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  1. #1
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    phase margin measurement

    As I recalled, all the books measure PM (phase margin) with respect to -180 degrees (at low freq, phase begin at 0 degrees)

    But for my case here, my phase begin with +180 degrees, should I measure phase margin with respect to 0 degrees or -180 degree? What is the phase margin of this plot? 85.05' or 265.05'?

    and why isn't my phase start at 0' but at 180'?

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    measure phase margin

    The phase margin is usually computed with respect to -180° because we expect the phase to start at 0.
    The meaning of the phase margin is "How much phase left before I get a phase shift of 180° with the input ?"
    So in your case it is about 80°...


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    bode plot phase

    The PM is about 80 degree. The phase doesn't begin at 0 degree because the negative input terminal is used in ac analysis.
    [size=2]Best Regards,[/size]
    [size=2]Hughes[/size]



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    bode plot + phase + starts + 0

    in your case ,the phase margin is about 80 degree,compared to the input ..



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    how to measure -3db from bode

    ur phase margin=180-85=95 degrees



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  6. #6
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    phase margin bode plot

    I agree with shpongle. Phase usually start from 0degree.



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    bode margin

    It would be helpful to post your design here. Maybe the curve is not phase but a phase shift from -180degree



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    measuring phase margin

    Hello all.
    There are always lots of different definitions for phase margin.... it becomes sometimes quite confusiong.
    The phase margin problem hapens when you have a feedback system.
    I believe that the best definition is:

    1- A feedback system becomes unstable if the total phase delay of the system is greater than or equal to 360 degrees (Barkhausen criterion).

    2- This means that a positive feedback system will always be unstable since the phase delay will always be more than 360 degrees.

    3- A negative feedback system can be stable under certain conditions. Due to the negative feedback the loop has already 180 degrees phase delay.

    4- There are still left 180 degrees.

    5- Normally, adding some more processing inside of the loop may improve the loop time or frequency response.

    6- The maximum phase delay allowed for all the processing that can be added is 180 degrees.

    7- After all the extra processing is added, the amount of phase delay that is left is then the phase margin of the system. 45 degrees is a good target value to have after all processing and compensation has beeing added.

    Could you show us your design so that people can give a better opinion?

    Greetings
    S.



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