# phase and gain margin (control)

by LM741
Tags: control, gain, margin, phase
 P: 130 hey guys. this has to do with determining the phase and gain margins of a system. i've read and understood the follwing: "To find GM, first find the phase crossover frequency, wGM. This is the frequency where the phase curve passes through -180. Get the magnitude at that frequency [looking at a bode plot] To find FM, first find the gain crossover frequency, wFM. This is the frequency where M = 1. Recall at M = 1, 20 log M = 20 log 1 = 0." what i don't understand is: in the bode plot - what if the phase plot does not go up to 180 degrees - does this mean there is no gain margin?also - i've only seen examples where the gain margin is from a negative dB number to 0dB. what if the the -180 degrees has a corresponding gain greater then 0dB , then will i have a negative gain margin? in the magnitude plot - what if the graph does not cut the w axis , does this mean there is no phase margin? thanks John
 P: n/a If the graphs don't cross the limits, the gain and phase margins are meaninless. You could say they are infinite.
 P: 130 ...........................thanks

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