Lee, When this topic first came up I knew it was going to be one of those religious discussions, exposing the philosophical rift regarding diff pair routing within the SI community. Last time this happened I = think was when you commented on grounding philosophy. Anyway its been = interesting reading. I was anyways in the camp that said implement diff pairs as = loosely coupled complimentary signals and then you don't have to worry about differences in common mode and diff mode termination impedance, or propagation velocity variance between single ended data and differential strobes. The last few years, however, I had almost been converted into the tighly coupled camp when you had to speak up and get everyone all riled up. All I know is this has been a good opportunity to stash away some very good argumnets for future consideration. Thanks for stirring things up a bit. A have to say that I'm still favoring tightly coupled these days, but I also make sure to have contiguous reference plane return paths. Contrary to your statemnt, however, I'm seeing better edge rates with tightly coupled pairs, but that may be a function of lower Zdiff and not the tighter coupling itself. I can't say I fully understand the phenomenon and I have not done any experiments comparing apples and apples. I may do so when I get a chance. =20 Brian P. Moran Signal Integrity Engineer Intel Corporation brian.p.moran@xxxxxxxxx -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Cheng Sent: Thursday, October 09, 2003 2:26 PM To: 'leeritchey@xxxxxxxxxxxxx'; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Diff.Pairs I would like you to answer a simple question : If tomorrow you are going into a client's office to consult on designing = a 2.4GB/s differential signal system. Will you recommend them to "routed thousands of differential signal where each member of the pair is on a different layer". Do you think that is good engineering practice ? Do = you think you can still keep your job as a consultant after making that statement ? We all cheat in some of our design, when spacing is tight we might = change the minimum separation between trace from x mils to y mils. And most of = the time we can get away with it because everything being worst case and = fail your margin is very rare. But that doesn't mean it is good design = practice or something you should brag about.=20 This is a public discussion forum and people are welcome to share = whatever ridiculous idea they have as long as they themselves believe in it and actually practicing it. I just want to make sure you "talk the talk and = walk the walk". -----Original Message----- From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx] Sent: Thursday, October 09, 2003 9:30 AM To: Doug Brooks; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Diff.Pairs ..........................This is a very common termination for 2.4 GB/S signal links.=20 It is time to stop representing differential signals as needing to be tightly coupled to each other in order to operate properly. It is = simply not so. I have routed thousands of differential signal where each = member of the pair is on a different layer. If this were not possible, 1 mm = pitch BGAs with differential signals would be un routable. There are tens of thousands of such parts being shipped every month on PCBs where they are routed apart from each other. This is all described in my recently published book, "Right the First = Time, A Practical Handbook on High Speed PCB and System Design". It is also covered in Howard Johnson's new book whose title escapes me at the = moment.. Lee ------------------------------------------------------------------ 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