Lee, I've constructed test boards to measure the effect of non-mitered = 90 degree bends and they are very visible with 50 Ohm traces on boards = 0.062 thick with 0.110 wide traces. Tom Dagostino Teraspeed(R) Labs 13610 SW Harness Lane Beaverton, OR 97008 503-430-1065 tom@xxxxxxxxxxxxx=20 www.teraspeed.com=20 Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 401-284-1827 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] = On Behalf Of Lee Ritchey Sent: Tuesday, July 25, 2006 1:04 PM To: Scott McMorrow; scott@xxxxxxxxxxxxx Cc: Matthias Bergmann; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: 90 degree turn in PCB tracks Way back in the dark ages when we used ECL to do high speed designs, I contacted the author of the book to see why we couldn't duplicate his measurements with an identical setup. The response was that the picture = was wrong, but technical publications was in a hurry to go to press, so they left it in! > [Original Message] > From: Scott McMorrow <scott@xxxxxxxxxxxxx> > To: <scott@xxxxxxxxxxxxx> > Cc: <leeritchey@xxxxxxxxxxxxx>; Matthias Bergmann <MBergmann@xxxxxxxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx> > Date: 7/25/2006 12:14:18 PM > Subject: [SI-LIST] Re: 90 degree turn in PCB tracks > > Y'all, > I just went and pulled up the old app. note. you can find it here=20 > http://www.onsemi.com/pub/Collateral/HB205-D.PDF > > I think the problem people have had is in not putting the old Motorola > measurements in perspective. Clearly if you read the previous pages=20 > you'll find that on pages 140 to 142 the design being tested is that = of=20 > a microwave hybrid divider on a substrate that was 62 mils thick and = had=20 > an Er of 5.3 (Er(eff)=3D 4.5 for microstrip). A hybrid divider takes = a 50=20 > ohm trace and divides it into two 100 ohm traces. If you do the=20 > calculations, that 50 ohm trace was somewhere around 110 mils wide. > > As we go on to page 144, we find the famous corner tests, where=20 > clearly > there are corner discontinuities shown by the TDR. If I were a = betting=20 > man, even though the material thickness and trace widths were not = shown,=20 > I'd bet that they are similar to the previous hybrid divider. So, we=20 > have a case where there is a really fat trace and therefore a big=20 > discontinuity. If we do the math, as I did previously, that=20 > discontinuity should be: > > t(discontinuity) =3D 85 x sqrt(2 x 4.5) x .11 =3D 28 ps. > > In a TDR this time will be doubled, due to the round trip across the > corner, so we would expect a hump with a duration of approximately 50 = to=20 > 60 ps. Even though the plots do not show a scale, I'd guess that = around=20 > a 50 to 60 ps pulse duration would be about right, when you add in = the=20 > additional TDR rise time, itself. > > My conclusion is that there is nothing wrong with the old Motorola=20 > application note. In fact, if you were to replicate this experiment=20 > with 100 mil lines on a thick FR4 substrate today, you'd get pretty=20 > much > the same result. The problem is that people have generalized the data = > to cases with smaller line widths without scaling. If you scale the=20 > Motorola measurements by 1/22 (5 mils/110 mils), those corner=20 > discontinuity impedance bumps will just blend into the noise. They=20 > still exist ... but they are really, really tiny ... and not important = > for modern digital board design ... just as Lee advocates. > > > > regards, > > Scott > > Scott McMorrow > Teraspeed Consulting Group LLC > 121 North River Drive > Narragansett, RI 02882 > (401) 284-1827 Business > (401) 284-1840 Fax > > http://www.teraspeed.com > > TeraspeedR is the registered service mark of > Teraspeed Consulting Group LLC > > > > Scott McMorrow wrote: > > Lee > > The experimental design used in Ultracad corner test is quite > > sub-optimal, given the incredible launch discontinuity. I'd be=20 > > surprised if better than a 35 to 50 ps rise time edge actually makes = it=20 > > into the board. Then, given the length of the traces involved, I=20 > > suspect that the actual bandwidth seen at the corners is somewhere=20 > > around 3 GHz, at best. Unfortunately, there do not appear to be any = > > structures available to determine the actual measurement bandwidth. = > > Just because Todd Hubing was responsible for the measurements, does = not=20 > > mean that the experimental design is as good as an 18ps launch and = 20=20 > > GHz makes it sound. If you can get a fast launch into a board, and = can=20 > > use low loss materials, it is possible to see corners. Gus Panella = and=20 > > I did so about 8 years ago on a test vehicle that we designed using=20 > > Rogers 4350 material. > > > > Having said that, 3GHz bandwidth is typical of the fastest standard > > logic signals on most conventional PCBs. Your assertion that = corners do=20 > > not matter for digital designs is true. > > > > The