The Designer's Guide Community
Forum
Welcome, Guest. Please Login or Register. Please follow the Forum guidelines.
Oct 17th, 2019, 4:24pm
Pages: 1
Send Topic Print
How to get Nyquist plot with VIVA ? (Read 181 times)
pwm
New Member
*
Offline



Posts: 6

How to get Nyquist plot with VIVA ?
Sep 14th, 2019, 6:39pm
 
Using Spectre and VIVA. I need Nyquist plot with real and Imaginary axis to analyze stability. Thanks !
Back to top
 
 
View Profile hoonsk   IP Logged
Andrew Beckett
Senior Fellow
******
Offline

Life, don't talk to
me about Life...

Posts: 1713
Bracknell, UK
Re: How to get Nyquist plot with VIVA ?
Reply #1 - Sep 17th, 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
Back to top
 
 
View Profile WWW   IP Logged
pwm
New Member
*
Offline



Posts: 6

Re: How to get Nyquist plot with VIVA ?
Reply #2 - Sep 17th, 2019, 6:52am
 
Andrew Beckett wrote on Sep 17th, 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



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 ?
Back to top
 
 
View Profile hoonsk   IP Logged
Andrew Beckett
Senior Fellow
******
Offline

Life, don't talk to
me about Life...

Posts: 1713
Bracknell, UK
Re: How to get Nyquist plot with VIVA ?
Reply #3 - Sep 22nd, 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
Back to top
 
 
View Profile WWW   IP Logged
pwm
New Member
*
Offline



Posts: 6

Re: How to get Nyquist plot with VIVA ?
Reply #4 - Sep 22nd, 2019, 5:34pm
 
Andrew Beckett wrote on Sep 22nd, 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
Back to top
 
 
View Profile hoonsk   IP Logged
Andrew Beckett
Senior Fellow
******
Offline

Life, don't talk to
me about Life...

Posts: 1713
Bracknell, UK
Re: How to get Nyquist plot with VIVA ?
Reply #5 - Oct 4th, 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.
Back to top
 
 
View Profile WWW   IP Logged
Andrew Beckett
Senior Fellow
******
Offline

Life, don't talk to
me about Life...

Posts: 1713
Bracknell, UK
Re: How to get Nyquist plot with VIVA ?
Reply #6 - Oct 4th, 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
Back to top
 

resultsBrowser.png
View Profile WWW   IP Logged
Pages: 1
Send Topic Print
Copyright 2002-2019 Designer’s Guide Consulting, Inc. Designer’s Guide® is a registered trademark of Designer’s Guide Consulting, Inc. All rights reserved. Send comments or questions to editor@designers-guide.org. Consider submitting a paper or model.