Hi, Graham. In response to the question of "... I don't understand where this 50 ohms came from ..." here is a link to an article by Dr. Howard Johnson on the subject: http://www.sigcon.com/Pubs/edn/why50.htm In practice, 50 Ohms seems to be a readily achievable impedance for most layer stacks both today and in the recent past, avoiding both: a) very wide tracks that restrict the layout unnecessarily, and present a low, heavy load, and b) very narrow tracks that cause problems with manufacturing. As manufacturing capabilities improve, allowing ever thinner tracks, the same improvements lead to thinner cores and prepregs, meaning that the 2 changes (impedance-wise) tend to balance each other out - even with 30+ layers in less than 3mm thickness, and pin densities close to 200/sq in (no HDI, just normal PCB technology) leading to high density routing, 50 Ohms is a readily achievable goal. What is interesting though (if I am right), is that the first typical 4 and even 6 layer boards would have tended towards more like 65-75 Ohms. But perhaps, back then, 50 Ohms was neither readily achievable (for typical industrial applications) nor desirable? Regards, ____________________________________ Sol Tatlow, M.Eng. (Oxon) ProDesign Electronic & CAD Layout GmbH Product Developer Albert-Mayer-Str. 16 D-83052 Bruckmuehl Phone: +49 (0) 8062-808-302 Fax: +49 (0) 8062-808-333 Mailto:sol.tatlow@xxxxxxxxxxxxxxxxxxxx www.prodesign-europe.com ____________________________________=20 ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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