original source of the corner information comes from the=20 > > Microwave > > literature. Gupta, et. al., Microstrip Lines and Slotlines, is a = good=20 > > reference. It has been well known that, in Microwave design, right=20 > > angle bends are not a good thing. If you're trying to decrease = return=20 > > loss in a microwave design, you will most assuredly use radiused corners=20 > > or the optimal chamfer. Why? Because microwave boards use low loss > > materials, like PTFE, and very wide trace width on thick substrates = to=20 > > reduce total power loss. The corner discontinuity on a 5 mil wide logic=20 > > signal trace on a conventional high density PCB may not be an issue, but=20 > > on a low loss microwave board that uses 50 to 200 mil wide=20 > > microstrip > > traces, that silly little corner is a killer. > > > > The magnitude of the corner discontinuity is proportional to it's > > duration. A corner discontinuity lasts for approximately 85 x = sqrt(2 x=20 > > Er(eff)) x w ps: > > > > Where > > Er(eff) =3D is the effective Er of the material > > w =3D the width of the trace in inches. > > > > If we run the numbers, > > > > For a 5 mil stripline trace on FR4 with and Er(eff) of 4, the > > discontinuity lasts for 1.2 ps. > > For a 100 mil microstrip trace on Duroid with an Er(eff) of 2, = the > > discontinuity lasts for 17ps. > > > > If we then use the rule of thumb that a discontinuity is important=20 > > only > > when it approaches 1/10th of the risetime, then our little 5 mil = corner=20 > > has an effective operating bandwidth of .35/(1.2 e-12 x 10) =3D 29 = GHz. =20 > > Clearly out of the region where we are interested for digital logic. > > > > But for the 100 mil trace, we have an operating bandwidth of=20 > > .35(17e-12 > > x 10) =3D 2 GHz. This is quite frankly not a very good microwave = design. > > > > > > Best regards, > > > > Scott > > > > Scott McMorrow > > Teraspeed Consulting Group LLC > > 121 North River Drive > > Narragansett, RI 02882 > > (401) 284-1827 Business > > (401) 284-1840 Fax > > > > http://www.teraspeed.com > > > > TeraspeedR is the registered service mark of > > Teraspeed Consulting Group LLC > > > > > > > > Lee Ritchey wrote: > > =20 > >> The paper published by Ultracad tests the effects of right angle=20 > >> bends in a > >> logic signal trace with an 18 pSec edge which is equivalent to=20 > >> about 20 GHz. There is no detectable effect. These tests were=20 > >> conducted by Todd > >> Hubing of UMR in a well equipped lab and are to be trusted. There=20 > >> have been amy other similar tests done with the same result. > >> > >> I believe the notion that right angle bends are a source of=20 > >> problems stems > >> from an error in the Motorola ECL handbook published in 1974 and=20 > >> still in > >> print with the error. Now O Semiconductor publishes it. > >> > >> This may be one of those cases where simulation shows a change in=20 > >> the field > >> distribution around the right angle bend, which we expect. The question is > >> whether the change is significant. One of our jobs is to=20 > >> distinguish between visible and significant. > >> > >> Once again, we have lies, damn lies and simulations. Simulations without > >> validation may well be more dangerous than no simulations at all in some > >> cases. > >> > >> > >> =20 > >> =20 > >>> [Original Message] > >>> From: Matthias Bergmann <MBergmann@xxxxxxxxxxxxxxxxxx> > >>> To: <si-list@xxxxxxxxxxxxx> > >>> Date: 7/25/2006 1:42:10 AM > >>> Subject: [SI-LIST] Antwort: Re: 90 degree turn in PCB tracks > >>> > >>> Hello, > >>> > >>> Following the advices I read the article about 90 degree bends on=20 > >>> ultracad.com as well as the appropriate chapter in Eric Bogatins=20 > >>> book. I am quite surprised about the conclusion that 90 degree=20 > >>> corners don't matter. I remember that once in a simulation of a 50 = > >>> ohm microstrip-lin=3D > >>> e > >>> with > >>> a chamfered 90=3DB0 corner in ADS Momentum, I never got a S11 = better than=3D > >>> 15 dB > >>> at frequencies higher than 15 GHz, even 10 GHz made problems. The=20 > >>> mentioned articles don't consider the frequency respectively just=20 > >>> consider designs where the right-angle bend is electrically=20 > >>> smaller tha=3D > >>> n a > >>> rising edge. > >>> Would be interesting to know how two 45 degree corners behave at=20 > >>> higer frequencies. > >>> > >>> Regards, Matthias > >>> > >>> ____________________________________ > >>> > >>> > >>> __________________________________________________________________ > >>> _ > >>> Diese E-Mail enth=3DE4lt vertrauliche und/oder rechtlich = gesch=3DFCtzte > >>> Informationen. Wenn Sie nicht der richtige Adressat sind oder = diese E-M=3D > >>> ail > >>> irrt=3DFCmlich erhalten haben, informieren Sie bitte sofort den Absender =3D > >>> und > >>> vernichten Sie diese Mail. 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