gajam.m

am调制解调

clear all;
close all;
clc;
dt=0.0001;
t=[0:dt:1];
fc=50;
phi0=0;
ct=cos(2*pi*fc*t+phi0);         %载波信号
fm=5;
a0=2;
mt0=cos(2*pi*fm*t+phi0);
mt=a0+mt0               ;        %调制信号+直流a0
s_am=1.5*mt.*ct;
figure(1)
subplot(3,1,1),plot(t,mt),title('mt----调制信号+直流a0')
subplot(3,1,2),plot(t,ct),title('ct----载波信号')
subplot(3,1,3),plot(t,s_am),title('s_am----已调信号')

[m,n]=size(s_am);
ni=0.05*randn(m,n);
s_am0=s_am+ni;
figure(2)
subplot(3,1,1),plot(t,ni),title('ni----高斯白噪声')
subplot(3,1,2),plot(t,s_am0),title('s_am0=s_am+ni----已调信号+高斯白噪声')

w1=2*dt*40;
w2=2*dt*60;
[b,a]=butter(4,[w1,w2],'bandpass');
s_am1=filter(b,a,s_am0);
subplot(3,1,3),plot(t,s_am1),title('s_am1----信号进入带通滤波器')

s_am2=abs(hilbert(s_am1));
figure(3)
subplot(3,1,1),plot(t,s_am2),title('s_am2----信号经包络检波后的波形')

B=2*fm;
wn3=2*dt*B;
[b,a]=butter(4,wn3,'low');
s_am3=filter(b,a,s_am2);
s_am4=s_am3-a0;
subplot(3,1,2),plot(t,s_am3),title('s_am3----解调后的信号')
subplot(3,1,3),plot(t,s_am4),hold on,title('解调后的信号与原调制信号比较')
plot(t,mt0,'r')

dt=t(2)-t(1);                   % 采样周期
f=1/dt;                         % 采样频率(Hz)
X=fft(s_am);                    % 计算x的快速傅立叶变换X
N=1/dt;
F=X(1:N/2+1);                   % F(k)=X(k)(k=1:N/2+1)
f=f*(0:N/2)/N;                  % 使频率轴f从零开始
figure(4)
subplot(3,1,1),plot(f,abs(F)),title('am调制信号频谱图') ,axis([0,100,0,18000])
xlabel('Frequency');
ylabel('|F(k)|')