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Simulators >> Circuit Simulators >> inconsistance between simulations
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Message started by Aigneryu on May 4^th, 2007, 4:08pm

Title: inconsistance between simulations
Post by Aigneryu on May 4^th, 2007, 4:08pm

Hi,

Recently while I use spectre6.1.1 to simulate an osc with PSP model, I found the results can vary in a very strange fashion. My testbench is like following:

1. I use "width" as a variable for transistor width of which the default value is 10 (um).
2. Use parametric to sweep the width from 6 to 16 with step of 2.
3. I found the results of "width=10" are different in 1. and 2. And the difference can be up to 2 dB.

I did make sure the osc has enough time to settle, and set the accuracy requirement to 1e-6 relative, and 1e-8 absolute. I also check the netlist directly, but cannot find any discrepency.

Does anybody have any clue this kind of problem could happen? Maybe this is due to the computer or other things rather tahn simulator?

Sincerely,

Title: Re: inconsistance between simulations
Post by Aigneryu on May 4^th, 2007, 8:31pm

I found something: if I do "killall spectre" everytime before I run the simulation, the result will be the same. I tried this one different machines with different version of linux, they are all like this. And also, this technology has both BSIM and PSP, only PSP model has this problem. Does that mean this version of MMSIM has some bugs?

Sincerely,

Title: Re: inconsistance between simulations
Post by Andrew Beckett on May 7^th, 2007, 4:21am

It's quite possible (especially with an oscillator) to have a sensitive dependency on the initial conditions. When you're doing a sweep, the dc solution will start from the previous solution.

You might want to ensure that the transient or pss analysis you're using has "restart=yes", and also set the simulation option "rebuildmatrix=yes" (I think that's the parameter - I don't have access to spectre at the moment). "spectre -h options" will tell you what it's called.

Regards,

Andrew,

Title: Re: inconsistance between simulations
Post by Scott Flinchbaugh on May 7^th, 2007, 10:53am

All,

The Spectre options parameter is "rebuild_matrix=yes".

Thanks,
-scott

Title: Re: inconsistance between simulations
Post by Andrew Beckett on May 8^th, 2007, 7:10am

Ah, close but no underscore...

Thanks Scott for correcting my memory!

Regards,

Andrew.