[SI-LIST] : Re: Charge moving from decoupling capacitors

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From: Barry Ma (barry_ma@altavista.com)
Date: Tue May 16 2000 - 13:29:29 PDT


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
bma@ANRITSU.com

--------
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)

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