[SI-LIST] Re: Time domain AMS-SI tools

Hi Mark:

I think you are on the right track with the notion of "higher"
parasitics, but it might help to formalize this notion by looking at the
frequency-domain description in more detail.

To get all the complexity of an arbitrary waveform ("unknown" rise time,
fall time, etc.) signal, you will at least have to start by assuming
that the signal is somewhat bandwidth-limited so that you do not have to
worry about the signal going from "DC to daylight".  So, let's assume
that your data rate is of the order of 2f and that you get 2 bits per
cycle, so the fundamental frequency of your signal is f.  Furthermore,
lets assume that your rising and falling edge are 0.1/f.  If you do a
Fourier analysis of this you will see significant signal content out to
tens (or hundreds) of f, depending on where your noise floor or
crosstalk spec is.

So the solution is to create a model which contains all of these
frequencies...all the harmonics of your fundamental, rising edge, and
falling edge.  This would give you a model that exists at many more
frequencies than just the fundamental, the rising edge, and the falling
edge.

It is relatively straightforward to get such a model, as was pointed out
earlier, by doing some sort of EM analysis.  The problem comes when
taking the results of the EM analysis, normally in the form of
s-parameters, and getting them into a time-domain tool.  Two of the
problems associated with this are the fact that s-parameters in
time-domain tools can sometimes violate causality and they may not be
passive (due to interpolation errors).  Simulation times can also be
excessive when the time-domain simulation starts going into
convolutional technqiues to handle the distributed effects.

There are methods using rational approximations which can minimize these
issues and make it much easier to bring broadband, distributed models
into time-domain tools.  You may want to look at the following and the
listed references:

Transient Simulation at RF
http://www.appwave.com/src/news/articles/AR0503.pdf

Hope that helps,
MikeH

Dr. Michael Heimlich
Director, University Programs
Applied Wave Research, Inc.


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Mark Burford
Sent: Wednesday, November 30, 2005 8:08 AM
To: Andrew.Burnside@xxxxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Time domain AMS-SI tools=20

OK so the tools for the most part I assume now to use FEM and a
frequency is perhaps used to allocate mesh spacings, higher =3D
freq=3D3Dsmaller mesh=3D3Dmore accurate simulation.  So can anybody tell =
me
exactly how the tool would deal with and allocate say resistances /
losses as a random (and when I say random I mean the tool doesn't know
what is coming) signal passes through the model with varying edge
rates.=3D20

The interconnect for example would cause a slower rise time of the
signal if the signal had a faster edge rate as the faster edge rates
would be attenuated more due to skin effect as one of the hindrances to
the signal. How would the tool deal with a fast edge and a slow edge in
the same simulation and hence be able to show ISI for example. I know
most time domain tools allow ISI to be explored but how can varying edge
rates be accommodated in a simulation that is only aware of a single
given frequency?

Surely more (higher) parasitics need to be modelled and simulated for
faster edge rates and less (lower) parasitics need to be modelled and
simulated for slower edge rates.

How do tools cope with this?

Mark

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Andrew Burnside
Sent: 30 November 2005 12:53
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Time domain AMS-SI tools=3D20

Mark.
:As is initial stab at understanding to include such effects one must
:tell the tool at which frequency we would like the simulation run and
an :internal 2D field solver will allocate appropriate parameters for
skin :effect etc at that frequency and then run waveforms through the
model.

The frequency is often used to set the mesh spacing of the 2D field
solver, as this is a finite element process.
Obviously, smaller mesh sizes take longer to calculate. Also, knowing
the frequency will allow enable the use of low frequency approximations
where appropriate e.g. remove small parasitics.

Regards

Andrew


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