[SI-LIST] Re: Even mode and common mode
- From: "Knighten, Jim L" <JK100005@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
- To: Doug Brooks <doug@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Tue, 5 Mar 2002 13:00:15 -0500
Doug,
This question used to bug me as well. It all hinges on the fact that the
ground plane (signal return) is not perfectly conducting, but has some
impedance. Return currents on the non-zero impedance of the return plane
causes EMI mischief.
So, the way this is often described, a single ended transmission line has a
differential-mode current and a common-mode current. By differential-mode
current, they mean the normal signal current that travels on the PCB trace
and returns to the source on the ground plane in the opposite direction as
the current on the trace. (The is not the same thing as a differential
transmission line's differential mode, except that both are the intended
signal.) The return current on the ground plane causes a voltage drop on
the ground plane that can drive current on the ground plane away from the
original source and in the same general direction as the original trace
current - hence, the term common-mode current. If the ground plane is
extended (bigger than the area under the trace - which it usually is) and a
cable shield is attached to the ground plane, then this common-mode current
can exit the board, and perhaps the chassis, and radiate EMI. This is a
simplified description, but I think it captures the essence.
Research work on this topic has been done by several people, but some of the
notable work has been done by people at the University of Missouri - Rolla.
A couple of references published IEEE papers on this are seen below. There
are other papers published, but these will get you started on a better
understanding of the phenomenon.
Hockanson, D.M., et. al., "Investigation of Fundamental EMI Source
Mechanisms Driving Common-Mode Radiation from Printed Circuit Boards with
Attached Cables," IEEE Transactions on Electromagnetic Compatibility, Vol.
38, No. 4, November 1996, pp. 557-566.
Hockanson, D.M., et. al., "Experimental and numerical investigations of
fundamental radiation mechanisms in PCB designs with attached cables," IEEE
1996 International Symposium on Electromagnetic Compatibility, Santa Clara,
CA, 1996, pp. 305-310.
These papers are copyrighted by the IEEE and I am not an author, so I don't
feel free to send you electronic copies. The Transactions paper can be
gotten form the IEEE web site if you are a member. I can direct you to one
of the authors, if you need, who may be able to provide you with a copy.
Hope this helps.
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: Tuesday, March 05, 2002 9:13 AM
To: Knighten, Jim L
Subject: Even mode and common mode
Jim,
Thanks for a good response.
Now ...... Other than by radiation (or coupling) from another source, how
might "common mode" signals be generated on a single stripline trace (and
its associated ground plane)? This is a question that has bug'd me for a
long time. If I understand that, then I think I got it!
Doug
At 08:19 PM 3/4/2002 -0500, you wrote:
>Doug,
>
>Great question! You have uncovered one of the semantic confusions in the
>transmission line/SI/EMC arena. The term "common-mode" is used in more
than
>one context, so that you can have common-mode currents in single ended
>t-lines on printed circuit boards and common-mode currents on differential
>t-lines, but they really aren't the same thing.
>
>Common-mode currents on a single ended t-line on a PCB usually refers to
>currents on the ground plane that are in the same direction as currents on
>the trace. This can drive EMI currents off the board and out the chassis
>and, voila, you have radiated EMI. This is a very common source of
radiated
>EMI. There is lots of IEEE literature on this. The University of
>Missouri-Rolla guys have done a lot of work in this area.
>
>What we have been discussing concerning differential t-lines is a different
>situation. Here we call the situation (mode) where both lines of the
>differential pair are driven in the same direction to be a common-mode
>current. What is similar, is that the common-mode current is usually the
>EMI issue (and the differential-mode current is the intended signal).
>
>Confusing, isn't it?
>
>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, March 04, 2002 5:03 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Even mode and common mode
>
>
>Thank you all for a very extensive and informative discussion re even and
>odd mode.
>
>I still have one remaining question. Is it possible to have a "common
>mode" signal in a single transmission line system (such as a stripline
>transmission line)? Or is what we commonly call "Common Mode" actually
>restricted to "even mode" currents in a two (or more) line system?
>
>After all the effort you went to in the previous thread, I hope this
>question is at least framed correctly!!
>
>Doug Brooks
>
>
>
>_____________________________________________________________________
>UltraCAD Design, Inc. Celebrating 10 years of design excellence!
>www.ultracad.com
>
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_____________________________________________________________________
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www.ultracad.com
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