Jon, I am pretty sure that I saw a paper on this a few years back on the same point. There is also some coverage of it in Dr. Johnson's book showing how tight meanders cause dispersion through adjacent coupling. A group of tight meanders can also make a pretty effective patch antenna. You can avoid this by using wide meanders. Regards, Steve. At 01:01 AM 3/12/2003 -0800, Jon Powell wrote: >My problem with 10 mil matching and serpentines nets is I think it >is all a self deception. The many curves of the serpentines and the self >coupling >envolved (tell me you serpentine with ground trace guards) adds change of >delay to the >nets that far out weigh any 10 mil length matching. I think the only way the >length matching is >going to be accurate to anything like 10 mils is if you do near perfect >mirroring of the the matching trace routes. > >Not saying you shouldn't do the length matching. Just don't believe you are >really matching to 10 mils. > >so shoot me. > >jon > > >-----Original Message----- >From: si-list-bounce@xxxxxxxxxxxxx >[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Robert Haller >Sent: Tuesday, March 11, 2003 5:40 AM >To: jmartinson@xxxxxxxxxxxxxxxxx >Cc: si-list@xxxxxxxxxxxxx >Subject: [SI-LIST] Re: Length matching of source synchronous busses. > > > >I too find myself compelled to add to this interesting thread. I see a >paralel >to the controlled impedance coupon thread. > >First off if timing specifications are critical , design and build a >simple test board >representative of your actual board and characterize the technology. > >Having spent a few summers working in a Board shop (prior to becoming an >engineer) >I got a good appreciation for boards manufacturing processes and >variables first hand. >There is no replacement for a good symmetrical stackup design, with >input from your >manufacturing engineer. > >I have measured good designs that yielded variations of less then 1 >ps/inch across multiple >layers, lots and boards at a particular edge rate. When working at very >high speeds or with >source synchronous interfaces, you will find that (propagation delay) >Tpd is one of the >most important electrical parameters. > >The best way to Measure Tpd is to send an edge down a line of some know >edge rate. >The edge rate defines the frequency content and the Tpd is a function of >frequency. >The challenge is zeroing out the contribution of fixturing. > >I designed controlled impedance boards with ECL back in the early 80's . >Initially we >found many board manufacturers resistant to impedance and propagation >specifications. >Subsequently we specified mechanical dimensions and listed impedance and >prop delay >targets and ranges as are reference. Eventually with the onset of source >synchronous >(i.e. Clock forwarding as the term was originally call back at DEC, ) >critical timing >specifications (i.e. Z0 and Tpd) became paramount and requirements. > >There is a tendency in the industry for people to over design for >characteristic impedance >when propagation delay is often a more critical parameter. > >So get into the Lab early and Often, and you will avoid sitting in the >lab debugging problems >with a manager sitting over your shoulder asking for daily status >reports :-) > >my $.02 > >bob > >Robert J. Haller (rhaller@xxxxxxxxxx) >Principal Consultant >Signal Integrity Software Inc. >6 Clock Tower Place, Suite 250 >Maynard, MA 01754 >Phone: (978) 461-0449, ext 15 > > > > > > > > > > > > > > > > > >Jerry Martinson wrote: > > >While reading this thread, I was reminded of an issue I had seen about > >two years ago that goes along the same line of needing to take > >everything into account when trying to length or delay match nets. When > >matching rules are as close as 0.010", many non-obvious effects are > >often more significant than the residual length difference. > > > > >... > >------------------------------------------------------------------ >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 > ------------------------------------------------------------------ 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