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

From: Hassan Ali ([email protected])
Date: Wed May 17 2000 - 07:02:07 PDT

Steve/Barry,

The EM field surrounding a current-carrying material (plane, wire, etc) is a
boundary-value problem. Both the metallic plane(s) and the dielectric (which
is in contact with the plane), influence the current flow and field
distribution when there is a charge imbalance on a metallic plane. In other
words, you would have different field distribution if the plane were
floating in air or immersed in a dielectric medium. Picture a plane charge
imbalance as converting the plane into a dipole antenna (not quite unless
the charge imbalance varies in an AC fashion at a sufficiently high
frequency). And then reason out the difference if the plane were touching a
dielectric layer or not. I mean, there will be a difference!

Hope this doesn't confuse you further :-)

Hassan.

```--
Hassan Ali <[email protected]>
Equipment & Network Interconnect, Nortel Networks
2 Brewer Hunt Way, Kanata ON, K2K 2B5 Canada
Tel: 613-765-1410 (ESN 395) Fax: 613-765-5512 (ESN 395)
-----Original Message-----
From: sweir [mailto:[email protected]]
Sent: Tuesday, May 16, 2000 9:40 PM
To: [email protected]
Subject: Re: [SI-LIST] : Re: Charge moving from decoupling capacitors
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
>[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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```

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