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

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From: Christian Schuster ([email protected])
Date: Thu May 11 2000 - 23:04:19 PDT

Hi Barry Ma,

let me try to answer your 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.

Consider the analogy of sound waves propagating through the air:
Are they accompanied by molecules moving at the speed of
sound (approx. 330 m/s)? No, they are not. What is moving there
at the speed of sound is the change in density and pressure of the
medium air (the molecules itself make only very tiny movements).

In the case of a guided electromagnetic wave the charges istelf are not all
moving at the speed of light (actually -- if they did -- what about all
these expensive charge accelerators?). They, too, make very tiny (and slow)
movements. Nevertheless the accompanying change in charge density moves
at the speed of light (in that medium)! And that is what's making up your signal!
> (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.

I think time of "flight" is of minor concern here. It's mainly the inductance
of the current loop cap-chip that increases with larger distances and
which reduces or even annihilates the effect of the cap.

I hope this helps.

Best regards,

Ch. Schuster

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