Hello SI gurus, I have been in a debate recently regarding how close differential signal pairs should be routed in edge coupled stripline. The debate is centered around routing loosely coupled lines on a backplane (long lengths) and potentially on a circuit card (short lengths). With the benefits obtained from having tightly coupled lines when considering cross talk , EMI and routing density, one might never consider spacing the traces as far apart as lets say 5W. The augment for loosely coupled lines contend that because of this separation, 40dB of cross talk isolation is can be obtained (for a given geometry, Dk...) and thus EMI concerns are satisfied as well. In addition, loosely coupled differential transmission lines have lower loss per unit length, in the order of .4 db/m (again for a given geometry). This last argument bothers me the most, as I have yet to see a direct comparison of test results illustrating the performance of the two routing methods. Moreover, the opposition to tightly coupled lines suggest that increased current density occurs at the edges of the differential pairs, and that this is the fundamental contributor to increased loss. I know that return current density is higher directly under the high speed trace. It would certainly follow that in tightly coupled lines a larger proportion of return current flows in the differential pairs as opposed to the ground plane. But it also is clear that the sum of all return currents must be be equal to the source current, thus reducing return current density in the diff pair return. So how can increased current density in the diff pair return be proportional to increased loss? Does skin effect further increase the loss? If so, what is the nature of the interaction (if any) between increased current density at the edge of the diff pair traces and skin effect? In exploring the loss argument, is there any data, studies or formulas that illustrate these relationships? Thanks, Chris -- Chris Mesibov Sr. Hardware Engineer Fujitsu Network Communications, Inc. Phone:(845)731-2037 2 Blue Hill Plaza, 6th Floor Fax: (845)731-2011 Pearl River, NY 10965 ------------------------------------------------------------------ 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 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