Mary, I agree that the concepts are best understood by examining homogeneous media. Then, one can look at the additional complexities of the inhomogeneous microstrip. Even and odd modes are electric and magnetic field distributions encompassing the coupled lines. To take issue with some previous posts, differential lines, such as the stripline, can support both even and odd modes. The mode(s) that exist are determined by excitation. (This is no different than a cavity or a waveguide, etc.) Hence, one can have either and even or odd mode, or both simultaneously. As you say, the "intentional" signal is the odd mode. But, the mischief lies in the even mode. 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: 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 -----Original Message----- Each propagates undistorted, but at different velocities? Timothy J. Christman Test Engineer Tel 651.582.3141 Fax 651.582.7599 timothy.christman@xxxxxxxxxxx Guidant Corporation 4100 Hamline Ave. N. St. Paul, MN 55112 USA www.guidant.com -----Original Message----- From: David G Haedge [mailto:haedge@xxxxxxxxxxxx] Sent: Tuesday, February 26, 2002 3:56 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion Eric and all, My understanding of a transmission line that has +1 volt signal on one line and 0 volts on the other is actually the superposition of an even mode signal of +0.5 volt / +0.5volt and an odd mode signal of +0.5volt/-0.5volt, giving you the +1volt/0volt signal on the line, in which case each mode should propagate undistorted. Is this not what in fact is occurring in a line excited in this nature? David Haedge Raytheon In a nutshell, odd and even modes, and any modes in general, are special voltage patterns that propagate undistorted down a pair of transmission lines. For example, in a pair of microstrip traces, if you send a +1 v on one line and a 0v signal on the other, the actual voltage on the two lines will change, as the signals move down the line. The 0v line will see a growing negative signal as the far end cross talk builds up and the +1v signal will drop and distort as it looses energy to the quiet line. This voltage pattern is not a mode. It is just a particular driven voltage pattern. There is nothing special about it. However, there are two special voltage patterns that you can impose on the lines which will not change as the signals propagate down the lines. If you put a +1v on each line, wrt the return plane below, there will be no voltage difference between the two signal lines and the voltage pattern will continue undistorted. The other voltage pattern is a +1v and a -1v applied to the two lines, wrt the return plane. ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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