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

Grasso, Charles (GrassC@LOUISVILLE.STORTEK.COM)
Fri, 27 Aug 1999 17:21:37 -0600

Typically, increasing the capacitance will
lower the Zo of a xmission line - so look for
stray effects. Thats my guess...

Thank you
Charles Grasso
Advisory Engineer
StorageTek
1 StorageTek Drive
Louisville CO 80027 M/S 4247.
Tel:303-673-2908
Fax:303-661-7115
email:grassc@louisville.stortek.com
RMCEMC Web Site: http://www.ewh.ieee.org/r5/denver/rockymountainemc/

-----Original Message-----
From: Allen Wang [mailto:Allen_Wang@asus.com.tw]
Sent: Friday, August 27, 1999 11:17 AM
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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