%-----------------------------------------------------------------------------
% exa090300_2.m,   for fig. 9.3.2,
%-----------------------------------------------------------------------------
clear all;

t0=.1; 
ts=.001;
fc=250;
t=[-t0/2:ts:t0/2];
m=sinc(100*t);
c=cos(2*pi*fc*t);

%得到幅度調制信號；
u=m.*c;
f=-0.5:1/256:0.5-1/256;
M=fft(m,256);
U=fft(u,256);

% 畫出調制信號和其對應的解析信號的頻譜；
subplot(322)
plot(f,abs(fftshift(M)));grid;
U=abs(U);
for i=1:256/2
   U(i)=2*U(i);
   U(i+128)=0;
   end
subplot(321)
plot(f,fftshift(U));grid;