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

• From: Steve Corey <steve@xxxxxxxxxxxxxx>
• To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
• Date: Tue, 26 Feb 2002 07:20:25 -0800

```Sainath -- I will give you the short answer, and refer you to the texts
for the long answer.  The even and odd modes are TEM modes, or at least
quasi-TEM.  TEM vs. higher order modes describes the field distribution
across a single transmission line or waveguide structure.  Quasi-TEM is
a basic assumption of the transmission line theory on which we rely,
including characteristic impedance and RLGC matrices.  It requires that
to first order, no field has a component in the direction of
propagation.  I usually rely on "Analysis of Multiconductor Transmission
Lines" by Clayton R. Paul when I have questions of this sort.

The higher order modes are significant to transmission line theory for
the most part in that we try to make sure they don't exist, since they
each propagate with their own velocities and make a mess of our nice
clean square waves.  Any E/M text with a waveguide section will describe
the physics behind this.

On the other hand, all TEM modes propagate.  Analysis of the different
TEM modes supported on a multiconductor transmission line structure is
from my perspective more of a mathematical tool for breaking down a
system into its most basic components.  In the case of even/odd, this
has a very practical application in differential signaling, but once we
step outside of the symmetric coupled two-line regime, the math gets
pretty complicated to try and define signaling schemes which coincide
with any particular modes.

So-called "full-wave" analysis, which essentially means "non-TEM
analysis" for our purposes, is necessary for situations where we can't
count on quasi-TEM assumptions.  These situations occur when we step
outside of the strict geometries of uniform transmission lines, or into
certain loss regimes.  Again, I would refer you to the book by Clayton
R. Paul.

-- Steve

-------------------------------------------
Steven D. Corey, Ph.D.
Time Domain Analysis Systems, Inc.
"The Interconnect Modeling Company."
http://www.tdasystems.com

email: steve@xxxxxxxxxxxxxx
phone: (503) 246-2272
fax:   (503) 246-2282
-------------------------------------------

> To all,
>
> Sainath addressed his question to me, but I am going on vacation
> and don't have the time to answer it.  Could someone please
> give him a good explanation to his questions?  Thanks,
>
> =================================================================
>
> -----Original Message-----
> From: sainath@xxxxxxxxxxxxx [mailto:sainath@xxxxxxxxxxxxx]
> Sent: Monday, February 25, 2002 11:18 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion
>
>
>
>
> This two T-lines vs one T-line mode distinction is confusing to me.
> What makes the two T-lines support even or odd kind of modes? and the one
> T-line support the TEM kind of modes?
> Is it not possible to have TEM kind of mode propagation in a two T-line
> system?
> How would you define a MODE when it is meant even or odd mode ? and when it
> is meant TEM mode?
>
> Thanks,
> Sainath
>
> ----- Original Message -----
> To: <si-list@xxxxxxxxxxxxx>
> Sent: Monday, February 25, 2002 10:26 AM
> Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion
>
>
>
>>What may be confusing to some is that the word "mode" is used
>>in different ways in different contexts, both involving
>>transmission lines.
>>
>>So far most responses mentioned the even or odd mode, where
>>the signals of ***two*** T-lines go either in the same or the
>>opposite direction.
>>
>>The other usage of this word involves only one T-line (or
>>wave guide or optical fiber) and describes how the electro
>>magnetic waves propagate inside that line, one of the most
>>familiar mode being the TEM (Transverse Electro Magnetic)
>>mode.  Don't confuse the two meanings of the word MODE!
>>
>>Intel Corporation
>>=================================================================
>>
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