From: Larry Miller ([email protected])
Date: Fri May 12 2000 - 07:37:50 PDT
At 03:49 PM 5/11/00 -0700, 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.
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.
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