Gain/phase margin calculation and the gain/phase cross over frequency

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Mahabba Almheiri 2020-12-5

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https://ww2.mathworks.cn/matlabcentral/answers/679883-gain-phase-margin-calculation-and-the-gain-phase-cross-over-frequency

评论： Mahabba Almheiri 2020-12-5

Hello..

I used the below code to calculate the gain and phase margin. Also to calculate gain and phase cross over frequency but I didn't get any answer in the command window. So, what is the correct code.

syms s
G = tf([7 14],[1 10 24])
[Gm,Pm,Wcg,Wcp] = margin(G)
grid on

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Answer 1

Alexandre Riebel 2020-12-5

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https://ww2.mathworks.cn/matlabcentral/answers/679883-gain-phase-margin-calculation-and-the-gain-phase-cross-over-frequency#answer_566113

I'd say your code is working correctly. Phase margin is measured at the frequency at which the gain crosses the 0 dB line (gain crossover frequency, Wcg) and gain margin is measured at the frequency at which the phase crosses the -180 deg line (phase crossover frequency, Wcp). So if your gain never crosses 0 dB then wcg = NaN and Pm = Inf which seems to be what's happening here.