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?

  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
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?