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

  • From: "Knighten, Jim L" <JK100005@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
  • To: fred@xxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Wed, 27 Feb 2002 15:37:13 -0500

Fred,

Coupling is indeed the key.  Without the lines being coupled, there is no
meaning to even and odd modes.  You see that your equations for even and odd
mode impedances become the same when L12 and C12 are zero.  Some people are
tempted to treat the two sides of a differential pair as individual
transmission lines - but you can't do that.

To take this discussion a step farther, because the even and odd mode
impedances are different, it takes different impedances to provide matched
loads.  So, you would expect to have different reflection coefficients for
the two modes.  A common practice in differential signaling is to use a
single resistor between the two lines as a load - with the resistor value
being the differential impedance.  The result of this is to provide an
impedance match for the odd mode, but an open circuit (maximum reflection)
for the even mode.  Since, as I stated earlier, the mischief is in the even
mode, designing in the maximum reflection for it may not be the best design
practice.

Jim


Jim Knighten, Ph.D.
Teradata, a Division of NCR             http://www.ncr.com
17095 Via Del Campo
San Diego, CA 92127
USA
Tel: 858-485-2537
Fax: 858-485-3788
jim.knighten@xxxxxxx

 -----Original Message-----
From:   Fred  Balistreri [mailto:fred@xxxxxxxxxxxxx] 
Sent:   Wednesday, February 27, 2002 10:52 AM
To:     doug@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject:        [SI-LIST] Re: Even mode, common mode, and mode conversion


There have been excellent responses on this subject. Perhaps one may better
understand the
meaning of odd, even modes by looking at the equations from where they
originate:
Given that there are 2 lines wrt gnd or some plane or in the microwave
description 3 lines, with
one being a reference and given the Maxwell's LC matrix then:

Odd = sqrt  L11-L12/C11+|C12|
Even = sqrt  L11+L12/C11-|C12|

Note that the loop inductance is minimized and capacitance maximized for odd
mode. The opposite is
true for the even mode. Thus the differences in impedance. The coupling is
the key.
Note that in Maxwell's C matrix the mutual capacitance is desginated as
negative, therefore the absolute brackets.

Best Regards,

----- 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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