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Nyquist stability test using stb analysis

DCashen

I'd like to do a Nyquist test for stability. I can generate a Nyquist plot from the loop gain analysis. That'll give me the number of encirclements of -1. Now I need the poles of the loop gain. Is there a way to extract that from the stb results?

Thanks

  • Andrew Beckett
    stb analysis does not provide this information. There have been enhancement requests for this in the past, but this is currently not available (it's not entirely straightforward). There is the pz analysis which will give you the poles and zeros of the entire circuit (not just a specific loop).
  • DCashen

    Hi Andrew,

    I forgot to ask. In formatting the stb results for generating the Nyquist plot we need to multiple the real portion by -1. I wouldn't have realized to do this without viewing the video on creating a Nyquist plot. Why do we need to multiple the result by -1?

    Thanks again

  • Frank Wiedmann

    You can find the explanation for the multiplication with -1 at http://www.designers-guide.org/Forum/YaBB.pl?num=1124688329.

    By the way, what is shown in the video (http://support.cadence.com/wps/mypoc/cos?uri=deeplinkmin:VideoViewer;src=wp;q=Training/Custom_IC_Design/TrainingBytes/CreateNyquistPlots.html;DocumentType=trng,norcfeat,trngbyte,video) is really only half the Nyquist plot. In order to get the complete plot, you should plot the imaginary part of the loop gain (without multiplication by -1) as a third curve (before selecting "Y vs Y").

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