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stb analysis / phase margin & gain margin functions

david0526

hello,

i have a question on stb analysis in spectre.  i can successfully perform the analysis, and can plot loop gain's phase/magnitude without issues.  however, when i try to select other functions, like PM Frequency, GM Frequency, Gain Margin or Phase Margin, i get a notice that the particular

"... metric is not found.  Perhaps the stb frequency sweep range was no adequate".  

i tried many different sweep setting, but can't get this function to work.  is there something obvious i'm missing?

thanks in advance.

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  • Hi David,

    >  I tried many different sweep setting, but can't get this function to work.  is there something obvious i'm missing?

    Perhaps you have already verified this or it is obvious, but are you sure that the system you are simulating shows a phase shift of - 180 degrees? For example, if the open loop phase does not cross -180 degrees, then the phase margin is not defined. This suggests that the phase margin frequency and gain margin frequency are also not defined.

    You might try plotting the open loop gain and open loop phase from your analysis and visually inspect the two to verify the existence of a phase shift of -180 degrees and the gain margin (if it exists).

    I hope this helps David,

    Shawn

Reply

  • Hi David,

    >  I tried many different sweep setting, but can't get this function to work.  is there something obvious i'm missing?

    Perhaps you have already verified this or it is obvious, but are you sure that the system you are simulating shows a phase shift of - 180 degrees? For example, if the open loop phase does not cross -180 degrees, then the phase margin is not defined. This suggests that the phase margin frequency and gain margin frequency are also not defined.

    You might try plotting the open loop gain and open loop phase from your analysis and visually inspect the two to verify the existence of a phase shift of -180 degrees and the gain margin (if it exists).

    I hope this helps David,

    Shawn

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