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Simulators >> Circuit Simulators >> Strange problem in AC/PAC analysis with Spectre
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Message started by cri on Dec 2^nd, 2005, 1:09am

Title: Strange problem in AC/PAC analysis with Spectre
Post by cri on Dec 2^nd, 2005, 1:09am

Hi all!

I have got a problem during my AC analysis of op-amps.
When I try to perform the analysis of the open-loop gain of the amplifier, the resulting Bode diagrams show very strange shape. In particular, both the magnitude and the phase of the open-loop gain show the typical low-pass shape but at medium-to-high frequency appear saw-toothed curves on the graphs. This saw-tooth profile is very sharp since it may cause the magnitude to change (randomly) of 10-20 dB in just few hertz. For this reason I do not think that is the real behaviour of the op-amp.
These sharp sawtooth profiles lay upon the familiar low-pass open-loop curve but are very tiresome since, usually, they start around the unity-gain frequency where the phase-margin should be calculated.
I tried to change the number of frequency points for the simulation but the result is the same.
Probably it depends on the system I used for breaking the loop: indeed using ideal switches (AC position = open) the problem never appears. On the contrary, using AHDL or VerilogA switches, the saw-tooth usually appears.

The same problem results during PAC analysis of switched-capacitor circuits. Unfortunately the ideal switches in "analogLib" library do not work for PAC analysis so I have to use AHDL/VerilogA switches for PAC analysis.

Should I change some simulator options?
Could anybody of you help me?
Thanks in advance!

Title: Re: Strange problem in AC/PAC analysis with Spectr
Post by Ken Kundert on Dec 3^rd, 2005, 8:14am

In general breaking the loop is a bad idea. You can find alternative approaches in "The Designer's Guide to Spice and Spectre", by going to Prof. Middlebrook's website (ardem.com), or http://www.thekunderts.net/ken/docs/c%26d2001-01.pdf.

-Ken

Title: Re: Strange problem in AC/PAC analysis with Spectr
Post by Andrew Beckett on Dec 3^rd, 2005, 1:08pm

There is also the "stb" analysis in spectre (which is a means
of analysing stability with the loop kept closed). There's also
a "pstb" analysis which is currently present as an engineering option in spectre for doing the same kind of thing with periodic circuits.

Ah, I see the paper that Ken was referencing was for the "stb" analysis in spectre, or at least the background to it.

Andrew.

Title: Re: Strange problem in AC/PAC analysis with Spectr
Post by Cri Azzolini on Dec 4^th, 2005, 7:48am

Hi Ken, hi Andrew,

thanks for your replies.
I read Ken's book about Spectre and Spice and I know that the Middlebrook method is the best one for open-loop gain evaluation but I was not sure how to apply to circuits with a periodic steady-state.

I am going to try the "pstab" analysys as suggested by Andrew.

Bye,
Cri

Title: Re: Strange problem in AC/PAC analysis with Spectr
Post by Cri Azzolini on Dec 9^th, 2005, 1:11am

Andrew,

in your last reply to my post, you talked about a periodical stability analysis (pstb). Actually I am not able to find it in Spectre5.033: is it an analysis added in the 6.0 version?
What do you mean with "engineering option"?

Thanks,
Cri

Title: Re: Strange problem in AC/PAC analysis with Spectr
Post by Andrew Beckett on Dec 12^th, 2005, 12:26pm

My mistake; I thought it had made it into MMSIM60 USR2 - but currently it's only available to selected customers rather than it being generally released. We tend to do this approach to allow something to be selectively introduced rather than releasing something before it's been properly validated.

Andrew.

Title: Re: Strange problem in AC/PAC analysis with Spectr
Post by Frank Wiedmann on Dec 15^th, 2005, 1:38pm

If in the meantime you would like to set up some kind of pstb analysis by hand, you might want to take a look at the thread starting at http://www.designers-guide.org/Forum/?board=power-electronics;action=display;num=1107716368;start=0 (please note that the thread spans 3 pages).

Title: Re: Strange problem in AC/PAC analysis with Spectr
Post by Ken Kundert on Nov 2^nd, 2006, 7:57pm

The paper I cited in my earlier post has moved. You can now find it at http://www.kenkundert.com/docs/cd2001-01.pdf.

-Ken

Title: Re: Strange problem in AC/PAC analysis with Spectr
Post by Andrew Beckett on Dec 12^th, 2006, 2:57pm

pstb is in MMSIM61, by the way.

Regards,

Andrew.