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Bode Plot stability analysis for nonminimum phase systems

ronaldomponte
ronaldomponte over 2 years ago

Hi everyone,

It is well known that assessing the stability of a nonminimum phase feedback system (e.g., circuits with RHP zeros and time delays) may fail using the frequency analysis of the Bode plot. That's why many textbooks instead recommend the Nyquist test.
However, there is no Nyquist test available in Cadence.
Does this mean that the STB analysis from Cadence is insufficient to evaluate my circuit's relative stability?

I very much appreciate your taking the time to answer my question. 

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  • Frank Wiedmann
    Frank Wiedmann over 2 years ago

    You can access the (negative) loop gain with getData( "loopGain" ?result "stb" ) and you can for example plot it in a polar chart (see https://designers-guide.org/forum/YaBB.pl?num=1124688329 for the sign convention issue; make sure if you need to look for encirclements of -1 or of +1).

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  • Andrew Beckett
    Andrew Beckett over 2 years ago

    If you go to the results browser, you can plot the loopGain (from the stb analysis results) on a polar plot, which will then give you a Nyquist plot. You can then look at the Nyquist criterion that way (including the angle at which it crosses the unit circle and the magnitude at which it crosses the x-axis - you can use the trace info assistant to show the gamma in dB to see that as you move along the trace). Is that what you want?

    Andrew

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  • ronaldomponte
    ronaldomponte over 2 years ago in reply to Andrew Beckett

    Yes, that's it. Thank you very much for your time on this matter, Andrew.

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  • ronaldomponte
    ronaldomponte over 2 years ago in reply to Frank Wiedmann

    Many thanks, Frank.

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