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interpolation for xmin calculation

RobinCommander

As an addition to phase margin & gain margin in some AC open loop simulations I have been also been calculating the vector margin which can be useful.

I calculate it thus:

stb_x = xmin((abs((v("/Ampout" ?result "ac") - complex(-1.0 0.0)))**2))

stb_dB = (- db20(value(v("/Ampout" ?result "ac") stb_x)))

To get more accurate results I need to add more points/decade which can slow things down in swept simulations, I assume this is because only the data points are used in the xmin calculation. I was wondering if interpolation could somehow be used in the xmin calculation so I can speed up the simulations. Or maybe there is some other way of doing this calculation.


Thanks,

Robin

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  • Andrew,

    I didn't think there would be an easy answer for the interpolation, but thought it worth trying here.

    I think I got the expressions for evaluating the vector margin from here:
    www.circuitdesign.info/blog/2012/02/creating-a-nyquist-plot-with-cadence

    I didn't check if the **2 was useful, so maybe it isn't needed.

    Sometimes when sweeping a parameter in ac analysis I found the phase margin would improve but gain margin reduce so I started looking for another measure of stability for a third opinion.
    Also phase & gain margins are measured at single points which might miss any peaking between these two measurement points.

    Regards,
    Robin
Reply
  • Andrew,

    I didn't think there would be an easy answer for the interpolation, but thought it worth trying here.

    I think I got the expressions for evaluating the vector margin from here:
    www.circuitdesign.info/blog/2012/02/creating-a-nyquist-plot-with-cadence

    I didn't check if the **2 was useful, so maybe it isn't needed.

    Sometimes when sweeping a parameter in ac analysis I found the phase margin would improve but gain margin reduce so I started looking for another measure of stability for a third opinion.
    Also phase & gain margins are measured at single points which might miss any peaking between these two measurement points.

    Regards,
    Robin
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