[SI-LIST] Even mode and common mode- SI and EMC worlds
- From: Eric Bogatin <eric@xxxxxxxxxxxx>
- To: Brooks Doug <doug@xxxxxxxxxx>
- Date: Wed, 06 Mar 2002 09:11:26 -0600
Doug-
You keep highlighting sources of confusion in the industry! It's
not just you- a lot of folks are confused about the terms we use,
and because so many folks use them incorrectly, the noise level
is high and it makes things even more confusing.
In the last thread, we had been using the terms odd and even
modes as we use them in signal integrity applications. They refer
to the special voltage patterns for a differential pair of
transmission lines. When we talk of differential driven and
common driven signals, we are always looking at the voltages (and
of course the currents) between the signal traces and how they
are driven wrt the return plane. I wrote before that we often
incorrectly use the terms differential mode and common mode
voltage patterns. I think we need to start eliminating these
terms in the SI world, to increase the signal to noise ratio.
However, there is another whole group of engineers who have been
using the terms common mode currents and differential mode
currents, who we have no chance of ever changing. It is the EMC
folks. They use these terms, in a completely different way than
us signal integrity folks.
To a signal integrity engineer, a common signal is one that is
common to both the signal traces in a diff pair, and the actual
voltages we are worrying about are measured wrt the return plane.
To an EMC engineer, a common mode current is a current that is
common to the signal and return path in a single transmission
line, which radiates like an electric dipole antenna. The same
word "common" is used in both worlds, but refers to different
components.
To a signal integrity engineer, a differential signal is one that
is differential to both the signal traces in a diff pair, and the
actual voltages we are worrying about are measured wrt the return
plane. To an EMC engineer, a differential mode current is a
current that is differential to the signal and return paths of a
single transmission line. To us, we might see a single ended
transmission line and call it a single ended transmission line.
The concepts of common or differential driving just would not
apply. To an EMC engineer, they will look at a single ended
transmission line and call it a differential mode current, which
radiates like a magnetic dipole antenna.
In the EMC world, a common mode current- a current common to the
signal and return path, will radiate about 10k x more than a
differential mode current- differential between the signal and
return paths. This is why common mode currents are so critical to
control in EMC engineering.
I think it is partly this different use of the same words between
the EMC and SI worlds that there is so much confusion and the
correct design principles for reduced common mode currents are so
poorly recognized in the industry. If we understand that the
"common mode" currents the EMC guys talk about are not the same
as the "common" signals that might arise in a diff pair, we will
re-calibrate our intuition and look for the real sources of
radiation-driving common mode currents, the total inductance of
the return path.
We cover these and other topics in our GTL250 High Speed Design
Class. I've also been invited to give a talk at the next IEEE EMC
conference on this issue of bridging the gap between the SI and
EMC worlds. I'll post my paper on the GigaTest web site after the
conference. If you have any comments about this topic feel free
to contact me off line.
--eric
**************************************
Eric Bogatin
CTO, Giga Test Labs
v: 913-393-1305
f: 913-393-1306
e: eric@xxxxxxxxxxxx
26235 W. 110th Terr. Olathe, KS 66061
corporate office:
408-524-2700
134 S. Wolfe Rd Sunnyvale, CA 94086
web: www.gigatest.com
**************************************
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
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