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Simulators >> RF Simulators >> Questions on phase noise simulation
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Message started by jlee on Dec 23^rd, 2007, 6:39pm

Title: Questions on phase noise simulation
Post by jlee on Dec 23^rd, 2007, 6:39pm

Dear all,

I) When PSS+Pnoise is used to simulate oscillator phase noise, some warnings are shown in the output log:

1. Warning from spectre during PNoise analysis `pnoise'.
"Internal (bsource)" 0: The Floquet eigenspace computed by Virtuoso(R) Spectre(R) PSS analysis appears to be ill-defined. PNOISE computations may be inaccurate. Consider re-running the simulation with smaller reltol, different tstab(s) and method=gear2only'. Check the circuit for unusual components."

2. Warning from spectre at freq = 10k Hz during PNoise analysis `pnoise'.
"Internal (bsource)" 0: Relative residual of linear system is significantly above tolerance. Results may be inaccurate. To get more accuracy, you can set maxacfreq (PSS option) to a value greater than 40*fundamental (default value). Or set the solver method in your Periodic small signal analysis option to "std" (standard).

My question is
a. What is the meaning of these warnings?
b. Why simulator shows these warnings?
c. Some suggestions are given in these warning, such as "set maxacfreq (PSS option) to a value greater than 40*fundamental (default value)" , why these suggestion can solve the problems?

II) Another problem is when simulating phase noise of oscillators, I can always see the message in the output log:

"The estimated phase-noise saturation frequency is 2.456787e+01 Hz"

My question is
a. What's the meaning of "saturation frequency " ?
b. I have observed that the smaller the saturation frequency is, the better the phase noise in 1/f^3 is. Is there any relation between these two factors?

Thanks

Jason Lee

Title: Re: Questions on phase noise simulation
Post by sheldon on Dec 23^rd, 2007, 7:43pm

Jason,

In answer to Question 2, hopefully, I will get the nomenclature correct:-)

When plotting L{jw}, the spot noise power of the output voltage, the power
of the phase noise can not exceed the power of the carrier, so at low frequency
the spot noise power, L{jw}, flattens out. The saturation frequency is the
frequency at which the L{jw} stops increasing. Since the area are under the curve
is always unity, that is, L{jw} is normalized to the carrier power, the lower the
phase noise is the lower the value of the saturation frequency is.

Best Regards,

Sheldon

Title: Re: Questions on phase noise simulation
Post by jlee on Dec 23^rd, 2007, 8:22pm

Got it! Thanks for your useful answer, Sheldon. :D

Regards

Jason