[PCB_FORUM] Diff pair phase tolerance
- From: "Jean Bratton" <jean.bratton@xxxxxxxxxxxxxx>
- To: <icu-pcb-forum@xxxxxxxxxxxxx>
- Date: Mon, 18 Apr 2005 13:17:34 -0700
Hi all,
We're running into something and would like to know if there's a better
way of handling this...
We have a diff pair going to 4 pins. Lets call the pins A+, B+, C+, D+
on the one net and A-, B-, C- and D- on the other net of the pair. The
pin escape lengths at the A+/- C+/- and D+/- pins are identical in
length on each net, but the pin escape on B+ is 50 mils shorter than
that of B- due to the location of the pins on the part and the desire
for their vias to be adjacent so that the overall transmission line is
smooth. (I've tried to sort of draw a picture below, but don't know how
much it'll help.) The phase tolerance we're really concerned with is the
over all distance from A+ to D+ compared with the distance from A- to
D-, so the extra pin escape length at B- really doesn't bother us.
However, it seems like the system is forcing the overall length of each
segment to match. Is there any way to define a pin pair that the phase
tolerance will look at, and only that pin pair? Our current solution is
to jack up the phase tolerance number so it doesn't holler, then create
a pin-to-pin match group of just these two nets to force the overall
lengths to match. Is there a better way? We have a bunch of these and
this seems kind of kludgy...
Thanks for any feedback.
Jean
A+ B+ C+ D+
| | | |
*------*------------*------*
*------*------------*------*
| | | |
A- | C- D-
|
B-
Jean Bratton
Sr. Printed Circuit Designer
Freedom CAD Services, Inc.
603-864-1300 x1349
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