fs=1200;
N=300;
n=0:N-1;
t=n/fs;
fL=20;
fH=200;
s=cos(2*pi*fL*t)+cos(2*pi*fH*t);
subplot(121);plot(t,s);
title('输入信号');xlabel('t/s');ylabel('幅度');
sfft=fft(s);
subplot(122);
plot((1:length(sfft)/2)*fs/ length(sfft),2*abs(sfft(1:length(sfft)/2))/length(sfft));
title('信号频谱');xlabel('t/s');ylabel('幅度');%设计低通滤波器
Wp=50/fs;Ws=100/fs;
[n,Wn]=buttord(Wp,Ws,1,50);
[a,b]=butter(n,Wn);
[h,f]=freqz(a,b,'whole',fs);
f=(0:length(f)-1)'*fs/length(f);
figure(2);
plot(f(1:length(f)/2),abs(h(1:length(f)/2)));
title('巴氏低通滤波器');xlabel('频率/Hz');ylabel('幅度');
grid;
sF=filter(a,b,s);
figure(3);
subplot(121);
plot(t,sF);
title('输出信号');xlabel('t/s');ylabel('幅度');
SF=fft(sF);subplot(122);
plot((1:length(SF)/2)*fs/length(SF),2*abs(SF(1:length(SF)/2))/length(SF));
title('低通滤波后的频谱图');xlabel('频率/Hz');ylabel('幅度');
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