% Figure 10.18  Nyquist plot  
W=100; 
dw = 0.1;  
w=0:dw:W; 
s= i*w;      
N = 1; 
D = s-1;
H = N./D; 
N = 1; 
D = s-2;       
P= N./D;  
G= H.*P;
r = abs(G);     
theta = angle(G); 
polar(theta,r,'k')
hold;
polar(-theta,r,'k--')