A=8;
a=0.7;
n=0:200;
k=0:300;
w0=20;
b=30;
w=(2*pi/300)*k;
xn=A*a.^n.*cos(w0*n+b);
X=xn*(exp(-1j*n'*w));%转置
subplot(3,1,1);
plot(k/300,abs(X));
title('DTFT1');
z=exp(-1j*w);
Yz=(cos(b)-z.^-1*cos(b-w0))./(1-2*z.^-1*cos(w0)+z.^-2);
subplot(3,1,2);
plot(k/300,abs(Yz));
title('DTFT2');
Xz=A*(cos(b)-a*z.^-1*cos(b-w0))./(1-2*a*z.^-1*cos(w0)+a^2*z.^-2);
subplot(3,1,3);
plot(k/300,abs(Xz));
title('DTFT3');
