为什么这个程序在7.0中没有频谱?

[梦龙]

r=0.02;
t=-2:r:2;
f=Heaviside(t+1)-Heaviside(t-1);
W1=2*pi*5;
N=500;K=0:N;W=K*W1/N;
F=f*exp(-j*t'*W)*r;
F=real(F);
W=[-fliplr(W),W(2:501)];
F=[fliplr(F),F(2:501)];
subplot(2,1,1);plot(t,f);

xlabel('t');ylabel('f(t)');
title('f(t)=u(t+1)-u(t-1)');
subplot(2,1,2);plot(W,F);
xlabel('w');ylabel('F(w)');
title('f(t)的傅立叶变换F（w)');