The question is, what is your nearest return via ? Are you surrounding your single ended trace via with ground vias to = ensure true common mode behavior or the nearest via is actually the = other signal of the differential pair. You need to consider the ground via placement also to define your common = mode via impedance at multi-giga speed.=20 Indepedent on whether you believe you have common or differential mode = vias, what you really want is not just 50ohm single ended or 100ohm = differential impedance at 1-5GHz. You really want to know : 1) What is the S11 (reflect) 2) What is the S21 (thru) 3) What is 1-|s11|^2-|s21|^2 (loss to the plane cavities) Common mode vs. differential mode makes a huge difference in all of the = above. And I assure you differential mode vias win hands down. -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Joel Brown Sent: Wednesday, January 09, 2008 8:30 PM To: wolfgang.maichen@xxxxxxxxxxxx; luant@xxxxxxxxxxx Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx Subject: [SI-LIST] Re: 50 Ohm Via? To Chris: I have been reading several places that recommend using loosely coupled differential pairs, that is why I mentioned 50 ohms. I know there are religious beliefs about tightly coupled vs loosely coupled pairs. The material I read regarding loosely coupled pairs mentioned advantages = such as wider trace widths for a given impedance and avoiding degradation of = rise time caused by coupling between signals within a pair. Thanks - Joel ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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