[SI-LIST] spice math question
- From: "John Ellis" <jellis@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 30 Oct 2002 09:07:35 -0500
I've got a question about how spice mathematically handles long transmission
lines. I was recently comparing the output of different spice structures
for a 40" long transmission.
For modal structures, I found that a single modal element produced a delay
that was a little fast for the structure. The signal arrived about 300 ps.
before it should have. At 158 ps/inch I was expecting a delay of 6.32 ns.
The result was 6.02ns. When I cascaded 10 4" lengths the delay was more
accurate, giving me a believable 6.38ns.
I saw this same effect when I modeled with W-elements as well. I vaguely
remember an app. note on W-elements describing something akin to this.
I then looked at ladder models for the same structure and found the same
slightly short delay. For this structure I cascaded >750 ladder elements to
build the model.
There seems to be some numerical inaccuracy native to spice. These results
were seen in Hspice and Berkeley spice as well.
Is it rounding error or something else?
Thanks,
John Ellis
Vice President
Interconnect Technology
TriCN, Inc.
3605 Vartan Way
Suite 301
Harrisburg, PA 17110
Phone:717-657-1002
Fax: 717-221-1185
email:jellis@xxxxxxxxx
www.tricn.com
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