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Simulators >> Circuit Simulators >> How to get Nyquist plot with VIVA ?
https://designers-guide.org/forum/YaBB.pl?num=1568511590

Message started by pwm on Sep 14^th, 2019, 6:39pm

Title: How to get Nyquist plot with VIVA ?
Post by pwm on Sep 14^th, 2019, 6:39pm

Using Spectre and VIVA. I need Nyquist plot with real and Imaginary axis to analyze stability. Thanks !

Title: Re: How to get Nyquist plot with VIVA ?
Post by Andrew Beckett on Sep 17^th, 2019, 4:06am

You can do this by running a stability analysis (stb) in Spectre, and then using the Results Browser go to the "stb" results and picking that the graph type will be either "Polar" or "RealvsImag" rather than "Default" (which is "Rectangular") and then double clicking on the "loopGain" signal within the stb results.

It's probably polar that you want rather than real vs imaginary because that then shows both the unit circle and also the angles - so it's easier to assess the stability that way.

Regards,

Andrew

Title: Re: How to get Nyquist plot with VIVA ?
Post by pwm on Sep 17^th, 2019, 6:52am

Andrew Beckett wrote on Sep 17^th, 2019, 4:06am:

Thanks Andrew !
It's strange that I could not find the RealvsImag in the Result Browser, only the Polar is available.
The case I have is strange, the phase margin is >60deg, Gain margin > 15dB and yet I have more than 3 rings in the transient. I don't think it is due to large signal instability i.e. it tripped some comparator. I notice that the Q factor due to the complex pole can be a factor, i.e. the higher the gain-peaking, the worse the ringing it will be. Maybe there's another better way to assess the stability due to the gain-peaking that can be recommended ?

Title: Re: How to get Nyquist plot with VIVA ?
Post by Andrew Beckett on Sep 22^nd, 2019, 6:00am

Which IC version are you using? (the precise number that's in Help->About in any Virtuoso window will help).

Hard to know what your stability issue is without more details.

Regards,

Andrew

Title: Re: How to get Nyquist plot with VIVA ?
Post by pwm on Sep 22^nd, 2019, 5:34pm

Andrew Beckett wrote on Sep 22^nd, 2019, 6:00am:

Which IC version are you using? (the precise number that's in Help->About in any Virtuoso window will help).

Hard to know what your stability issue is without more details.

Regards,

Andrew

Virtuoso 6.1.7-64b

Title: Re: How to get Nyquist plot with VIVA ?
Post by Andrew Beckett on Oct 4^th, 2019, 7:32am

That's only the first part of the version number - I asked for the entire version number (which is shown on the Help->About form)

If it makes it easier, type in the CIW: getVersion(t)
and then post the result here.

Andrew.

Title: Re: How to get Nyquist plot with VIVA ?
Post by Andrew Beckett on Oct 4^th, 2019, 7:38am

Actually, I just checked in the base release (IC6.1.7-64b.78) and RealVsImag is there - so I'd be surprised if it went missing in a later sub-version.

Here's what it looks like for me in the results browser...

Andrew