What?!?! You don't think I have it memorized? Ok, you caught me... As far as I can see, it doesn't specifically say whether the trace = dimensions were adjusted. In fact, it doesn't say how he brought the = trace closer to the ground plane. I was trying to emphasize that = Dockey's paper concludes only that traces farther away from the = reference plane (decreasing Mgs - mutual inductance between signal and = ground - in his paper) create more EMI; he doesn't talk about the = relationship between impedance and EMI (is there one?). Jeff Loyer -----Original Message----- From: Grasso, Charles [mailto:Charles.Grasso@xxxxxxxxxxxx] Sent: Thursday, October 16, 2003 4:26 PM To: Loyer, Jeff; 'MikonCons@xxxxxxx' Cc: 'si-list@xxxxxxxxxxxxx' Subject: RE: [SI-LIST] Re: Traces don't cause EMI - really? Hi Jeff, It sounds like you have Dockeys paper in front of you. If I remember rightly, the trace dimensions were scaled to maintain a constant 50 ohm impedance. Best Regards Charles Grasso Senior Compliance Engineer Echostar Communications Corp. Tel: 303-706-5467 Fax: 303-799-6222 Cell: 303-204-2974 Email: charles.grasso@xxxxxxxxxxxx; =20 Email Alternate: chasgrasso@xxxxxxxx =20 -----Original Message----- From: Loyer, Jeff [mailto:jeff.loyer@xxxxxxxxx]=20 Sent: Thursday, October 16, 2003 2:06 PM To: MikonCons@xxxxxxx Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Traces don't cause EMI - really? Thanks for sharing! This ("a 100-Ohm trace radiated >6 dB more than a 50-Ohm trace") seems = =3D to agree with Dockey's paper - the farther away you are from the =3D = reference plane (and, consequently, the higher the impedance), the more =3D EMI = you radiate. The question I'm confronted with at this point is: do "well-behaved" =3D single-ended signals generate appreciable EMI? Dockey's paper outlined = =3D the effect, but with geometries (90mil wide trace over a 62mil =3D dielectric) that aren't likely to be used. He then demonstrated the =3D reduction in EMI when the trace was brought closer to the reference =3D = plane. Given current geometries, is it likely for a bus of properly =3D = designed single-ended traces to cause failing EMI? My =3D experience/impression = is no. =3D20 If that's incorrect, please let me know. On the other hand, as I understand it, differential traces are much more = =3D "forgiving", EMI-wise, of non-idealities. And these we often induce, = =3D knowingly or not. So, tightly-coupled differential traces are =3D = desireable, EMI-wise, given our imperfect world and the issues we must =3D deal = with. That's not to say there aren't other reasons, besides EMI (increased =3D = loss, and some yet to be discovered/published), why we may NOT want to =3D = route differential traces closely-coupled. Unless I'm incorrect about current single-ended traces and their =3D = tendency (or lack of) to generate EMI, I'd stick with my original =3D assertion, = that closely coupling differential signals has benefits, but =3D is not = absolutely necessary for their entire length (but compensation =3D for impedance = change due to reducing or increasing coupling might be =3D necessary). Thanks for your time/energy. Actually, I've been contracted by the =3D fishees to distract you from pursuing them :-) Jeff Loyer -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx = [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of MikonCons@xxxxxxx Sent: Thursday, October 16, 2003 11:33 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Traces don't cause EMI - really? In a message dated 10/16/2003 10:14:54 AM Pacific Standard Time,=3D20 jeff.loyer@xxxxxxxxx writes: Just to be clear - are you saying that if I connected 2 properly =3D designed=3D20 chips (driver and receiver) = together with many properly designed =3D single-ended=3D20 transmission lines, they = would likely fail FCC standards? Yes, but only with qualifications. Picture a single-ended, 50-Ohm =3D microstrip,=3D20 not multiple closely spaced = traces. If excited with a 1 ns risetime =3D signal at=3D20 33 MHz clock frequency = from a 50-Ohm generator, and terminated with a =3D 50-Ohm=3D20 chip resistor, = expect failure. For example, the EMCAD1 software =3D application=3D20 (circa = 1992) is based on radiated emissions equations and predicts ~39 =3D dBuV at=3D20 = 33 MHz at 30 meters antenna distance. The first few harmonics are also=3D20 = predicted to be out of limits. Note that this software uses worst-case =3D predictions.=3D20 The same software also indicates an increase in the single-trace =3D emission level=3D20 by SQRT(N), where N =3D3D the = number of traces; however, the assumption is =3D that=3D20 all traces are carrying = the same excitation (which is unrealistic).=3D20 Now, if you place grounded = planar areas around the trace, or have a =3D densely=3D20 routed board, the = additional copper provides a substantial reduction in =3D the=3D20 measured = emissions from that single trace because many field lines that =3D would=3D20 otherwise = leave the board terminate on the added copper (even though =3D they may = be=3D20 additional 50-Ohm traces). And, in a normal board design, many of the = =3D signals=3D20 on these surface traces will generate fields that cancel = each other.=3D20 Therefore, the bare (i.e., unshielded, and unenclosed) PCB = MAY or MAY =3D NOT fail an FCC=3D20 or CISPR radiated emissions test.=3D20 The IBM paper I mentioned recorded up to 20 dB variation in = radiated=3D20 emissions from a single microstrip on a bare board as the trace was =3D = moved in-board=3D20 from the PCB edge. This measured reduction also = illustrates the =3D field-capturing=3D20 effect of surrounding copper, reference planes, = or other conductors. The PCB I created contained significant open surface space around the = =3D traces=3D20 (including the guarded trace structures) which tended to = increase the =3D radiated=3D20 emissions relative to a real board design, but = this was on purpose to=3D20 illustrate the effect. As an added note, I found that the radiated emissions from a single =3D trace=3D20 increased more than linearly with the impedance of the line; = i.e., a =3D 100-Ohm=3D20 trace radiated >6 dB more than a 50-Ohm trace. This = result makes sense =3D as a better=3D20 match to the 120*pi =3D3D 377 Ohms of = free space is achieved with the =3D 100-Ohm=3D20 line. Mike Michael L. Conn Owner/Principal Consultant Mikon Consulting *** Serving Your Needs with Technical Excellence *** ------------------------------------------------------------------ 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: =3D20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages=3D20 Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =3D20 ------------------------------------------------------------------ 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: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages=20 Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =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: //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