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

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From: Barry Ma (barry_ma@altavista.com)
Date: Tue May 23 2000 - 10:22:14 PDT


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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