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

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From: George_Tang@Dell.com
Date: Tue May 23 2000 - 12:19:40 PDT


The current can flow from the cap, but it will get to the IC at the wrong


-----Original Message-----
From: Barry Ma [mailto:barry_ma@altavista.com]
Sent: Tuesday, May 23, 2000 10:22 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


I am impressed by your attitude to pursue the correctness, and glad to
discuss with you further on “How does a decoupling capacitor support an IC?”

Here is my two cents worth. The decap supplies necessary charge to the IC
during Tr through a transmission line. As you mentioned before: “The current
is an impulse function, although the voltage waveform is a step function.”
This impulse function, actually a bell-like function on Tr, happens every
time period T when the IC gate switches from low to high. The corresponding
frequency spectrum contains lots of frequencies. There must be some
frequencies making the transmission line a 1/4, 3/4, ... wavelength. It is
hard for me to be convinced that currents of those frequencies cannot flow
from the decap to the IC. ... Pleas correct me if misunderstood. Thanks.

Barry Ma

On Mon, 22 May 2000, George_Tang@Dell.com wrote:

> Barry,
> I need to make a correction. I was rushing to lunch on Thursday, so I did
> not read over what I wrote. Here is the correction for the 2nd comment
> below:
> At 1/4 wavelength, the charges are 180 degrees out of phase, so they are
> working against the IC current draw. 1/8 wavelength (90 degrees out of
> phase) is what I consider to be acceptable.
> Regards,
> George Tang
> george_tang@dell.com


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