FreeLists / si-list / [SI-LIST] Re: transmission line model

[Steve Corey <steve@xxxxxxxxxxxxxx>]

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
 >
 >
 >
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 >


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