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Figure 2-5.  Stability Margin

Now examine the effect on the stability margin of discretizing H(s) at 
100 Hz.

dt = 0.01;

Hd = discretize(H,dt);

margD = smargin(Hd,delb);

100 Hz is a high discretization frequency for H, so the stability margin 
is unchanged in the discrete-time case. The new plot is not much 
different from Figure 2-6. Again, minimum margin is about 8 dB 
at about 1/2 Hz.