Jim, note that MMTL will give you the single ended impedance for each trace in addition to the differential impedance, that would be another thing to compare (note that the Zo for the diff pair will be SLIGHTLY lower than the Zo for an isolated stripline of same dimensions, because in the diff pair the second trace adds a little metal). 2 Ohms difference is quite close. After all, standard tolerance when doing PCB traces is around 10% for impedance, and 4 - 5 Ohms if you pay extra money. Wolfgang otter30@xxxxxxxxxxxxx Sent by: si-list-bounce@xxxxxxxxxxxxx 10/21/2008 09:09 AM To "Kotson, Michael" <mkotson@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx> cc Subject [SI-LIST] Re: Diff Z for asymmetric coupled striplines Hi Mike, Thanks to you and everyone else for their suggestions. I tried 2 open source field solvers on the same configuration. MMTL gave a diffZ of 91.5ohms, and mdtlc gave 93.57ohms. These results seem reasonable, and both tools were very easy to use. Structure Parameters --------------------- line width = 3mils line spacing = 4mils (edge coupled) trace thickness = .7mils (1/2 oz cu) h1 = 3.9mils (dist to upper ref plane) h2 = 7mils (dist to lower ref plane) er = 4.3 Zo = 53.5ohms (obtained from a formula for a single asymmetric stripline) Thanks for your help, Jim -------------- Original message from "Kotson, Michael" <mkotson@xxxxxxxxxxx>: -------------- > Hi Jim, > Give MMTL a try. I have had pretty good luck with the results, and > it's free. > > http://mmtl.sourceforge.net/ > > Best of luck, > -Mike > > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] > On Behalf Of otter30@xxxxxxxxxxxxx > Sent: Monday, October 20, 2008 10:16 AM > To: si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Diff Z for asymmetric coupled striplines > > Hello, > I need to determine the differential impedance of pair of coupled > stripline traces. They are edge coupled and are asymmetric in the sense > that the distances to the 2 reference planes are different - both traces > are > 4 mils from the upper plane and 7mils from the lower. > > Our pcb fabricator told us that this configuration would have a diff Z > of 100 ohms, but I have gotten a value from the cross section calculator > in Cadence Allegro of 79 ohms. I'd like to find another method to settle > this, as our board does have SI problems. > > The diff Z formula for this configuration isn't in any of the well known > SI or TL texts, although the symmetric case (H1 = H2) is. Can anyone > either: > 1) Point me toward the correct equation > 2) Suggest a free field solver that would let me calculate this value? > > I've downloaded the student version of Ansoft Maxwell, and am trying it > out. If it can extract the mutual > capacitance and inductance, is there a general method of getting the > diff Z from these? > > Any help is greatly appreciated, > Jim > > > ------------------------------------------------------------------ > 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 technical documents are available at: > http://www.si-list.net > > 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 > > > > ***************************************************************** > ************* > > CONFIDENTIAL NOTE: This e-mail and any attachments may be > confidential and protected by legal privilege. 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