Jason -- your deduction based on the earlier posts is essentially correct, that once a feature is electrically longer than some fraction of your risetime, you will likely need to use more than a single L and a single C to represent it. The actual fraction varies according to a number of factors, the most important being what your accuracy requirements are. Another is the shape of your signal transition, since this affects the frequency content it contains. There is also the nature of the feature you are modeling -- some types of interconnects behave more like a few L's and C's than like a transmission line, down to a surprisingly fast risetime. So there's really no hard number that is right for every application. We tell our customers 1/6 as a generic rule of thumb to use as a starting point, but to get it right you have to decide based on your application. As was stated in earlier posts, the best way to answer your question with certainty is to take some measurements and compare with simulations, and decide what you can tolerate. If you can't get measurements, at least run some simulations and see if they hold up to a sanity check. You can compare various segmentation schemes, and also compare against truly distributed models. This is a good way to build some confidence in the validity of the engineering approximations that we all make when it comes time to model something for simulation. On a somewhat peripheral note, a clarification of terms: in common usage, a long transmission line would be called an "interconnect" just as much as a shorter piece of interconnect would be. I won't even attempt a rigorous definition of the term, since that Pandora's box should be the subject of a separate thread, which I have no intention of launching... -- 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 ------------------------------------------- Jason D Leung wrote: > Hi all, > Earlier we have some discussions about the lumped model and distributed > model, and thanks for everyone who has participated the > discussions.However, I still have a question wanted to ask ,when should > we say a interconnect is not interconnect any more , but indeed a > transmission line model. > ( it would be nice if I can get a clear picture ,such as a formula) > Because in the previous threads ,if the time delay of a interconnect is > longer than 1/10 of the rise time, we should use the distributed model > to represent that transmission line. > > > happy holidays > thanks again > Jason Leung > > > > ------------------------------------------------------------------ > 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 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 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