Phase and gain margin (control)

  1. 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. jcsd
  3. SGT

    SGT 0

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