Doug, I have some real heartburn with some of those representations, particularly the fluid analogy that speaks of current as the flow of electrons. When I grew up current was defined as time variation of electric flux. When an E/M field impinges a chunk of metal the resulting interaction concentrates the field forming a wave guide. All practical wave guides leak, be they a microstrip over a plane, a stripline, or whatever. Some, like a good semirigid coax leak only a little tiny bit. When they leak too much creating excessive disturbance in nearby wave guides, we have cross talk problems. I hope that this is what you were trying to convey. Regards, Steve. At 10:06 AM 8/5/2005 -0700, Doug Brooks wrote: >Some of the answers you received may have made this sound more complex than >the issue really is. > >First: Current is the flow of electrons. Electrons MUST flow in a closed >loop. The way they find their way "back" is called the "return path." If >you have a simple battery with two wires attached to it, one carries the >outgoing current and the other carries the return current. > >Second: Sometimes the return is constrained to a specific wire. Then there >is no question where it is. Sometimes the return is on a plane. Then there >CAN be a question where it is, since it can spread around the plane in a >variety of ways. > >Third: WHERE it is on a plane is determined (at least in part) by >electromagnetic coupling between the signal that generates the return and >the return itself. The return will follow the path of least IMPEDANCE. For >high frequency components the lowest impedance path is directly underneath >the trace carrying the signal. For low frequency components, the return >will follow a path of least RESISTANCE, which can be a DIFFERENT path than >for the higher frequency components. Therefore, different parts of the >return path (different frequency harmonics associated with the return >current) may flow back to the source on different paths. > >Fourth: Since the lowest impedance path (for the higher frequency >harmonics) is on the plane directly under the trace, this part of the >return current will "want" to be on the "closest" plane, whether it is a >power plane or a ground plane. For HIGH frequency harmonics there is no >difference in these planes. (If that is hard to understand, think of all >the bypass caps there are connecting the two planes!) > >Finally: An issue for signal integrity "can" be (depending on >circumstances) how the return current actually gets to the plane it wants >to be on. Sometimes (not always) the way it gets there can cause SI >problems (especially EMI or crosstalk.) It is not unreasonable to state >that MOST of the signal integrity issues on boards today are caused by a >failure to control return currents. Knowing where your return currents are, >and making proper provision for them, is one of the primary ways of >controlling signal integrity. > >I cover a lot of this in my book "Signal Integrity Issues and Printed >Circuit Board Design," Prentice Hall. > >Doug Brooks > > > > > > > > > > >At 10:31 PM 8/4/2005, you wrote: > >Hello Experts, > > > > > > > >I have a question on return current. I was going through the book "Signal > >Integrity Simplified" and came to know that return current path can be a > >Power Plane or Ground Plane. I am still confused about return path. Let me > >describe what I understood. > > > > > > > >The Signal Path is the active path and the Electric field from Signal will > >terminate in the return path. The Magnetic field will form a circular loop > >around the signal and it will be coupled with return path so that equal > >and opposite current will flow in return path. Normally if we have Power > >Plane or Ground place, the return path will be easy to find out. Let's > >assume, if we have a 2 layer board with no copper shape drawn on it, how > >to find the return path for the signal? Please help me understanding this. > > > > > > > >Thanks, > > > >Darshan Mehta > > > > > > > > > >__________________________________________________ > >Do You Yahoo!? > >Tired of spam? Yahoo! Mail has the best spam protection around > >http://mail.yahoo.com > > > >------------------------------------------------------------------ > >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 > > > >____________________________________________________________________________- >Check out UltraCAD's new presentation videos and new skin effect calculator >at http://www.ultracad.com > > >------------------------------------------------------------------ >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 > ------------------------------------------------------------------ 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