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

• From: "Knighten, Jim L" <JK100005@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
• To: mary@xxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
• Date: Tue, 26 Feb 2002 19:30:25 -0500

```Mary,

I agree that the concepts are best understood by examining homogeneous
media.  Then, one can look at the additional complexities of the
inhomogeneous microstrip.

Even and odd modes are electric and magnetic field distributions
encompassing the coupled lines.  To take issue with some previous posts,
differential lines, such as the stripline, can support both even and odd
modes.  The mode(s) that exist are determined by excitation.  (This is no
different than a cavity or a waveguide, etc.)  Hence, one can have either
and even or odd mode, or both simultaneously.

As you say, the "intentional" signal is the odd mode.  But, the mischief
lies in the even mode.

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:   Mary [mailto:mary@xxxxxxxxxxxx]
Sent:   Tuesday, February 26, 2002 4:04 PM
To:     si-list@xxxxxxxxxxxxx
Subject:        [SI-LIST] Re: Even mode, common mode, and mode conversion

Don't confuse the issue by referring to what happens in an
inhomogeneous medium. I believe Mr. Haedge's point is valid.
After all, aren't there an infinite number of ways to divide
a signal on three conductors into two complete-orthogonal modes?
The even/odd mode description is convenient for many reasons.
However, I don't think there's anything magical about these
modes. They do not propagate down a transmission line
independent of one another. It's true that if you launch an
odd (or even) mode signal down a symmetric pair of traces you
will theoretically get an odd (or even) mode signal at the
termination. However, if you launch an odd and an even mode
signal at the same time, you no longer have the symmetry that
was responsible for the "single-mode" propagation.

I don't believe it's proper to assume that the odd-mode
propagation and even-mode propagation can be analyzed
independently. Yet there is a tendency on this list to ignore
what happens to the even mode component when the "intentional"
signal is all odd mode.

Mary

-----Original Message-----

Each propagates undistorted, but at different velocities?

Timothy J. Christman
Test Engineer
Tel 651.582.3141  Fax 651.582.7599
timothy.christman@xxxxxxxxxxx
Guidant Corporation
4100 Hamline Ave. N.
St. Paul,  MN   55112  USA
www.guidant.com

-----Original Message-----
From: David G Haedge [mailto:haedge@xxxxxxxxxxxx]
Sent: Tuesday, February 26, 2002 3:56 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion

Eric and all,

My understanding of a transmission line that has  +1 volt signal on one
line and
0 volts on the other is actually the superposition of an even mode signal
of +0.5 volt / +0.5volt and an odd mode signal of +0.5volt/-0.5volt, giving
you
the +1volt/0volt signal on the line, in which case each mode should
propagate
undistorted.  Is this not what in fact is occurring in a line excited in
this nature?

David Haedge
Raytheon

In a nutshell, odd and even modes, and any modes in general,
are special voltage patterns that propagate undistorted down
a pair of transmission lines. For example, in a pair of
microstrip traces, if you send a +1 v on one line and a 0v
signal on the other, the actual voltage on the two lines
will change, as the signals move down the line. The 0v line
will see a growing negative signal as the far end cross talk
builds up and the +1v signal will drop and distort as it
looses energy to the quiet line. This voltage pattern is not
a mode. It is just a particular driven voltage pattern.
There is nothing special about it.

However, there are two special voltage patterns that you can
impose on the lines which will not change as the signals
propagate down the lines. If you put a +1v on each line, wrt
the return plane below, there will be no voltage difference
between the two signal lines and the voltage pattern will
continue undistorted. The other voltage pattern is a +1v and
a -1v applied to the two lines, wrt the return plane.

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```