t=0:0.1:1;
s=square(2*pi*25*t);  %周期為0.04
x=sin(t);
y=cos(t);
f=abs(fft(s,128));    %對方波作FFT變換后取其幅值
a=0:1:127;
x=sin(a/128);
y=cos(a/128);
stem3(x,y,f,'b','fill');  %用菱形桿頭標記，用藍色填色