On Sun, 08 Dec 2013 10:33:44 -0800, John Larkin
wrote:
On Sun, 08 Dec 2013 10:41:37 -0700, Jim Thompson
wrote:
Anyone have a decent Spice model for a twisted pair... lossy, not the
ideal model in circulation?
...Jim Thompson
There's enormous variation in twisted pairs, even among "standard" things like
compliant CAT5 and CAT6 cables.
The Spice lossy delay line implements The Telegrapher's Equation, but ignores
skin loss variation with frequency. The best thing to usually do is get a chunk
of the actual cable of the expected length and note its step response on a
scope, then fiddle the Spice lossy line parameters to match. An extra RC or two
sometimes helps fine-tune the model to reality.
Multi-pair cables, like CAT5/6, can be expected to have various amounts of
crosstalk and pair delay skews, too.
https://dl.dropboxusercontent.com/u/..._Risetimes.pdf
https://dl.dropboxusercontent.com/u/...tonewall_2.JPG
https://dl.dropboxusercontent.com/u/...P_CAT6A_Tr.pdf
Simple RC equalization helps, if the cables are repeatable.
Thanks! Good collection of data there in your examples!
I do remember doing compensation BC (before CAD) using a pre-emphasis
leading-edge pulse... on a thousand foot chunk ;-)
I'll look at how PSpice does their lossy T-line, and maybe I can
adapt. I did some lecturing on skin-effect modeling for Intel eons
ago... I'll review my notes.
...Jim Thompson
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