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

• To: si-list@xxxxxxxxxxxxx
• Date: Mon, 25 Feb 2002 10:26:40 -0800

```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
=================================================================

-----Original Message-----
From: Dima Smolyansky [mailto:Dima@xxxxxxxxxxxxxx]
Sent: Monday, February 25, 2002 9:54 AM
To: doug@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion

Doug:

The distinction is subtle, and the two mode impedances are defined as :
- Differential mode impedance is impedance between two traces when the
traces are driven with differential stimulus
- Odd mode impedance is impedance of one of the traces when the traces are
driven with differential stimulus

In practical terms, if you need to measure odd mode impedance with TDR
oscilloscope, you would simply drive the trace pair differentially and
measure one channel only (no waveform math). If you need to measure
differential mode impedance, you would drive the trace pair differentially
and subtract your voltage waveforms on the two channels, or add the
are symmetric, then the differential impedance is simply twice the odd mode.

We have an FAQ entry on our web site at
http://www.tdasystems.com/faq/faqTDR.htm#even_odd_modes.

Thanks,

===================
Dima Smolyansky
TDA Systems, Inc.
11140 SW Barbur Blvd., Suite 100
Portland, OR 97219
(503) 246-2272
(503) 246-2282 (fax)
(503) 804-7171 (mobile)
www.tdasystems.com

----- Original Message -----
From: "Doug Brooks" <doug@xxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Monday, February 25, 2002 9:31 AM
Subject: [SI-LIST] Even mode, common mode, and mode conversion

>
> Gurus,
>
> I've been following the thread on mode conversion and suspect I am not the
> only one with this question. Can someone explain, in layman's terms:
>
> The distinction between even mode and common mode?
>         between odd mode and differential MODE (not to be confused with
> differential traces)
>
> And does the existence of differential traces add any complications to
> these distinctions?
>
> Thanks for bringing the dummies along with you!
>
> Doug Brooks
>
>
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```