One obvious benefit is the reduction in EMI because of reduced loop area. I have verified this through simulation. However, I am not able to understand the phenomenon of edge degradation due to close coupling. I ran some quick simulations on six inch long differential Microstrip nets under the following conditions: Case 1: Trace width 9 mils, separation 4 mils, differential impedance 99.3 ohms Case 2: Trace width 12.5 mils, separation 40 mils, differential impedance 98.8 ohms The driver had a rise time of 250 ps. The only difference I observed between the two waveforms was that in the first case the propagation time was 63 ps less than the second case. This is understandable since the signals in the two branches of differential net have opposite polarity, the coupling effect will speed them up. Next I tried the same simulation for Stripline nets. In this case, there was practically no difference between two waveforms (less than 5 ps difference in propagation delay). Am I missing something here. Regards, Ravinder Server PCB and Flex Development Hitachi Global Storage Technologies Email: Ravinder.Ajmani@xxxxxxxx "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx> Sent by: si-list-bounce@xxxxxxxxxxxxx 10/03/2003 11:41 AM Please respond to leeritchey To: "Duane Takahashi" <duanet@xxxxxxxxxxxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx cc: From: si-list-bounce@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Diff.Pairs More than that, it does not have any benefit. Tight coupling of differential pairs forces the traces to be narrower increasing the skin effect losses. Also, this tight coupling is going to result in good old cross talk that actually degrades the edges. How the notion of tight coupling of differential pairs as beneficial got started is a mystery to me. There are several references that show that tight coupling is not beneficial, one of them is Howard Johnson's latest book, at least one column he has written and my recently released book. Lee Ritchey > [Original Message] > From: Duane Takahashi <duanet@xxxxxxxxxxxxxxxxxxxxxx> > To: <si-list@xxxxxxxxxxxxx> > Date: 10/2/2003 3:58:59 PM > Subject: [SI-LIST] Re: Diff.Pairs > > Hi Juergen: > > Aligning the stack up for the broadside coupled diff lines is expensive. > You can do this, but it drives up the cost of the board. > > Duane > > > Hi Juergen, > > You can find lots of application notes > > especially with respect to process variation > > on differential pairs here: > > > > > > www.polarinstruments.com/support/cits/cits_index.html > > > > In particular this one may be of interest: > > > > > > How measured impedance may vary from field solver calculations when > > using woven glass reinforced > > <http://www.polarinstruments.com/support/cits/AP139.html>laminates > > > > www.polarinstruments.com/support/cits/AP139.html > > > > > > And this note: > > > > Copper thickness, edge coupled lines and > > characteristic > > <http://www.polarinstruments.com/support/cits/AP151.html>impedance > > > > > > www.polarinstruments.com/support/cits/AP151.html > > > > > > > > Hope this helps.... > > > > > > Kind regards > > Martyn Gaudion > > www.polarinstruments.com > > T: +44 1481 253081 > > F: +44 1481 252476 > > M: +44 7710 522748 > > E: martyn@xxxxxxxxxxxxxxxxxxxx > > > > ============================================ > > Controlled Impedance & Signal integrity tools > > for the Printed circuit fabrication industry > > ============================================ > > > > > > > > > > > > > > At 19:00 02/10/2003, you wrote: > > > >>I am seeking help in finding enlightenment regarding electrical > >>performance pros and cons and how manufacturing tolerances play a role > >>when comparing side by side and tandem differential pairs. I'd appreciate > >>your opinion, experience, analysis, pointers to papers and articels, etc. > >> > >>In return, I would offer to share a summary of the finding/discoveries > >>with interested parties. > >> > >>Thanks > >> > >>Juergen > >> > >> > >> > >>------------------------------------------------------------------ > >>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 > > > > -- > Duane Takahashi phone: 408-720-4200 > Greenfield Networks fax: 408-720-4210 > 255 Santa Ana Court email: duanet@xxxxxxxxxxxxxxxxxxxxxx > Sunnyvale, CA 94085 > > * MOVING! Please note new numbers and address * > > ------------------------------------------------------------------ > 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