[SI-LIST] Re: [SI-LIST]: Which tool is the best - LINPARdisc ussion

Steve,
My question is, if the approximation is bad, what does it exhibit itself in
the simulation ?
If it results in "internal time step too small" or crazy oscillation, I know
immediately the model has problem and move on. If however, it generates the
wrong waveform with incorrect delay, I will be very worry.
Interconnect model impact on simulation memory and time is not a concern for
me, I believe my I/O circuits and package models dominates that by far.  

-----Original Message-----
From: Steve Corey [mailto:steven.corey@xxxxxxxxxxxxxx]
Sent: Friday, April 23, 2004 8:43 AM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: [SI-LIST]: Which tool is the best - LINPARdisc
ussion


Ray -- although your statement is correct, it is numerically difficult 
to achieve.  Not all algorithms are robust enough to fit long, nearly 
lossless delays.  Furthermore, simulations of electrically large, nearly 
lossless systems take longer as well.  How to extract delay and treat it 
separately when fitting such data is a current area of research.

Of course, this phenomena isn't isolated to rational approximations, but 
is a general problem with stiff systems -- those which have both 
microscopic and macroscopic behaviors excited simultaneously.  A long 
cable with little loss driven with a fast-risetime signal is a good 
example.  As a broad generalization, the wider the range of behaviors 
being simulated, the more memory and time that will be required to 
simulate it.

Different simulation and modeling techniques are optimized for different 
types of systems.  For example, good distributed transmission line 
models are often better for simulating long transmission lines at fast 
risetimes than are straight rational approximations.

   -- Steve

-------------------------------------------
Steven D. Corey, Ph.D.
Time Domain Analysis Systems, Inc.
"The Interconnect Analysis Company."
http://www.tdasystems.com

email: steven.corey@xxxxxxxxxxxxxx
phone: (503) 246-2272
fax:   (503) 246-2282
-------------------------------------------


Raymond Anderson wrote:
> Raj Raghuram wrote:
> 
> 
>>>I am not sure modeling with S-params would work for long transmission 
>>>lines i.e. metres in length. Most s-parameter simulators use a rational 
>>>fit which in the end is a lumped model. Maybe you can comment on this.
>>>   
>>>
>>
> 
> 
> The delay information required to model an electrically long structure 
> such as a transmission line is contained in the phase information of the 
> complex s-parameters.
> 
> Most instruments output the phase of s-parameters in the range of +180 
> to -180 degrees.  Plotted wrt frequency it resembles a sawtooth. This 
> modulo 360 phase info needs to be unwrapped into a linear phase 
> progression to interpret it as delay.
> 
> When a rational polynomial approximation is fitted to the unwrapped 
> s-parameter data, if the phase part of the approximation is the same as 
> the phase of the original s-parameter then I'd expect the resultant 
> macromodel to exhibit the same delay characteristics.
> 
> Comments ???
> 
> 
> -Ray Anderson
> 
> Sun Microsystems Inc.
> 
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