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

• From: Steve Corey <steve@xxxxxxxxxxxxxx>
• To: si-list@xxxxxxxxxxxxx
• Date: Wed, 27 Feb 2002 08:22:40 -0800

```Chris -- The nomenclature that Mary is using is standard in transmission
line theory, in which the ground plane is counted as a third conductor.
Transmission line theory at a high level treats the ground plane as an
additional conductor, the same as it would treat, for example, the second
wire of a twisted pair as a second conductor.  While each may have
different capacitive, inductive, and loss effects on the system, each is
nevertheless a conductor.  So two wires with no ground plane, e.g.
twisted pair, is called a two-conductor system, and has one mode.
Likewise, a single trace over a ground plane is also called a
two-conductor system, and has one mode.  Two traces running over a ground
plane is called a three-conductor system, and has two modes, as she
correctly stated.  In the general case, an (n+1)-conductor system is said
to have n modes.  Mary, please correct me if I have misconstrued your
usage.

-- Steve

Chris Cheng wrote:

> 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
> analysis.
>
> 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.
>
> Mary
>
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