Re: Modeling - Fast Tr

Dr. Edward P. Sayre (esayre@nesa.com)
Thu, 5 Sep 1996 15:58:21 -0400

I do not agree that s-parameter is the best approach since you rarely know
what bandwidth to consider the s-parameters over. The microwave idea of a 3
dB bandwidth has little or no meaning when one is considering the non-linear
nature and threshold of digital drivers and receivers. Based on many years
of high performance interconnect design for digital systems, I agree with
Fabrizio Zanella's approach. It is accurate, and if properly done, does not
ignore coupling or any other crosstalk effects and is suitable for use in
either IBIS or SPICE simulations.

The choice of how big to make the subsections is entirely dependent on the
"electrical size" of the interconnect and can be directly related to the
risetime of the signals. The rule of thumb of 10 time samples per risetime
translates into something like a small fraction of a wavelength of a fifth
harmonic of the fundamental. Those of us with a background in fields know
this corresponds to a "electrically small" object in the frequency domain.
Measurements directly compared to theory shows this to be an excellent
approach. On the cautionary side, one does not want to over sample the
connector. More is not always better since the Green's functions describing
the connector may not converge well of the subsection length is too short.
The rule of thumb stated above works for most field simulators and has the
theoretical basis for being correct.

ed sayre

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