This is repost since I did not get any responses and it seems the list was down for awhile. I was able to get an s-parameter model from the manufacturer, so I will try simulating with it. It also turns out from looking at a graph of differential impedance vs frequency, the impedance varies widely from 100 ohms at 1Ghz, 230 ohms at 1.6Ghz, 30 ohms at 3 Ghz. We have been using common mode chokes on USB and DVI interfaces. We don't know if we need them or what effect they are having on EMI or signal quality. They were added because they were recommended on some application notes and design guidelines. The specification of one of the chokes is 90 ohm common mode impedance, 20 ohm differential impedance, 3 ohm DC resistance. Although common chokes are supposed to have low differential impedance, it seems the 20 ohm differential impedance would be a discontinuity when used in a 100 ohm differential system. Since the part is only 0.05 inches in length it seems this discontinuity would be negligible at DVI data rate of 1.6 GBPS. Also it seems that the 3 ohms of resistance would cause some attenuation by acting as voltage divider in series with the termination resistance. I have asked the manufacturer for a spice model so I can do some simulation, but I am not sure if they will provide one. Is there a simple spice model I can create based on the specifications? It seems that capacitance would be one parameter that is missing. On our next design we plan to have traces shorting across the choke so we can test with and without it. 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