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Simulators >> RF Simulators >> Convergence
https://designers-guide.org/forum/YaBB.pl?num=1208694646

Message started by bobbie on Apr 20^th, 2008, 5:30am

Title: Convergence
Post by bobbie on Apr 20^th, 2008, 5:30am

This a broad topic but can someone shed some light on what would be a "systemic approach" to dealing with convergence simulating RF circuits. While convergence can be related to transistor modeling and circuit wiring and values, which produce discontinuities, what kind of things can be done from an Spectre/SpectreRF analysis options point of view to start a simulation that is having trouble, or avoiding false convergence? I guess this could be categorized by circuit type and analysis (for example oscillators have special considerations).

Title: Re: Convergence
Post by Ken Kundert on Apr 21^st, 2008, 7:05pm

The first thing to do is to ignore all of the advice Cadence gives on setting the tolerances tighter than the default. I have used SpectreRF since before it was released, and I always start the default tolerances and they are sufficient almost all of the time. Most to the time when people complain about convergence, I strip off the tight tolerances and the convergence problems go away. And the simulation run much faster.

Second, keep an eye on the convergence norm. If it drops steadily until close to one (one represents convergence), but then stalls, it suggests that your tolerances are too tight.

Third, make sure your circuit is periodic. The easiest way to do this is to turn on strobing and strobe at the period of the fundamental frequency. If you then plot the strobed waveforms they should become constant valued. If they don't, if there is some noise or oscillation, then your circuit is not periodic and PSS will not converge.

Fourth, increase tstab. If you have plotted the strobed waveforms, set tstab to the time when they are close to being constant valued.

That is the process I use, and I very rarely have convergence problems with SpectreRF.

-Ken

Title: Re: Convergence
Post by RFICDUDE on Apr 22^nd, 2008, 4:06am

But there is always that somewhat fuzzy line between convergence and accuracy.
Can you comment on the accuracy, especially for IP3 measurements, of just staying with default settings?
Approaches I've seen in the past are to crank down on the tolerances until the measurement results don't change and then hope you don't have convergence problems.

Comentary - After working with RF and ICs for 20 some years, I find solving (steady state) simulator convergence issues more of a black art than RF design. While the simulation engines have greatly improved convergence performance, they have not gotten much better at providing analysis and specific feedback as to the nature and probable causes of the convergence issues. Spectre spits out a list of things to check and if that doesn't help then you are out of luck (back to trial and error troubleshooting - spray and pray).

Title: Re: Convergence
Post by Ken Kundert on Apr 22^nd, 2008, 2:48pm

I use the small-signal IP3 method described in http://www.designers-guide.org/Analysis/intercept-point.pdf and have never had a problem with accuracy.

-Ken

Title: Re: Convergence
Post by RFICDUDE on Apr 22^nd, 2008, 8:46pm

I should have mentioned that most trouble applications I have encountered are class AB transmitters operating at levels where small signal IP3 isn't valid.

Title: Re: Convergence
Post by Ken Kundert on Apr 23^rd, 2008, 12:21pm

Why would the small-signal approach to IP₃ not be valid on class AB transmitters?

-Ken

Title: Re: Convergence
Post by RFICDUDE on Apr 23^rd, 2008, 7:58pm

For the driver amplifier the linearity at rated output power is being optimized. Typically the IM3 is being optimized near compression where the IM3 slope is not 3:1 (i.e. the third-order coefficient is not dominant). Higher order terms and second-order interaction contributions to third-order response matter considerably when evaluating the performance.

I could have someone take a look at how the technique compares with PSS two-tone simulations for a CMOS transmitter and let you know the difference.

Title: Re: Convergence
Post by Ken Kundert on Apr 23^rd, 2008, 8:58pm

Even order terms play no role in IP₃ as they generate responses that are at the wrong frequencies. The high order odd terms could play a role, but generally IP₃ is not used when they are significant. That is usually ACPR or EVM territory.

-Ken

Title: Re: Convergence
Post by pancho_hideboo on Apr 24^th, 2008, 12:33am

Sometimes we have to apply true two large tone input drive simulation for IM3 evaluation.

See http://www.designers-guide.org/Forum/YaBB.pl?num=1190971685/9#9

Envelope changes which DUT undergoes are different between two tone drive and one tone drive.
This difference could cause different AM/AM and AM/PM characteristcs, for example, due to memory effects which is well known in power amplifier.

Title: Re: Convergence
Post by RFICDUDE on Apr 25^th, 2008, 5:26am

A modulated carrier input to a cascade of two second-order nonlinearities generates a third-order intermodulation term which is typically denoted as a "second-order interaction" contribution to third-order distortion.

If the low frequency baseband terms of the first source of second-order distortion experience a filter function then positive and negative baseband frequencies will have a complex conjugate relationship. When the filtered baseband tones remix with the desired signal through the subsequent second-order nonlinearity, third-order intermod products are generated which have conjugate symmetry. These second-order interaction products can add in and out of phase with the primary third-order products producing the classic asymmetric third-order intermodulation terms which are indicative of the so called "long term memory" effects in RF power amplifiers.

I can post a more elaborate expanation; however, there is a good reference on the topic with clear derivations.

K. A. Remley, D. F. Williams, D. M. M.-P. Schreurs, and J. Wood, "Simplifying and Interpreting Two-Tone Measurements," IEEE Trans. Microwave Theory and Techn., vol. 52, no. 11, 2004, pp. 2576-2584.

Sections III.B and III.C discuss the generation of third-order products from second order interaction with baseband and second harmonic products.

Title: Re: Convergence
Post by RFICDUDE on Apr 25^th, 2008, 10:17am

Ken,
You are absolutely correct that 2nd order interaction is mostly relavent to large signal nonlinearity which mostly impacts ACPR and EVM. My original question on convergence stems from convergence issues for IM/ACPR simulations operating near compression and the relationship between accuracy and tolerances for these simulations.
I wasn't clear in stating this above.
Thanks