Jim, You failed to mention a couple of other critical parameters. Dielectric material (and weave) of upper and lower dielectrics Copper weight Spacing of pairs With a generic starting point of 4 mils, and 7/4 heights, one can achieve 100 ohms but at a specific spacing and dielectric. You might try the Polar calculator as well. MMTL gives good results but you'll need to know the above info to get a practical result with any tool. Regards, Bill -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Kirby Goulet Sent: Monday, October 20, 2008 1:12 PM To: otter30@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Diff Z for asymmetric coupled striplines Another open source free solver is mdtlc: http://mdtlc.sf.net It has a gui and also includes factors such as etch taper of different Er values for different layers. These factors may account for the differences you are seeing. Its based on the 'atlc' solver which has been available for some time. -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Grasso, Charles Sent: October-20-08 12:36 PM To: otter30@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Diff Z for asymmetric coupled striplines Try the Differential calculator from UltraCad -----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 ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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