[SI-LIST] Re: "common mode noise conversion"
- From: "Knighten, Jim L" <JK100005@xxxxxxxxxxxxxxxx>
- To: damonjbowser@xxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 8 Mar 2004 14:18:14 -0500
Damon,
There is no magic involved in differential to common-mode conversion in
coupled transmission lines (such as a differential transmission line).
Differential transmission lines are simply two individual transmission lines
that are electromagnetically coupled. Typically, designers try to make the
two lines identical. You can excite these lines in the odd-mode (equal and
opposite voltages on each line, or differential signaling) or in the
even-mode (equal voltages on each line, or common-mode signaling).
Excitation that is not exactly balanced is some combination of odd and
even-mode excitation, so imbalanced excitation produces both modes.
Similarly, if the two lines are not identical (often called "imbalanced
lines"), then both modes are required to meet the boundary conditions.
Hence, if you excite an imbalanced or asymmetrical pair of lines with a
perfectly balanced differential excitation, some common-mode will be created
by the asymmetry of the transmission line. This is usually what people mean
by the term "common-mode noise conversion.
There are probably exceptions to what I've said about perfect symmetry, but
in the world of differential transmission lines on printed circuit boards,
what I have described is typical.
One reference source on coupled transmission lines can be found in
Jon C. Freeman, Fundamentals of Microwave Transmission Lines, John Wiley &
Sons, Inc. 1996, ISBN 0-471-13002-8
The even and odd modes of behavior of a pair of coupled transmission lines
can be described in terms of s-parameters. In this case, you have 4-ports,
so you have 16 s-parameters. It is common to convert these into what is
called "mixed-mode" s-parameters which can describe the differential
excitation to differential output behavior, along with differential
excitation to common-mode output, etc.
An Agilent presentation on the web can be found at
http://img.cmpnet.com/enen/seminars/802/webcast.ppt and although it focuses
more on device characterization than transmission lines, the mathematics is
the same. Another reference on the mathematics of mixed mode s-parameters
is
Robert J. Weber, Introduction to Microwave Circuits, Radio frequency and
Design Applications, IEEE Press, 2001 ISBN 0-7803-4704-8, pages 89-103 are
the most pertinent.
Hope this helps.
Jim
_____________________
James L. Knighten, Ph.D.
Teradata, a division of NCR http://www.ncr.com
17095 Via del Campo
San Diego, CA 92127
tel: 858-485-2537
fax: 858-485-3788
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Damon Bowser
Sent: Monday, March 08, 2004 9:58 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] "common mode noise conversion"
Hi all,
I have heard people refer to the term "common-mode
noise conversion" as any common mode noise that is
generated as a result of the differential signaling
environment not being matched precisely. Is there a
more formal definition or mathematical definition of
this term?
Thanks!
Damon
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