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

From: Barry Ma ([email protected])
Date: Tue May 23 2000 - 10:22:14 PDT

George,

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.

Regards,
Barry Ma
[email protected]

--------
On Mon, 22 May 2000, [email protected] 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
> [email protected]

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