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Phase and gain margin (control) 
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#1
Sep1406, 02:25 PM

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 


#2
Sep1606, 07:26 AM

P: n/a

If the graphs don't cross the limits, the gain and phase margins are meaninless. You could say they are infinite.



#3
Sep1606, 11:33 AM

P: 130

...........................thanks



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