Extract small-signal parameters at a dynamic operating point during .TRAN

[Renaud GILLON]

.AC analysis usually starts with computing a DC operating point.

However in some cases, it is necessary to extract AC parameters at a dynamic operating point reachable only with a .TRAN simulation.

Several commercial simulators offer options allowing to run AC analyses at selected time-points of a transient analysis. For some of them you have to disable the DCOP associated with the AC analysis explicitly, for others it is by default in this case.

From the XYCE documentation, it looks like it is not possible to stop a .TRAN, save the dynamic operating point and use it to linearize the circuit and run the AC analysis.
Can you confirm ? Any idea how to do this ?

The extraction of small-signal matrices is very usefull to investigate stability of NL systems, or to characterize NL blocks to build behavioral models, etc.

[xyce-users]

Hi Renaud,

Unfortunately, there isn't an easy way to save an operating point from a transient time point in Xyce.  We should probably add it, and I'll add it to our issue tracker.

I can see where this would be useful for an AC calculation.

thanks,

The Xyce Team.

[Marcel Hendrix]

On Monday, December 7, 2015 at 12:36:23 AM UTC+1, xyce-users wrote:

Hi Renaud,

Unfortunately, there isn't an easy way to save an operating point from a transient time point in Xyce.  We should probably add it, and I'll add it to our issue tracker.

I can see where this would be useful for an AC calculation.

It does not work for some important  classes of non-linearity.

E.g., for switching circuits the best you can hope for is a representation

of the linear system that corresponds to the switch configuration in

effect for calculating the DC operating point.

-marcel

[Renaud GILLON]

Hello all,

I would like to come back on the initial request for getting the ability to :

(1) stop a .TRAN at some time-point, store the dynamic operating point;

(2) load the dynamic operating point and run an .AC analysis directly from that operating point, not attempting to run a .DC analysis (as is the default now).

My impression today is that point (1) is now covered, however the .AC command has no "UIC" or "NOOP" option allowing to disable the default .DC analysis that is usually coupled to it.

The extraction of AC matrices at operating points (either static ones after a successful .DC analysis, or dynamic ones obtained from a .TRAN analysis) is a very powerful tool to characterize the non-linear response of a circuit (by the collection of a set of such matrices at various OP's). It allows to build data-driven behavioural models, it allows to perform fault grouping for analogue circuits and identify their failure modes. 

Personally I have been using this method of extracting AC matrices in many commercial simulators (SPECTRE, ELDO, ADS, HSPICE) and find it quite frustrating that because (2) is apparently not available, the method can not be used in Xyce. So I would like to request the addition of a "UIC" or "NOOP" option on the AC simulation that would then be allowed to run from the last known operating point, for example loaded by .IC command. The essential feature being that no .DC analysis would be run implicitly before the linearization step for the .AC command, so that the impact of eventual dynamic currents on the OP are not lost.

Looking forward to receive your feedback,

Regards,

Renaud

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