Transfer function nyquist diagram problem

Why does case (1) appear to scale/wrap infinity so that it is visible on the plot, while case (2) does not? Both systems are marginally stable. This is most likely some theory I don't understand but is there a way to wrap the nyquist plot so that I can see what is going on with the rest of the plot for case (2)?

case (1):

numg=[1 5];
deng=conv([1 6 100],[1 4 25]);
G=tf(numg,deng);
Gzpk=zpk(G)
nyquist(G)

case (2):

numg=poly([-3 -5]);
deng=poly([0 -2 -4]);
G=tf(numg,deng);
Gzpk=zpk(G)
nyquist(G)

[Azzi Abdelmalek]

appear to scale/wrap infinity: What does that mean?

Answer 1

[Star Strider]

I am not certain that I understand your question, but the second one has a pole at infinity while the first on does not. Extending the imaginary axis in the second one would not likely provide any additional useful information.

According to the documentation, you can extend the frequency axis if you want to:

• nyquist(sys,w) explicitly specifies the frequency range or frequency points to be used for the plot. To focus on a particular frequency interval, set w = {wmin,wmax}. To use particular frequency points, set w to the vector of desired frequencies. Use logspace to generate logarithmically spaced frequency vectors. Frequencies must be in rad/TimeUnit, where TimeUnit is the time units of the input dynamic system, specified in the TimeUnit property of sys.

Answer 2

[Craig]

If I understand what you mean by scale/wrap. The first system has finite magnitude for all values of jw. The second system has an integrator, and therefore the magnitude goes to inf as jw approaches 0. This is why you do not see part of the nyquist plot for the second system at freqeuncies near 0.