down_frecon.m

fm中频调制中实现数字正交上变频和正交下变频。

%数字下变频
% Ts:采样间隔
% wc:载波频率
% x:输入信号
% f_i,f_q：同向，正交两路输出
% f_cutoff:调制信号频率
function [f_i,f_q]=down_FreCon(x,Ts,wc,f_cutoff)
N=length(x);
n=1:N;Fs=1/Ts;
SFMI=x.*cos(wc*n*Ts);
SFMQ=-x.*sin(wc*n*Ts);

df=1000;%频率分辨率
[S_I,sfmi,df1]=fftseq(SFMI,Ts,df);  
S_I=S_I/Fs;
n_cutoff=floor(f_cutoff/df1);
H=zeros(1,length(S_I));
H(1:n_cutoff)=ones(1,n_cutoff);
H(length(S_I)-n_cutoff+1:length(S_I))=ones(1,n_cutoff);
f_i=real(ifft(S_I.*H))*Fs;
f_i=f_i(1:N);

[S_Q,sfmq,df1]=fftseq(SFMQ,Ts,df);  
S_Q=S_Q/Fs;
f_q=real(ifft(S_Q.*H))*Fs;
f_q=f_q(1:N);