Ok, I will give an example of both transmission line emitting EMI and why tight coupling differential pairs solve the problem. Take your best package with the lowest possible lead inductance. Run your signal trace on the surface as microstrip. Reference it to a perfect solid plane as you like to mention. Except the solid plane is tied to a power plane that has nothing to do with the I/O power or ground. Guess what, you will see both pwr/gnd bounce and large EMI emission because the image current flowing on the reference plane does not have a low impedance path to return to the I/O source. This is a classic example of so many experts in this message board like to sell you thin dielectric core bypass planes because it is the best way to provide the low impedance for the image current to return after you make the mistake of referencing signals to the wrong plane. You can dig up lots of papers that claim how EMI is changed with the additional of thin core. What the paper won't tell you is if you reference the microstrip with the proper I/O power or ground and proper on die decoupling, the EMI and SSO noise will be even smaller. I have seen this in real design and fixed the problem with the proper referencing plane. Now, to further the claim of advantage of coupling differential signals, run a differential pair on top of the same reference plane that has nothing to do with the I/O power or ground. The SSO and the EMI will be substantially decreased. Why ? The image current of the differential signals on the reference plane cancels each other out due to the tight loop and there is no large return current other than the common mode which can be decoupled at the terminator. I am not a brag but in designs where layers count is tight and I am forced to bump signals up to top layer with core power planes, I have success in taking all the differential clocks out to that layer while keeping the single ended highspeed buses as striplines with proper reference plane inside. The key is tight coupling on the differential clock on the outer layer. What do you think ? -----Original Message----- From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx] Sent: Monday, October 13, 2003 7:07 AM To: bdewitt@xxxxxxxxxxxxx; Charles Grasso; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Traces don't cause EMI - really? Brent, I've tried several times to measure significant emissions from a single transmission line travelling over a plane at the distances above the plane one uses to create signal paths for logic circuits, the topic of concern in this thread, with no luck. I welcome measurements from others that supports the statement that traces over planes are significant sources of EMI. So far, no one has presented such evidence. Lacking such evidence, how can anyone make such claims? True, this thread concerns differential signals, but how are they different from single ended signals? True, Doug Brooks' paper didn't set out to measure emissions from traces on outer layers, but it did state that measured emissions were 30+ db below what a mono pole would emit. In all my measurements, the lead frames of the ICs are the big radiators. They make nice antennas. They stick up above the PCB and they have significant transient currents flowing through them, especially the power leads. Check out a PLCC with a near field probe sometime. Time for some measurements from those who want this discussion to seriously come to any conclusion. Anyone have any? Lee > > "Second, traces that run over continuous planes don't cause EMI, > > differenrtial or otherwise. Again, there are many papers that demonstrate > > this as well." ------------------------------------------------------------------ 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