Here's a document by Doug Brooks that gives a nice (and brief) = description of differential impedance. Enjoy.... http://www.ultracad.com/diff_z.pdf robert Robert M Washburn Signal Integrity Group Dell Computer Corporation 512.723.6025 robert_washburn@xxxxxxxx -----Original Message----- From: Alexis LESTRA [mailto:alestra@xxxxxxxxxx] Sent: Tuesday, June 12, 2001 10:58 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Even/Odd mode impedance Dear all, I am new in the field of signal integrity and would like to submit a=20 basic problem. I am currently designing a backplane for differential=20 signals at 2.5 Gbit/s. The receiver is differential, with 100 Ohm=20 termination. For coupled transmission lines, which characteristic=20 impedance should match 50 Ohm, the even-mode impedance or the even-mode = impedance ? After reading some documentation on the topic, I am=20 confused. On one hand, I read that differential signals propagate in=20 odd-mode since coupled transmission lines are driven with equel=20 magnitude and 180=B0 out of phase ("High-speed Digital System Design", = S.H. Hall, G.W. Hall, J.A. McCall, p.53) . On the other hand, I read=20 that since differential signals are launched from silicon devices, the = gates that driver these signals are both referenced to ground,=20 resulting in even-mode coupling =20 (http://www.amp.com/simulation/files/papers/98060A 5.pdf p.19). Can anyone help me to clarify this point. Thanks in advance. Alexis Alexis Lestra Tachys Technologies Les Algorithmes, Batiment Aristote A 2000, route des Lucioles F-06901 Sophia-Antipolis Cedex ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: = //www.freelists.org/archives/si-list Old list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list Old list archives are viewable at: http://www.qsl.net/wb6tpu