2.调幅(AM)信号 Amplitude Modulation 乡m(t)带有直流分量 s(t)=m(t)cos at -mo cosa t+m(t)cosa t S(o)=/moS(O-o+So+o I +[M(o-0)+M(o+o) 29 2.调幅（AM）信号 Amplitude Modulation m（t）带有直流分量 s t m t t m  c ( ) = ( )cos m t m t t  c  c cos ( )cos 0 = +  ( ) [ ( ) ( )] S m  =m0   − c +  + c [ ( ) ( )] 2 1 M   c M  + c +  − + 