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

From: "Knighten, Jim L" <JK100005@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
To: martyn.gaudion@xxxxxxxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
Date: Wed, 27 Feb 2002 13:20:02 -0500
Mary,

This is an interesting application note.  Thanks for pointing us to it.

The microsection figure clearly shows that the inhomogeneities in FR-4
(glass weave, etc.) make differential transmission lines in it inherently
imbalanced.  Therefore, it is impossible to propagate a purely differential
signal on it.  There will be conversion of some of the differential mode
(odd mode) into a common-mode signal (even mode).  

Of course, the amount of mode conversion is more pronounced as you get
higher in frequency.

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:   Martyn Gaudion [mailto:martyn.gaudion@xxxxxxxxxxxxxxxxxxxx] 
Sent:   Tuesday, February 26, 2002 4:17 PM
To:     si-list@xxxxxxxxxxxxx; mary@xxxxxxxxxxxx
Cc:     timothy.christman@xxxxxxxxxxx
Subject:        [SI-LIST] Re: Even mode, common mode, and mode conversion


One reason odd and even modes may travel at
differant velocities, is that composites of glass and
resin tend to have resin rich areas between the pair.

Odd mode signals will "see" more resin with a lower
Er and fly faster than the even mode which "sees" more
of a glass resin mix.  This is true for any woven glass
reinforced base material..

More here:

www.polarinstruments.com/support/cits/AP139.html

Kind regards
Martyn Gaudion
www.polarinstruments.com


At 18:04 26/02/2002 -0600, you wrote:


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

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