Earlier in my career, I had occasion to run various designs at Xerox through FCC Class B EMI certification. Many of these were IBM PC like motherboard systems where four layer PCBs were the rule. Hence all signals were Microstrip. All of these systems were in metal enclosures some of which had bays for installable devices such as Hard drives. Some of these opening were left open initially, essentially covered by plastic only panels. Measurements were made. Virtually none of these initially passed FCC Class B without some work. Sometimes the Drive areas could be left open whereas very small openings seemed to act as wave guides for higher frequency emissions. Running the clock spread spectrum (dithering) allowed many of these systems to pass as the emissions were spread over a wider spectrum with respect to time hence passed levels of Class B. Where it not for metal cases or some means of effective containment, none of these systems would pass emissions. FCC class B is more stringent than FCC Class A. If your electronics in going in a good EMI envelope, spending a lot of extra time on controlling board emissions may have diminishing financial rewards. But. . . Card to card coupling: If you have two cards side by side, say in adjacent card slots (0.800 inches away) and one of those cards has a highly susceptible circuit, you may need to revert to shielding to gain successful noise margins or increase separation distance or both. Steve Salkow Lockheed Martin -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Lee Ritchey Sent: Sunday, October 19, 2003 5:06 PM To: Larry Barnes; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Traces don't cause EMI - really? I downloaded a copy of this paper a few minutes ago. The conclusions of the paper do not state that microstrip traces emit significant EMI. All that is stated is that the analysis and chamber measurements agree within measurement accuracy of what little energy is detected. The actual amount is not stated in the paper. There is certainly no claim that the detected emissions are large enough to cause concern among EMC engineers. This is likely one of those cases where the difference is between detectable and significant. There is no claim in this paper that the emissions are significant, only detectable. If we are to accept that surface traces are significant sources of EMI, there needs to be credible demonstration of this. The reason is, designers are expected to expend cost avoiding microstrip traces. To do this, there needs to be demonstrated emissions that are worth the extra cost to control. My challenge to the proponents of this claim is to provide this demonstration. I and many others have designed hundreds, maybe thousands of PCBs with traces on outer layers with all the products successfully passing all emissions tests. That could not happen if these claims are true. Lee > [Original Message] > From: Larry Barnes <larry.barnes@xxxxxxxxxx> > To: <si-list@xxxxxxxxxxxxx> > Date: 10/18/2003 5:42:14 AM > Subject: [SI-LIST] Re: Traces don't cause EMI - really? > > I just couldn't resist after all this discussion. I would refer you to = > a study done by Dr. Dave Hill of NIST. I demonstrates that microstrips = > indeed radiate. The title of the paper is "Radiated Emissions and = > Immunity of Microstrip Transmission Lines: Theory and Reverberation = > Chamber Measurements" IEEE Trans on EMC May 1996.=20 > > Larry > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =3D= > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > Lawrence C. Barnes > QLogic Corporation > Aliso Viejo, CA 92656 > > > ------------------------------------------------------------------ > 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