Can anyone provide guidance on expected impedance variations along the length of a differential grounded coplanar transmission line built with foil construction? I have made differential TDR measurements on several samples of a nominally 100ohm, 10" long line on prepreg, and besides seeing a difference in the average impedance, on some coupons, I see 'triangular' variations of >10ohms occurring over 500ps of round trip delay. Another sample, built with core construction showed no anomalous behavior - just a slight rise of impedance that I attribute to conductor resistance. I've been told by a board shop that for the outer layer, foil construction is cheaper than core construction, so I'd like to avoid core construction unless I have to. Have others seen this behavior? What specifications have you made on fab drawings to avoid it? Thanks, Mike Kauffman Infinera Confidential: This message is intended only for the use of the individual or entity, to which it is addressed and may contain information that is privileged, confidential, and/or exempt from disclosure by applicable law or court order. If the reader of this message is not the intended recipient, or the employee or agent responsible for delivering the message to the intended recipient, you are hereby notified that any use, dissemination, distribution, or copying of this communication is strictly prohibited. If you have received this communication in error, please notify us immediately by return e-mail at the above address and delete all copies of this message from your computer systems. Thank you. ------------------------------------------------------------------ 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 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