% Routine 3.2      Bode plots
clear all;
fS= 720;
T = 1/fS;
f = 0:fS/2;
o = 2*pi*f;
w = exp(-i*o*T);
N = 1-2*w*cos(pi/6)+w.*w;
D = 1-1.8*w*cos(pi/6)+0.81*w.*w;
H = N./D;
r = abs(H);
phi = angle(H);
%plot(f,r)
%plot(f,20*log10(r))
plot(f,360*phi/(2*pi),'k-')