I can't totally concur with the previous poster. Using a well known 2D solver I show a variance of only about 1.5% with the shift that was mentioned. One *attempt* to reduce this shift is to make sure that the traces are etched on a core. However, your fabricator may prefer to use the trace/plane pair as the core for etch balance. Contact them and find out. If you can use the trace/trace as a core you can reduce the shift. That said the following produced a 100 ohm diff line. Plane to trace 1: 6 mils Trace 1 to trace 2: 14 mils (yuck) Trace 2 to plane: 6 mils Trace 1,2 thickness: 0.7 mils each (1/2 oz Cu) Trace 1,2 width 5.5,5.25 (trapezoidal in nature) Er: assumed constant @ 4.0 Traces aligned: 100 ohms Traces shifted 2.5 mils (ea): 101.34 If you think about it the trace to plane return doesn't change. Only the field between traces *stretches* just a hair which is why you should NOT see such a drastic change in impedance. A while back I sat down with a top tier fabricator to go over this and their results matched what I am presenting. Their SI guys had intentionally produced shifts in broadside pairs to see the impedance change (that everyone was whining about) and it just wasn't enough to worry about. However... I use edge coupled diff pairs in 95% of the work that I do. I will be glad to give you the fabricator's contact info -- OFFLINE! Bill -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Javier DeLaCruz Sent: Thursday, January 30, 2003 11:37 AM To: doug.hopperstad@xxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: High-Speed GHz differential signals routed Broadside Coupled Doug, I've run a variety of simulations on this in the past=20 but a significant hurdle presented itself. While broadside=20 differential coupling appears to improve routability in=20 substrates/PCBs, it is sensitive to layer alignment. Layer- to-layer registration on substrates and PCBs are too loose=20 to be able to get a repeatable differential impedance. =20 I had, in the past, run 2D simulations of these=20 broadside coupled traces, and the space between the traces=20 was about double the space to ground respectively. In the=20 following diagram, the layers comprised of "G"'s are the=20 reference layer (ground usually) and the "S"'s are the=20 differential signals. The space between the "S" layers would=20 need to be a much thicker core layer. You would not be=20 able to use double prepreg nor a core+prepreg since the=20 thicknesses can vary easily. This issue can be overcome as=20 described, but the alignment issue remains. GGGGGGGGGGGGGGGGGGGGGGGGGGGGG SSSSSSS SSSSSSS GGGGGGGGGGGGGGGGGGGGGGGGGGGGG When the same simulations are run at the extremes of=20 the layer alignment windows, which can be as much as +/- 5mils on=20 PCBs, you will see that you cannot maintain a repeatable=20 impedance. This is illustrated below. This is why, I=20 assume, that this technique is not commonly used in the=20 industry. If a technology emerges for substrates or PCBs=20 that have much tighter tolerances on these layer=20 registrations, and variable layer thicknesses, then the=20 practice may re-emerge. =20 GGGGGGGGGGGGGGGGGGGGGGGGGGGGG SSSSSSS SSSSSSS GGGGGGGGGGGGGGGGGGGGGGGGGGGGG Thanks, Javier DeLaCruz -----Original Message----- From: Doug Hopperstad [mailto:doug.hopperstad@xxxxxxxxxx] Sent: Wednesday, January 29, 2003 9:58 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: High-Speed GHz differential signals routed Broadside Coupled I am looking for feedback if anyone has tried or is using Broadside =3D coupling for 100 ohm differential GHz signals? And if so, what trace =3D width and lengths are being used?=3D20 Thanks for your time D.M. Hopperstad ------------------------------------------------------------------ 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: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages=20 Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ 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 -- Binary/unsupported file stripped by Ecartis -- -- Type: application/ms-tnef ------------------------------------------------------------------ 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