f)(c) 载波和上边带信号频谱的卷积结果f00-f0f2f0-2f0M(f)HL(f

4、)12)()(2)(00ffMffMAfSs(t)调制信号m(t)Acos0tH H( (f f) )已调信号s(t)()()(2)(00fHffMffMAfS13基带信号m(t)接收信号s(t)cos0tr(t)H(f)()(2100ffSffS)()()(2)(00fHffMffMAfS)()()2()()()2(40000ffHfMffMffHfMffMA)()()(400ffHffHfMA14)()()(400ffHffHfMACffHffH)()(00mfffM当, 0)(mffCffHffH,)()(0015H(f + f0)-(f0+fm)0000ffff0-f0f0+fm-2f

5、02f0-2f02f0fm-fmfmfH(f)H(f - f0)H(f + f0) + H(f f0)mffCffHffH,)()(0016)cos()(cos)(00tAtAtcdttdti)()(0)()(dttti17)cos()(cos)(00tAtAtc)()(00tkmtt)(cos)(00tkmtAtsp)(t)0tmdtdkpi（18)()(0tmktfi000)()()(dttmktdtttfi)(cos)(00dttmktAtsff19角度调制波形i(b)(a)20ttmmcos)(tktmktmfficos)()(00sin)/(coscoscos)(00ttAtdtktAtsmmmff： mfmmfkffm21sin)/(coscoscos)(00ttAtdtktAtsmmmff)3cos()3)cos()2cos()2)cos()cos()cos(cos)()(00300200100ttmJttmJttmJtmJAtsmmfmmfmmfff为奇数时当为偶数时当nmJmJn