Hi, Barry, Responses below: At 03:49 PM 5/11/00 -0700, Barry Ma wrote: >Hi, > >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.
Due to skin effect at anything above a few tens of kHz, the current flows mainly on the surface of the trace, so it has to interact with the dielectric in accordance with Maxwell's equations for electromagnetic waves. >(2) The second question is regarding distance between the cap and the chip. Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it. > No, you have to let the wave propagate (see below). >Take the running water system for example. When we open, then close the water faucet within one second, does the water we've got in basin come from water tower (or water station, or reservoir)? No, it is the water that resides in the pipe. As a matter of fact, we have a very large pipe - pwr/gnd planes. Well, of cause you know, I did not mean we don't need water tower - the cap. ...... The running water analogy breaks down here (at AC). Another analogy would be that the current is comprised of many successive collisions between billiard balls, not the motion of one single ball. Yet another analogy would be to look at the current as a game of Chinese checkers, where an individual electron can only move into a hole in the board vacated by another electron (if you overcome this you get superconduction!). As I recollect, the actual speed of an identifiable electron, assuming you could actually identify it, is on the order of a few meters/sec, though the electromagnetic wave caused by the transfer of energy between electrons travels at the speed of light in the medium. Larry Miller **** To unsubscribe from si-list or si-list-digest: send e-mail to [email protected]. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu ****
