Charles, Actually, these were edge coupled differential stripline pair that I presented in my table. To better read the columns, they are as follows in order from left to right: trace width trace edge-to-edge separation dielectric height (always a constant 14 mils) routed width (= 2*trace width + separation) conductor loss (dB/in) dielectric loss (dB/in) total loss (dB/in) loss for 20" trace loss for 30" trace frequency = 1.5625 GHz Impedance = 100 ohms Er = 4.2 tanDelta = 0.02 bottom line, the difference in loss for 30" of trace that is routed differentially with 7.5 mil lines and 2.5 mil lines, with impedance held at a constant 100 ohms, is -7.317 dB vs. -8.658 dB. This is a difference of -1.34 dB, which may or may not be significant for a given application. However, the difference in linear space required by the two are 45 mils vs. 8 mils for an improvement of 560%. So, to say that closely spaced differential pairs are always a "bad" design practice is just not true. If you can tolerate some slight increase in loss, they may be quite good. regards, scott Charles Grasso wrote: >Scott, >thanks for soing this. I have a question though. I infer >that this a stripine broadside coupled pair? > >How about a similar study for an edge coupled pair >ustrip configuration? > > >-----Original Message----- >From: si-list-bounce@xxxxxxxxxxxxx >[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Scott McMorrow >Sent: Friday, October 03, 2003 3:53 PM >To: silist >Subject: [SI-LIST] Re: Diff Pairs > > >All, >So, "just for fun" I decided to run a differential pair trace through a >field solver and put some numbers to our musings on differential pair >loss vs. width and spacing. As Lee Richey likes to point out, it is >always good to add a bit of actual science to our conjectures about >these things. > >For this particular experiment I assumed FR-4 with an Er = 4.2 and a >loss tangent of .02, which is representative of some of the resent >designs we have built and measured here at Teraspeed. The dielectric >width from plane to plane was kept at a constant 14 mils, and the >differential pair was constructed as stripline in the center of the >layer. Finally, the pair width and spacing was adjusted from weak >coupling to tight coupling, always keeping differential impedance >constant at 100 ohms. Here are the results for a frequency of 1.5625, >which is the Nyquist frequency for 3.125 Gbps binary data streams:: > >width separation height routed width conductor loss >dB/in > dielectric loss dB/in total loss dB/in 20" 30" > > > > > > > > > >7.5 30 14 45 0.0958 0.1481 0.2439 > 4.878 7.317 >6.79 13.89 14 27.47 0.0973 0.1481 0.2454 > 4.908 7.362 >6.05 9.62 14 21.72 0.101 0.148 0.249 4.98 7.47 >5.4 7.55 14 18.35 0.1052 0.1481 0.2533 > 5.066 7.599 >4.82 6.25 14 15.89 0.1098 0.1481 0.2579 > 5.158 7.737 >4.3 5.35 14 13.95 0.1147 0.148 0.2627 5.254 > 7.881 >3.84 4.67 14 12.35 0.1197 0.148 0.2677 5.354 > 8.031 >2.5 3.07 14 8.07 0.1406 0.148 0.2886 5.772 > 8.658 > >There are several things to note here: > >1) Since impedance is kept constant, dielectric loss is constant. >2) As pointed out by others, as trace spacing becomes smaller, and trace >width becomes narrower, and conductor losses increase. >3) Loss is dominated by dielectric loss. >4) For long traces there is just a titch less than 20% improvement in loss >by using a 7.5 mil conductor vs. a 2.5 mil conductor, with constant >impedance. >5) Trace linear density is improved by greater than 550% by using a narrower >conductor. >6) A wide trace is easier to manufacture and has better impedance control >than a narrow trace. > > >So, as with all things in engineering, there are tradeoffs. We can trade >off loss for density by changing the conductor width. We can also trade off >manufacturability, since a narrow conductor with small space is harder to >produce and control. We can trade off poor vs. good impedance control. All >of these must be evaluated for any design. > >One assumption that everyone makes is that loss is bad. This is not always >the case. Given a choice, I generally appreciate having a bit of conductor >loss in a design, as it tends to de-Q resonant circuits quite nicely. >Little things like package, connector, blocking capacitor and via >discontinuites can benefit from a bit of increased resistive loss. > >If you require the density, close spaced, narrow width differential pairs >win hands down, at a slight sacrifice in loss performance. The interesting >thing to consider will be the overall system performance. Due to other >discontinuities, it may be that overall performance is actually improved by >a little additional loss here and there. > > >regards, > >scott >(Who doesn't have a book.) > >-- >Scott McMorrow >Teraspeed Consulting Group LLC >2926 SE Yamhill St. >Portland, OR 97214 >(503) 239-5536 >http://www.teraspeed.com > >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 >>>>>============================================ >>>>> >>>>> > > > >------------------------------------------------------------------ >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 > > > > -- Scott McMorrow Teraspeed Consulting Group LLC 2926 SE Yamhill St. Portland, OR 97214 (503) 239-5536 http://www.teraspeed.com ------------------------------------------------------------------ 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