dtfn.m

N分别取45,50,55,60时的FFT频率响应曲线绘图.

%exa3.28
figure(1)
subplot(221)
N=45;
n=0:N-1;t=0.01*n;
q=n*2*pi/N;
x=2*sin(4*pi*t)+5*cos(8*pi*t);
y=fft(x,N);
plot(q,abs(y),'k')
title('FFT N=45')

subplot(222)
N=50;
n=0:N-1;
t=0.01*n;
q=2*n*pi/N;
x=2*sin(4*pi*t)+5*cos(8*pi*t);
y=fft(x,N);
plot(q,abs(y),'k')
title('FFT N=50')

subplot(223)
N=55;
n=0:N-1;
t=0.01*n;
q=2*n*pi/N;
x=2*sin(4*pi*t)+5*cos(8*pi*t);
y=fft(x,N);
plot(q,abs(y),'k')
title('FFT N=55')

subplot(224)
N=60;
n=0:N-1;
t=0.01*n;
q=2*n*pi/N;
x=2*sin(4*pi*t)+5*cos(8*pi*t);
y=fft(x,N);
plot(q,abs(y),'k')
title('FFT N=60')