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Simulators >> RF Simulators >> accuracy of phase noise sim
https://designers-guide.org/forum/YaBB.pl?num=1121371925

Message started by pletch on Jul 14^th, 2005, 1:12pm

Title: accuracy of phase noise sim
Post by pletch on Jul 14^th, 2005, 1:12pm

Hi,

I'm simulating phase noise for a pretty standard LC VCO and I get the following warning from the pnoise analysis:

**********************************************************************
Periodic Noise Analysis `pnoise': freq = 3.03189 GHz + (1 kHz -> 1 MHz)
***********************************************************************
Using the operating-point information generated by PSS analysis `pss'.

Warning from spectre at freq = 1 kHz during PNoise analysis `pnoise'.
Relative residual of linear system is significantly above tolerance.
Results may be inaccurate.

pnoise: freq = 1 kHz (0 %), step = 0 Hz (0 %)

The pss analysis seems to complete fine and the results seem reasonable. I have tried tightening simulator tolerances, increasing maxacfreq, and the standard pnoise solver and oscsolver modes (instead of turbo). I still get the warnings at almost every frequency step out to about 1MHz.

I'm not sure if I can trust these results or not because of the warning. Can anyone recommend a course of action to correct the warning? The message seems to suggest I need to increase tolerances, but I don't want inaccurate simulation results.

Thanks,
nate

Title: Re: accuracy of phase noise sim
Post by Ken Kundert on Jul 14^th, 2005, 5:37pm

Try sweeping from 1MHz to 1KHz.

-Ken

Title: Re: accuracy of phase noise sim
Post by pletch on Jul 15^th, 2005, 9:09am

Hi Ken,

Thanks for the tip. Unfortunately, I get the same result in reverse. The first couple of points at larger offsets (>1MHz), have no warnings. However, pnoise still generates the same warnings from 1MHz on down.

I tried more points per decade and starting from 10MHz and 1MHz, all with the same result. Any other possibilities I might try? This one's a tough one because the warning doesn't give any clear path to resolution, at least for me.

Thanks again for your help.

nate

Title: Re: accuracy of phase noise sim
Post by Ken Kundert on Jul 15^th, 2005, 5:05pm

My recommendation would be to report the problem as a bug and then send the circuit to Cadence to see if they can figure out what is going wrong.

-Ken

Title: Re: accuracy of phase noise sim
Post by pletch on Jul 15^th, 2005, 5:16pm

Sounds good Ken. I'm also pursuing that avenue but I thought I would post here in the meantime, just in case it was a simple fix.

Thanks,
nate