Priyawrat, Differential transmission lines (microstrip or other) can support two modes of propagation, the even and odd modes. Usually, the odd mode is the intended mode of operation, as you have noted. Differential circuits inevitably suffer some imbalance between the two sides of the differential circuit, giving rise to the odd mode. Odd mode current must return by some path - often on the ground, or if the circuit contains a differential cable, on the cable shield, or on another pair within the cable if the cable is not shielded (e.g. Cat5 cable). It is this even mode current (often referred to as common-mode) that's can be a source of radiated EMI. Some in the EMI community have come to refer to this as "signal-induced" EMI, since it's source is the signal on the cable or transmission line, rather than from other noise (clocks, etc.) that merely "hitch a ride" out on cable shields, etc.
Doug McKean is right - this is a signal integrity issue, but at the same time it is also an EMI issue, because differential imbalance is always present. For a reference on the EMI aspects of the common-mode generated by differential imbalance, see the following paper: Knighten, J.L., N.W. Smith, J.T. DiBene II, and L.O. Hoeft, "Experimental Analysis of Common Mode Currents on Fibre Channel Cable Shields due to Skew Imbalance of Differential Signals Operating at 1.0625 Gb/s," 1999 IEEE International Symposium on Electromagnetic Compatibility, Seattle, WA, August 1999, pp. 195-200. An electronic copy of this may be found at: http://www.emcs.org/99papers.html <http://www.emcs.org/99papers.html> Hope this is helpful. Jim ________________________________________________________ Dr. Jim Knighten e-mail: [email protected] <[email protected]> Technical Consultant - Design NCR 17095 Via del Campo San Diego, CA 92127 http://www.ncr.com <http://www.ncr.com> Tel: 858-485-2537 Fax: 858-485-3788 -----Original Message----- From: P.R.Dewasthalee [mailto:[email protected]] Sent: Monday, March 27, 2000 5:07 PM To: [email protected] Subject: Differential Modes - Even and Odd Importance: High Hi all, I feel the best way to close the argument about the "split" is to invoke a lot of discussion on EM related topics! Fortunately, I have a lot of doubts to clarify! *********************************** In CML/pCML/ECL/pECL logic families, we see a differential output with even mode operation. I mean, CML/pCML sink currents on both the lines(T/F) whereas ECL/pECL source current on both lines(T/F). (Even Mode). Comparing this with LVDS, the current is sourced out of one line and is returned on the other one. (Odd mode). I can understand the benefits of differential pair in the odd mode operation (Flux Cancellation). Question is: Does the same phenomenon occur in the even mode also? Please give me some clue, any further references to look. Thanks in advance, best regards, - Priyawrat. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
