Dear Marc, You have raised what is (to me) a very interesting question, one which deserves a good analytical answer. If we just deal with a group of ten signals surrounding one nearby ground return wire, there are some analytical tools that can help you predict the answer. First, you need to know the mutual inductance between signals in your configuration. Given an aggressive wire at position a, and a victim wire at position v, and one and only one ground wire at position g, all traversing z-axis distance h, if I recall correctly the formula for the mutual inductance shared between the aggressor and victim is: Lm = 5.08 * h * [ln(|v-g|/R) - ln(|v-a|/|g-a|)] where R is the radius of the wire, the dimensions h, |v-g|, |v-a|, |g-a|, and R are inches, the function ln() is the natural logarithm, and the answer Lm appears in nH As you can see the wire radius R appears as a logarithmic term in the equaltion and so changes in the wire radius do not strongly affect the outcome. The height h of the cavity between the boards (the wire length) affects the result directly and is thus your most powerful control variable. Let's do an example with the positions a, v, and g in a straight row with equal spacing s=0.1 inch. |v-a| = .1 |v-g| = .1 |g-a| = .2 R = .01 (radius of 24 AWG wire) h = .2 (from your note, although this sounds awfully tight) Lm = 5.08 * .2 * [ln(.1/.01) - ln(.1/.2)] Lm = 3.04 nH You should re-run this example assuming the only return paths are provided by the so-called "low inductance" ground pins in your backplane connector. Because they are located further away from the signals, the mutual inductance Lm will come out larger -- thus the importance of locating the return paths near the signals. The next issue has to do with your signal risetimes. The risetime (not the operating speed) determines the bandwidth of your signals. Assuming you are working with a modest CMOS process having rise/fall times on the order of Tr = 1 nS, the bandwidth (upper-bounded by approximately 0.5/Tr) works out to f = 500 MHz, at which frequency the impedance Zm represented by the mutual inductor Lm at frequency f equals Zm = 2*pi*f*Lm = 9.55 ohms. The crosstalk you receive has to do with the ratio of Zm to the circuit impedance Z0. If you were using end-terminated transmission lines with Z0=50 ohms, the numbers in this case indicate a crosstalk of approximately 9.55/50 = 19 % FOR EACH AGGRESSOR. Fortunately, in your application the wires are quite short compared to 1 ns and so they are likely not terminated, in which case the current drawn on each wire (and thus the mutual-inductive crosstalk) will be considerably less than 19 %. By the way, if you are using source-terminated lines the current waveform produced on the wire appears as two steps, one positive followed by one negative, with amplitudes equal to (1/2)Vcc/Z0 and separation in time approximately equal to the line delay (depending on where along this line your "connector" appears). If the line length, and thus the separation of the two opposing current waveforms, is small compared to the risetime the two opposing chunks of mutual-inductive crosstalk created in this configuration tend to cancel out. Lastly, the crosstalk that concerns you in this example bears a striking resemblance to the crosstalk that occurs when ordinary vias traverse a pcb. In analyzing those situations one takes into account all the nearby return paths, which tends to diminish the value of Lm quoted above for a single return path. Software for such calculations is provided by, among other companies, Sigrity. Best regards, Dr. Howard Johnson, Signal Consulting Inc., tel +1 509-997-0505, howiej@xxxxxxxxxx http:\\sigcon.com -- High-Speed Digital Design articles, books, tools, and seminars -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Godbout, Marc Sent: Wednesday, February 19, 2003 6:57 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Split planes and ground return wires I've got questions concerning ground return wire requirements and hope the list can help. First the configuration. Our system as got boards that are physically two separate PCB's, about 6" X 7", sandwiching, and bonded to, a metal plate. Signals need to traverse from one PCB to another. To accomplish this, we run wires through holes in the plate. These signals are basically microprocessor-type signals, 5V CMOS. The bus speeds are 25MHz or lower. Ground returns are added at about a 10:1 ratio, and placed somewhat near the groups of signals. The ground planes are also connected to each other through the backplane connector through 40 low-inductance pins. So I see this as a not-quite classical split plane problem. The split is crossed by 40 connector wires and maybe 15 z-axis wires, each 0.200" long. We've got some mechanical constraints that are making me look into the minimum requirements of the ground return wires. I feel that the "bigger is better" theory was used to design them in the first place, but I've got to cut back on that right now. So the question is, by how much? So what are the areas or major concerns that I should investigate? How do I go about calculating a minimum wire size? And are there any texts that would help me zero in on my analysis methods? Thanks for any help you could give. -Marc ------------------------------------------------------------ ------ 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 ------------------------------------------------------------------ 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