close all;
clear all;
dt=0.001;
fm=1;
fc=10;
T=5;
t=0:dt:T;
mt=sqrt(2)*cos(2*pi*fm*t);
%NO=0.01;
%AM modulation
A=2;
s_am=(A+mt).*cos(2*pi*fc*t);
B=2*fm;
%noise=noise_nb(fc,B,NO,t);
%s_am=a_am+noise;
figure(1)
subplot(311)
plot(t,s_am);hold on;
plot(t,A+mt,'r--');
title('AM調制信號及其包絡');
xlabel('t');
%AM demodulation
rt=s_am.*cos(2*pi*fc*t);
rt=rt-mean(rt);
[f,rf]=T2F(t,rt);
[t,rt]=lpf(f,rf,2*fm);
subplot(312)
plot(t,rt);hold on;
plot(t,mt/2,'r--');
title('相干解調后的信號波形與輸入信號的比較');
xlabel('t')
subplot(313)
[f,sf]=T2F(t,s_am);
psf=(abs(sf).^2)/T;
plot(f,psf);
axis([-2*fc 2*fc 0 max(psf)]);
title('AM信號功率譜 ');
xlabel('f');