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

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From: George_Tang@Dell.com
Date: Thu May 18 2000 - 12:30:45 PDT


Thanks for the comments. Here are my comments:

Ok, you put caps at a certain distance away from the IC because you only
want them to work at 100 MHz. But that distance turns out to be the 1/4
wave distance at 400 MHz, and you placed enough caps at the 1/4 wave
distance to cause board resonance. Now what? Do you tell the caps not to
work at 400 MHz because it's not their frequency?

For your 2nd comment:

I used the words "loosely define" for that reason. If you are interested in
high frequency decoupling and instantaneous current, you really want to have
all your charges moving in phase. At 1/4 wavelength, the charges are 90
degrees out of phase, so they will not do much for your instantaneous
current. 1/8 wavelength is what I consider to be acceptable. You can
certainly pick a different number.


George Tang

-----Original Message-----
From: Barry Ma [mailto:barry_ma@altavista.com]
Sent: Thursday, May 18, 2000 10:50 AM
To: George_Tang@exchange.dell.com
Cc: si-list@silab.eng.sun.com; emc-pstc@ieee.org
Subject: RE: Charge moving from decoupling capacitors


Thanks for your long input. I'd like to make some comments below.
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
> 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.

Barry Ma


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