interpolation for xmin calculation

Robin,

I don't think it's a case of getting xmin to interpolate - the minimum y value must inherently be a point that's in the original waveform unless you do some kind of spline fit of the curve. However, I think that probably you want to find the closest point on the Nyquist plot to -1,0 - and that may not be at a point. I'll have a think about that.

It wasn't obvious to me why you have the **2 (squared) term in your expression?

Also, assuming that an interpolated x-value could be found, I think you'd want to do value(db20(...) rather than db20(value(...)) because that way it would linearly interpolate something in db which I think would probably be more appropriate (even then it's not quite right because you really want to linearly interpolate on a log-log curve, I guess. Right now it doesn't matter because the x point you're using value() at is one of the original points, and so there's no interpolation in the value() either.

Anyway, I'll have a think about possibiities of calculating this vector margin. Potentially it could be a good enhancement for spectre's stb analysis to output this figure of merit too.

Regards,

Andrew.

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

p.s.

I think the formula I gave above isn't really the Vector Margin as it measures the distance
to the origin not to (-1,0).
I think it should be:
VectorMargin = abs(value((v("/Ampout" ?result "ac") - complex(-1.0 0.0)) stb_x))
I've also seen this called stability margin. Recommended values I've seen are >= 0.33 to 0.5.

A related measure is Vector Gain Margin which is somewhat similar to gain margin.

VectorGainMargin_dB = db20(1 - abs(value((v("/Ampout" ?result "ac") - complex(-1.0 0.0)) stb_x)))

You can also calculate Vector Phase Margin, although I'm not sure how useful that is:
VectorPhaseMargin = value(phaseDeg(v("/Ampout" ?result "ac")) stb_x)

Regards,

Robin