[SI-LIST] Re: "common mode noise conversion"
- From: "Michael E. Vrbanac" <vrbanacm@xxxxxxxxxx>
- To: jeff.loyer@xxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 09 Mar 2004 01:11:56 -0600
Jeff,
Good explanation as well as Jim's.
However, it still seems like the same thing only viewed from two different
perspectives.
1. Internal stimulus: The driver/receiver signaling pair and its
electrical balance without
considering external stimuli.
2. External stimulus: The signaling pair and the affect of external
stimuli on the receiver
performance.
In either case, the imbalance works against the signaling link performance.
To what degree and whether that is important will depend on the signaling
system characteristics.
In any case, all things being equal, in the external stimulus case, a
poorly balanced,
loosely coupled pair has more of a propensity to cause difficulties than a
well balanced,
tightly coupled one. That shouldn't be surprising since the loop/capture
area of a poorly
balanced, loosely coupled pair will allow greater amplitudes of energy to
be lost
as common mode energy into the environment or accepted into the poorly
balanced
pair as differential mode energy from external stimuli.
Michael Vrbanac
At 01:16 PM 3/8/2004 -0800, you wrote:
>I think Jim did a great job of describing much of this, but I would add
>a couple of things:
>
>I personally would consider the term "common-mode noise conversion" (of
>a differential pair) as ambiguous. To me, it could either mean:
>1) the conversion of a purely differential excitation into some level of
>common mode, or
>2) the conversion of common-mode excitation into differential-mode
>signal.
>
>In the first case, this is often the result of skewing the routing
>lengths of a differential pair (perhaps, even though the TOTAL lengths
>of the 2 halves are perfectly matched), or the result of differing
>material properties between the 2 halves. This is mathematically
>represented as SCD21, in mixed-mode S parameters. The net result is
>loss of relevant (differential) energy available to the differential
>receiver.
>
>In the second case, common-mode noise from an external source doesn't
>remain common-mode (which, ideally, wouldn't matter to a differential
>receiver). Instead, a significant portion becomes differential - which
>does matter to a differential receiver. The causes of this are the same
>as for the first case, but it's mathematically represented as SDC21.
>
>I believe most folks take the term to mean case 1, but I would be
>careful in using it. SCD21, or "differential-to-common-mode conversion"
>are less ambiguous terms.
>
>Jeff Loyer
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of Knighten, Jim L
>Sent: Monday, March 08, 2004 11:18 AM
>To: damonjbowser@xxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: "common mode noise conversion"
>
>
>Damon,
>
>There is no magic involved in differential to common-mode conversion in
>coupled transmission lines (such as a differential transmission line).=20
>
>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=20
>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. =20
>
>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
>
>__________________________________
>Do you Yahoo!?
>New Yahoo! Photos - easier uploading and sharing.
>http://photos.yahoo.com/
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at: =20
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
> =20
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at: =20
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
> =20
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
