RE: [SI-LIST] : PCB impedance variation v.s. layout pattern

Mellitz, Richard (richard.mellitz@intel.com)
Fri, 27 Aug 1999 11:57:21 -0700

'Not surprising at all. How may turns. We have model this effect. Also
there is change in FR4 weave that effect the impedance too.

...Rich

-----Original Message-----
From: Allen Wang [mailto:Allen_Wang@asus.com.tw]
Sent: Friday, August 27, 1999 1:17 PM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : PCB impedance variation v.s.
layout pattern

Dear all:

We are designing a Rambus related product which requires the
PCB to maintain 28 Ohm +/- 10% trace impedance.
However, what we have found that a straight line trace will
get a higher
impedance (through TDR, 1 Ohm or more) than
a trace with many turns on it. We know a turn on the trace
will change
the
impedance. However, is it the root cause
that make our trace impedance not consistent?

Two traces are on the same layer, with the same
trace width.


+---------------------------------------------------------------------------
------------------------------------+
Straight line

+-----------+ +------+ +---- \\\\ -----+
+------------------------+
| | | |
| |
| | | |
| |
+-----+ +----+
+---+
line with turns

Allen
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