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

  • From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>
  • To: "'mary@xxxxxxxxxxxx'" <mary@xxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
  • Date: Tue, 26 Feb 2002 16:51:36 -0800

Here is the problem. We are confusing the classical
definition of TEM odd/even mode (which only happen
in a two line transmission line system) and the 
generic SI term of odd/even mode in crosstalk 

A three conductors TEM mode transmission line system
has three not two modes. What you are describing is
the generic SI engineering term of analyzing the
worst case impact of aggressor to the victim lines
where the aggressors are switching in the same
direction as the victim (even) or opposite (odd).
There is no TEM odd or even modes involved. You
can even extend this definition to an N line
system. The propose of such analysis is to 
understand the worst case impact of a victim line
(signal degradation and propagation variation)
under the worst case impact of crosstalk.

May be we should drop the TEM mode definition so as
not to confuse people more ? 

-----Original Message-----
From: Mary [mailto:mary@xxxxxxxxxxxx]
Sent: Tuesday, February 26, 2002 4:04 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion

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.


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