FreeLists / si-list / [SI-LIST] Re: Even/Odd mode impedance

[Robert_Washburn@xxxxxxxx]

Here's a document by Doug Brooks that gives a nice (and brief) =
description
of differential impedance.  Enjoy....

http://www.ultracad.com/diff_z.pdf

robert


Robert M Washburn
Signal Integrity Group
Dell Computer Corporation
512.723.6025
robert_washburn@xxxxxxxx

-----Original Message-----
From: Alexis LESTRA [mailto:alestra@xxxxxxxxxx]
Sent: Tuesday, June 12, 2001 10:58 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Even/Odd mode impedance



Dear all,

I am new in the field of signal integrity and would like to submit a=20
basic problem. I am currently designing a  backplane for differential=20
signals at 2.5  Gbit/s. The receiver is differential, with 100 Ohm=20
termination. For coupled transmission lines, which characteristic=20
impedance should match 50 Ohm, the even-mode impedance or the even-mode =

impedance ? After reading some documentation on the topic, I am=20
confused. On one  hand, I read that differential signals propagate in=20
odd-mode since  coupled transmission lines are driven with equel=20
magnitude and 180=B0 out  of phase ("High-speed Digital System Design", =

S.H. Hall, G.W. Hall, J.A.  McCall, p.53) . On the other hand, I read=20
that since differential  signals are launched from silicon devices, the =

gates that driver these  signals are both referenced to ground,=20
resulting in even-mode coupling =20
(http://www.amp.com/simulation/files/papers/98060A 5.pdf   p.19).
Can anyone help me to clarify this point.

Thanks in advance.

Alexis

Alexis Lestra
Tachys Technologies
Les Algorithmes, Batiment Aristote A
2000, route des Lucioles
F-06901 Sophia-Antipolis Cedex

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