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

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From: Barry Ma (barry_ma@altavista.com)
Date: Thu May 18 2000 - 10:49:38 PDT


George,

Thanks for your long input. I'd like to make some comments below.
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On Wed, 17 May 2000, George_Tang@Dell.com wrote:

> Large parallel plates behave as transmission lines. A quarter wavelength
> transmission line with a short at the end has infinite impedance, so
> capacitors placed 1/4 wavelength away are bad.

Thatís why decaps work on low frequency portion. Letís set 100 MHz and below for decaps to cover. The wavelength at 100 MHz is 3 meters. A quarter of it is 75 cm. Itís long enough to ordinary PCB size. (The cap is directly connected to pwr/gnd planes.)

> This means that we can loosely define the largest usable board area capacitance as 1/8
> wavelength radius of copper surrounding the IC power pin. Charges stored on the planes
> further than 1/8 wavelength away are not very usable due to the time delay.
> At 500MHz in FR4, 1/8 wavelength is 1.5 inches. Is such a board capacitor
> good enough for your IC?

George, I beg for differentials. How did you jump from "capacitors placed 1/4 wavelength away are bad" to "the largest usable board area capacitance as 1/8 wavelength radius"?

Can I use the same token to infer from "caps placed one wavelength away are good" to "the largest usable board area capacitance is within 1/2 wavelength radius"? And so, and so on.

Regards,
Barry Ma
bma@ANRITSU.com

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