%顯示模擬調制的波形及解調方法AM
%信源
close all;
clear all;
dt = 0.001;          %時間采樣間隔         
fm=1;                %信源最高頻率
fc=10;               %載波中心頻率
T=5;                 %信號時長
t = 0:dt:T; 
mt = sqrt(2)*cos(2*pi*fm*t);  %信源
%N0 = 0.01;            %白噪單邊功率譜密度

%AM modulation
A=2;
s_am = (A+mt).*cos(2*pi*fc*t);
B = 2*fm;            %帶通濾波器帶寬
%noise = noise_nb(fc,B,N0,t);    %窄帶高斯噪聲產生
%s_am = s_am + noise;

figure(1)
subplot(311)
plot(t,s_am);hold on;     %畫出AM信號波形
plot(t,A+mt,'r--');       %標示AM的包絡
title('AM調制信號及其包絡');
xlabel('t');
%AM demodulation
rt = s_am.*cos(2*pi*fc*t);  %相干解調
rt = rt-mean(rt);
[f,rf] = T2F(t,rt);
[t,rt] = lpf(f,rf,2*fm);    %低通濾波
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');