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

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From: sweir (weirsp@atdial.net)
Date: Tue May 16 2000 - 18:40:15 PDT


Barry,

That imbalance in potential is a restatement of: "The electric potential is
different at one point in space versus another, and the plane connects
both." The difference in potential provides an accelerating force for
charge, and the plane provides a path for the charge to move. When the
charge accelerates the EM wave results. The EM wave travels in the
dielectric surrounding the plane. If the dielectric K were infinite then
even the slightest acceleration of the charge would create an opposing
field that would stop any propagation in its tracks.

Regards,

Steve.
At 01:29 PM 5/16/00 -0700, you wrote:
>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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