[SI-LIST] Re: Long simulation diverges
- From: Steve Corey <steve@xxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sun, 14 Apr 2002 23:35:42 -0700
Hassan -- I'm not a SigXplorer user, but I do have some experience in
simulation algorithms and modeling. These types of problems typically
arise from bad data passed into a model, or from a bad algorithm (or
algorithm implementation) in the simulator. In either case, error
builds up throughout the course of a transient simulation, and at some
point takes over the results. Fortunately, since the simulator is not
bound by any energy constraints, the numbers usually get so huge that
the error is very noticeable.
By running the same simulation with a lossless line, you have for the
most part ruled out the possibility of a bug or errant parameter in one
of your driver models.
If the transmission line model is parameterized (analytical equations
for RLGC parameters), and your parameters are within acceptable bounds,
then I would suspect the simulator and go to tech support. Transmission
line modeling and fast simulation in the presence of nonlinear devices
is a subject of continuing research and development, and these types of
problems are not terribly uncommon.
If the model is one in which you are supplying RLGC data points to the
simulator, I would start by ensuring that the RLGC values you are
supplying correspond to a physically realizable transmission line
system. The ability to supply RLGC data points to a transmission line
model is useful, but can come back to bite if used inappropriately. The
associated math is fairly involved, so again, I would run them past tech
support. If the data is good, then it points to a bug in the simulator.
As far as workarounds, if there were a bug in the simulator, I would
start by adding some loss -- either inline resistance or conductance to
ground -- which can often damp nonphysical numerical oscillations. If
you're worried about introducing error with the loss, design a filter
which only applies the loss at the frequency at which the explosive
oscillations are occurring. (You may, however, only end up finding
another oscillating mode at another frequency which manifests itself
later in the simulation record.) These are just ad hoc
grad-student-type tricks which certainly don't guarantee success in one
case much less all. Since, unlike graduate labs with donated software,
you have actually paid for it, I would work through tech support.
Good luck,
-- Steve
-------------------------------------------
Steven D. Corey, Ph.D.
Time Domain Analysis Systems, Inc.
"The Interconnect Modeling Company."
http://www.tdasystems.com
email: steve@xxxxxxxxxxxxxx
phone: (503) 246-2272
fax: (503) 246-2282
-------------------------------------------
Hassan O. Ali wrote:
>
> I'm using SigXplorer 14.1 to perform a very long simulation: 25,000 cycles
> with a 2.5GHz
> periodic (101010...) stimulus (period = 400ps). I have observed that the
> solution
> diverges badly (voltages in the order of kilovolts!) after the first 200ns
> (500 cycles).
>
> The circuit is very simple: just an ideal 3.3Vpp voltage source with 0-100%
> rise/fall
> time of 10ps connected to a 50ohm load through a 2-inch 50-ohm microstrip
> line. The
> microstrip line, however, is specified to be frequency-dependent for up to
> 18GHz. The
> same simulation runs fine if I use an ideal 50ohm transmission line (TLine).
>
> Has anyone encountered the same problem and found a solution for it?
>
> Regards.
>
> Hassan.
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