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

From: Yu Wang ([email protected])
Date: Wed May 17 2000 - 07:45:51 PDT

Hi, Barry,
I think we all do agree the 1st point you said.
But on the 2nd point, I have another opinion. Say, you
are right,"When an electrical potential imbalance
happens in a metallic plane, a current would flow on
the plane for regaining the equi-potential."
But we regard the metallic plane as a reference plane,
then, theoretically, there is no an electrical
potential imbalance happens in a metallic plane. In
fact, we can never get a ideal reference plane(even
the earth is not IDEAL). Based on this, I would say
"yes" on your question "Does it need EM field support
from the dielectric?". Still but, the need of the
support is usually slight if the electical size of the
metallic plane is big enough. because we know the
metallic plane is a equi-potential plane.

regards,
Yu Wang

--- Barry Ma <[email protected]> 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
> [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)
>
>
>
>
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=====
Yu Wang, Ph.D
U.T. MD Anderson Cancer Center
1100 Holcombe Blvd., Box 217
Houston, TX, 77030
Tel:713-745-1671

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