## [SI-LIST] Re: Even mode, common mode, and mode conversion

• From: "Knighten, Jim L" <JK100005@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
• To: doug@xxxxxxxxxx, si-list@xxxxxxxxxxxxx
• Date: Mon, 25 Feb 2002 13:08:55 -0500

```Doug,

Good question!  I also suspect that you are not the only one trying to
figure it out.

The term "common-mode" is often used in different contexts, so its use can
be confusing.

What this thread on "Even mode, common mode, and mode conversion" is all
about is signal behavior on differential transmission lines.  The discussion
has nothing to do with single (or single ended) transmission lines.

On differential traces, these two modes can exist.  Even mode is when both
lines are excited with the same voltage (common excitation).  Odd mode is
when the lines are excited with equal, but opposite voltages (differential
excitation).

Now, you would think that odd-mode impedance would be the same as
differential-mode impedance.  Not so.  To make things more confusing, the
industry conventions are such that even and odd mode impedance is related to
common and differential mode impedances by the following:

Odd mode impedance = 0.5 * Differential Mode impedance
Even Mode impedance = 2 * Common-mode impedance

These are not different phenomena, just two different conventions in the way
that you can look at the same transmission line.

Jim

Jim Knighten, Ph.D.
Teradata, a Division of NCR             http://www.ncr.com
17095 Via Del Campo
San Diego, CA 92127
USA
Tel: 858-485-2537
Fax: 858-485-3788
jim.knighten@xxxxxxx

-----Original Message-----
From:   Doug Brooks [mailto:doug@xxxxxxxxxx]
Sent:   Monday, February 25, 2002 9:31 AM
To:     si-list@xxxxxxxxxxxxx
Subject:        [SI-LIST] Even mode, common mode, and mode conversion

Gurus,

I've been following the thread on mode conversion and suspect I am not the
only one with this question. Can someone explain, in layman's terms:

The distinction between even mode and common mode?
between odd mode and differential MODE (not to be confused with
differential traces)

And does the existence of differential traces add any complications to
these distinctions?

Thanks for bringing the dummies along with you!

Doug Brooks

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