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Tip of the Week: Guidelines for simulating oscillators - phase noise simulations

By Tawna Wilsey on August 26, 2008Comments(5)Filed under: RF design, Virtuoso Spectre, Spectre, Spectre RF, Virtuoso Spectre Simulator XL

When simulating oscillators, it is important to choose the correct simulator engine (shooting Newton vs. harmonic balance.)  In general, we suggest that you use the HB (harmonic balance) engine as your first choice.  In addition, there are situations where you may need to use the augmented=yes pnoise option. 

 For LC type oscillators (mildly nonlinear), we strongly suggest using the HB engine.  For ring oscillators (or circuits that contain digital elements such as dividers), the HB engine may have trouble converging - and then should you use the shooting engine.


Below is the pnoise strategy that we suggest you use when simulating oscillators with SpectreRF:

1) Run PSS with the harmonic balance (HB) engine.  If it converges,

--goto step 2),

--else, goto step 3).
 
2) Run "normal" HB pnoise (i.e. do not use augmented=yes pnoise option).

--If the results are as expected, then you are done.

--If the results are not good (or you get "WARNING: Abnormal PPV value found, fix it" or other messages indicating your solution may have problems), try "augmented" HB pnoise.

--If that fails, goto step 3).
 
3) Try PSS using the shooting engine.  

--If it doesn't converge, contact Cadence Customer Support (http://sourcelink.cadence.com).  We will likely need to file a CCR, and if so we'll also need a testcase. 

--If shooting PSS converges, goto step 4).
 
4) Try "normal" shooting pnoise (i.e. do not use augmented=yes pnoise option).

--If the results are as expected, then you are done.

--If the results are not good (e.g. you get Floquet warnings or other warnings indicating that your solution may have problems), try "augmented" shooting pnoise.
 
5) If "augmented" pnoise fails in 2) and 4), contact Cadence Customer Support (http://sourcelink.cadence.com).  We will likely need to file a CCR, and if so we'll need a testcase.

 

For more information on augmented pnoise, see the http://sourcelink.cadence.com solutions:

11459288 Shooting PSS+Pnoise phase noise results
differ significantly from HB PSS+Pnoise results.
11459258 Warning:  Abnormal PPV value found, fix it.




Comments(5)

By maverick on January 26, 2009
I am getting a similar warning about 'abnormal PPV value' and my pnoise simulations fails when trying to simulate a cross-coupled LC tank oscillator.

I am using MMSIM62 with a ADE-L option and am not able to see this 'augmented' option in the pnoise form. Is there some way or a special setting to enable this option to show up in the simulator?

Thanks...


By Tawna on February 13, 2009
Hi Maverick,
First, for your type of oscillator, I recommend using the Harmonic Balance Engine.  Augmented pnoise is likely not available in MMSIM6.2 - it was added in one of the later ISRs of MMSIM 6.2.1 for the HB engine and in 7.0.1 for the Shooting engine.    I recommend downloading the latest version of spectre from downloads.cadence.com - at a minimum MMSIM 7.0.1, but I strongly recommend MMSIM 7.1.     If you continue to have issues with this, please file a Service Request using Sourcelink.cadence.com and one of the AEs on staff will be able to help you.
Best regards,  Tawna

By RizwanAhmed on November 5, 2009
Is there a step by step tutorial about "how to simulate divde-by-2" ?

By Tawna on November 16, 2009
Hi RizwanAhmed,
http://support.cadence.com solution 11571722 may be what you are looking for:
Guidelines for simulating frequency dividers in spectreRF
best regards,
Tawna

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