I remembered answering this at least twice a long time ago & dug this = out from SI archives-it also contains an older reference.=20 Hope it can help, Jeff LaT. =20 [si-list] || [Date Prev] [08-2002 Date Index] [Date Next] || [Thread = Prev] [08-2002 Thread Index] [Thread Next]=20 [SI-LIST] Re: PCB tracks From: "LATOURRETTE,JEFF (A-SanJose,ex1)" <jeff_latourrette@xxxxxxxxxxx>=20 To: "'johnlipsius@xxxxxxxxx'" <johnlipsius@xxxxxxxxx>,SI-LIST = <si-list@xxxxxxxxxxxxx>=20 Date: Thu, 8 Aug 2002 11:09:41 -0600=20 John, Philippe: Another reason for 45 degree angle or curved traces instead of 90 degree = bends=20 is to reduce what ends up being excess capacitance due to an abrupt = bend. This=20 capacitance occurs as the fields change from a nice even distribution on = straight microstrip to very concentrated on the inside and less = concentrated on=20 the outside of the bend. This problem is more pronounced with wider, = lower=20 (like 30 to 50-ohm) impedance lines, becoming much less significant at = 75-ohms=20 or 100 ohms. =20 If you are stuck doing 90 degrees and working at high frequencies, you = can=20 miter (remove metal) at the bend to reduce excess capacitance in your=20 high-speed signal traces. You can also think of it as slightly raising = the=20 impedance of the line at the bend, which when done correctly can = significantly=20 improve return loss. This is probably even harder for PCB houses than = 45=20 angles, so be sure your application really needs it. As a quick check you can take that corner, mitered off, calculate its=20 capacitance and then reactance at your frequency of interest. For most=20 frequencies below 1 GHz, this becomes insignificant and experiments can = show=20 that mitering does little good at very low frequencies. This technique = is used=20 on most microwave designs where they are ringing out every last dB of = Return=20 Loss performance from a design. A lot of digital systems & components = don't=20 even specify VSWR/Return Loss and may be able to tolerate more mismatch. From an MTT article on how to optimally miter a microwave bend, mitering = can=20 improve performance at frequencies as low as 1 GHz, so your comment on = not=20 needing to worry at 200 MHz is probably right on. It just depends what = kind of=20 mismatches (VSWR) your system can handle, and how many harmonics above = your=20 signaling rate you are designing to. I think I posted this reference here once before, but for more:=20 =20 "Experimental Study of Symmetric Microstrip Bends and Their = Compensation" =20 Douville & James, IEEE Transactions on Microwave Theory & Techniques, = Vol.=20 MTT-26. No. 3, March 1978, pp. 175-181 =20 This article, due to its age, only covers out to 3 GHz, but the = principles in=20 it are applied successfully up into 10's of GHz, although if you're up = in that=20 range, you can easily verify the effect on your field solver. I agree with John that the amount of energy launched into evanescent = modes is=20 probably a 3rd order effect and not worth worrying about. I'm not sure, however, I agree that you can ignore this at 10 Gb/s. = Since a 10=20 Gb/s signal carries with it some very high frequencies and seems to = start to=20 tax the limits of some PCB materials, cables and connectors, I'd think = you'd=20 want the best possible Return Loss in your signal traces. Now you have $0.03, Jeff LaT. =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: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: http://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