Hello everybody,

Recently, I got problems with convergence in PSS simulation lots of times. Then I read the manual of "Spectre Circuit Simulator Reference" Product Verion 5.0 on June 2003, pp718-731. I feel confused with certain point.

By the way, I am simulating a fold-cascode opamp with SC-CMFB with layout extraction, plus a common biasing circuit with layout extraction.

On this manual page 725 bottom, it says "One can set "steadyratio" to a smaller value to activate a tighter tolerance for the interative linear solver". But when I looked at page 726 bottom, which list the effect of "errpreset" on some parameters for driven circuits (which I posted here):

errpreset      reltol       relref            method      lteratio steadyratio noisefloor
liberal 1e-3 sigglobal traponly 3.5 0.001 -70dB
moderate 1e-3 alllocal gear2only 3.5 0.001 -120dB
conservative 1e-4 alllocal gear2only * 0.01 -200dB

where the "*" explains as  : lteratio=10.0 for conservative errpreset. Only if user specified reltol <= 1e-4 * 10.0/3.5,
it will change to lteratio=3.5.

My questions are:
1. I assume steadyratio=0.001 is a smaller value in errpreset=liberal or moderate than steadyratio=0.01 in conservative. But I thought errpreset= conservative is more tightened option, which is more accurate. It looks there is confliction between "tightening" and "loosening" in "steadyratio". Maybe I am just wrong. Please advise.

So, if I want to loosen "steadyratio", which is one of 6 suggestions in my spectre.out when my PSS simulation was failed with convergence. Which way I should set it if I pick "errpreset=conservative"? to go with 0.001 (or even smaller) or go beyond 0.01 (or even 0.1)?

2. Same point with "lteratio". I notice that "lteratio=10" for "conservative" case, and "lteratio=3.5" for liberal and moderate cases.

So, if I want to "tighten the normal simulation tolerances (maxstep, reltol, lteratio, and errpreset), which way for the value of "lteratio" should I go to resolve my convergence problem?
by the way, I already picked "errpreset=conservative" and it displayed in spectre.out that "lteratio=10", "reltol=100e-06".

3. Does the "finitediff" option give some difference in convergence with its three options (no, yes, refine)?

4. Occasionally, I met a bizarre phenomenon with simulation in certain cases.
Within one identical simulation setup, I got one simulation results which looked okay. then I re-run it (I didn't change anything on testbench and layout), the results just looked weird. If I clicked to run it again, it even displayed that some junction currents of the certain transistors exceeded "imelt" ...

So, what reason could be for this?

Appreciate your help.

ethan

3. There is one thing you can try to see if finitediff can help convergence.

You want to set steadyratio to a relative big number. That is loosening like Ken said. Try steadyratio=1. At the same time, you need to set finitediff=yes. Hopefully it converges.

Finite difference is run after shooting-Newton. Steadyratio controls shooting-Newton but not finite difference. By setting steadyratio=1 you give shooting-Newton a better chance to converge. However, the result may be inaccurate. Finite difference will make the result more accurate.

Baolin

Dear Ethan,

One reason is that when highorder=yes or finitediff=yes, the finite difference method is used which is almost like requiring steadyratio=0 in the shooting-Newton method. SpectreRF had a harder time converging because the convergence requirement became strict.

On the other hand, SpectreRF can converge with loose requirements. However, it seems like the accuracy took a hit.

Right now probably only the SpectreRF R&D can solve this problem.

Baolin