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 > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu