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

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From: Ron Miller (rmiller@Brocade.COM)
Date: Fri May 12 2000 - 10:04:22 PDT


Barry
> -----Original Message-----
> From: Barry Ma
> Sent: Thursday, May 11, 2000 3:50 PM
> To: EMC-PSTC; si-list@silab.eng.sun.com
> Subject: [SI-LIST] : Charge moving from decoupling capacitors
>
> Hi,
>
> As the speed of digital signals gets faster and faster, people begin being
> concerned with the distance for electric charge to move on power and
> ground planes of multilayer PCB during the signal rise time from a
> decoupling capacitor (cap) to a chip it serves. I would like to raise two
> questions.
>
> (1) The charge is moving in a metalic plane, not inside the dielectric
> between pwr and gnd planes. Please let me know why you have to use the
> propagation velocity in the dielectric, instead of that in the metal.
        [Ron Miller]
        Wrong.

        The voltage is stored in the dielectric on the surface of the metal

> (2) The second question is regarding distance between the cap and the
> chip. Do we really have to limit the distance letting the charge have
> enough time to move from the cap to the chip during the rise time
> interval? I doubt it.
        [Ron Miller]
        Of course we do. How else is the output voltage changed if not from
voltage from the supply
        and ground via the switching semiconductor.

> Take the running water system for example. When we open, then close the
> water faucet within one second, does the water we've got in basin come
> from water tower (or water station, or reservoir)? No, it is the water
> that resides in the pipe. As a matter of fact, we have a very large pipe -
> pwr/gnd planes. Well, of cause you know, I did not mean we don't need
> water tower - the cap. ......
>
        [Ron Miller]
        Smell the coffee.
> Regards,
> Barry Ma
> bma@ANRITSU.com
> Morgan Hill, CA 95037
> Tel. 408-778-2000
>
>
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