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. > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > > > -- ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu