Barry, You're hitting upon one of the basic fundamentals of how radiated emissions occur.
The situation that Andrew is describing occurs if we design our boards properly, with a low inductance return path for each current and with proper impedance matching; then our traces will act more like "waveguides" and "transmissions lines" where the charge flows in the conductors and the corresponding field stays mostly contained in the dielectric. The other situation that you're describing is the situation where there is no "loop" for return current. In this situation, the trace is acting more like an antenna which is more or less a conductor with a time varying potential at one end which causes charge to accelerate back and forth within the conductor. Since there is no readily available return path, the current is more of a common mode current which will find it's own "return path" by radiating a field (displacement current?) to another conductor. The problem frequencies will be determined by the frequency at which the charge accelerates (changes speed or direction) and by the distance that the charge accelerates (length of antenna). This is the main mechanism for the radiated emissions that we get paid to minimize. As for the velocity. The equations for a stripline may be used (too complex for me to write using an email editor). One good reference is pages 407-411 of "Fields and Waves in Communication Electronics" by Ramo, Whinnery and Van Duzer, published by Wiley. In short, you can only use the velocity of propogation from the dielectric if you have a perfect stripline (strip conductor sandwiched in dielectric between two very wide conductive planes). Correction factors need to be applied in other situations. Thanks for bringing up such a challenging topic. It helps keep the juices flowing. Chris Maxwell, Design Engineer GN Nettest Optical Division 109 N. Genesee St. Utica, NY 13502 PH: 315-797-4449 FAX: 315-797-8024 EMAIL: [email protected] > -----Original Message----- > From: Barry Ma [SMTP:[email protected]] > Sent: Tuesday, May 16, 2000 4:29 PM > To: EMC-PSTC; [email protected] > Subject: Re: Charge moving from decoupling capacitors > > > Hi Andrew, > > You said: "It is just like an ordinary transmission line such as > stripline. " > > Please allow me to say something different. > > (1) When a signal propagates along a transmission line, we could observe a > current loop from source to load through the transmission line. The signal > velocity is the same as the speed of light in the dielectric. You are > right. > (2) When an electrical potential imbalance happens in a metallic plane, a > current would flow on the plane for regaining the equi-potential. This > current looks different from the signal current. There's no current loop > here. Does it need EM field support from the dielectric? If not, should it > have a different velocity? That is my point. I have no answer, and > appreciate any input. Thanks. > > Bets Regards, > Barry Ma > [email protected] > > -------- > Barry Ma wrote: > As the speed of digital signals gets faster and faster, people begin > being > concerned with the distance for electric charge to move on power and > ground planes of multilayer PCB during the signal rise time from a > decoupling capacitor (cap) to a chip it serves. I would like to raise two > questions. > > (1) The charge is moving in a metalic plane, not inside the dielectric > between pwr and gnd planes. Please let me know why you have to use the > propagation velocity in the dielectric, instead of that in the metal. > ------ > Ingraham, Andrew wrote: > > The charge may be moving in the metal, but the energy (which makes the > charge keep moving) is primarily in the electro-magnetic field between > the > planes, in the dielectric. The charge won't move unless there is an E-M > field to push it. > > It is just like an ordinary transmission line such as stripline. The > propagation velocity of a trace is that of the dielectric, even though > the > charge moves only in the metal trace and planes. > > (Edited by BM) > > > > _______________________________________________________________________ > > Why pay when you don't have to? Get AltaVista Free Internet Access now! > http://jump.altavista.com/freeaccess4.go > > _______________________________________________________________________ > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: [email protected] > Michael Garretson: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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