Chris, I think you made the assumption on the drivers that return current has to be on the planes. What about designing a driver to return current on opposite trace on pcb, so we don't need to care about reference to planes. A similar example is eithernet Cat-5 cable, which has no ground refernce. The reason I asked is the scenario: diff pair with planes refernce on pcb --> coaxial cable --> other pcb From pcb diff pair to cable diff pair, there seems to have mismatch even though both have same impendence- one has reference to planes while the other don't. This really puzzled me. Please advise. Signal Hoss --- Chris Rokusek <crokusek@xxxxxxxxxxxx> wrote: > > Signal, > > Try thinking about it as two single ended traces. > Then in terms of return > current, a large proportion of the return current > for each diff trace > actually returns on the planes rather than the > opposite trace. So what you > really have in terms of currents is a separate > signal/return loop for each > diff half. These two loops are equal and opposite > but spaced very near each > other (in fact on the planes they overlap at all but > the highest frequencies > causing cancellation, less noise, good stuff). > Another way to visualize > this is that the E/H field lines are "contained" by > the planes. Thus > impedance will vary as the plane separation is > varied. > > Chris Rokusek > Innoveda > __________________________________________________ Do You Yahoo!? Make international calls for as low as $.04/minute with Yahoo! Messenger http://phonecard.yahoo.com/ ------------------------------------------------------------------ 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