% Routine 3.1      Polar plot
clear all;
fS= 1000;
T = 1/fS;
f = 0:fS;
o = 2*pi*f;
w = exp(-i*o*T);
N = (1 + w)/2;
D = 1;
H = N./D;
r = abs(H);
phi = angle(H);
polar(phi,r,'k-')