Equally important is that respect has to be paid to impedance matching ;
other wise, energy is reflected from the load and this will degrade the
risetime as well as cause some energy to be radiated. The dielectric
material must be low loss so that you ensure the signal is conducted with
minimum attenuation.and keeping the path length to the chip to a minimum
keeps the undesired switching products very low.
Ralph Cameron
EMC Consultant for Suppression of Consumer Electronics
(after Sale).
----- Original Message -----
From: "Jim Bacher" <jim_bac...@mail.monarch.com>
To: "Larry Miller" <ldmil...@nortelnetworks.com>; "EMC-PSTC"
<emc-p...@ieee.org>
Sent: Friday, May 12, 2000 11:24 AM
Subject: Re[2]: [SI-LIST] : Charge moving from decoupling capacitors
>
> Forwarded for Larry.
>
> ____________________Reply Separator____________________
> Subject: Re: [SI-LIST] : Charge moving from decoupling capacitors
> Author: "Larry Miller" <ldmil...@nortelnetworks.com>
> Date: 5/12/00 7:37 AM
>
> Hi, Barry,
>
> Responses below:
> At 03:49 PM 5/11/00 -0700, Barry Ma wrote:
> >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.
>
> Due to skin effect at anything above a few tens of kHz, the current flows
> mainly on the surface of the trace, so it has to interact with the
> dielectric in accordance with Maxwell's equations for electromagnetic
waves.
>
> >(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.
> >
> No, you have to let the wave propagate (see below).
>
> >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. ......
>
> The running water analogy breaks down here (at AC). Another analogy would
> be that the current is comprised of many successive collisions between
> billiard balls, not the motion of one single ball. Yet another analogy
> would be to look at the current as a game of Chinese checkers, where an
> individual electron can only move into a hole in the board vacated by
> another electron (if you overcome this you get superconduction!). As I
> recollect, the actual speed of an identifiable electron, assuming you
could
> actually identify it, is on the order of a few meters/sec, though the
> electromagnetic wave caused by the transfer of energy between electrons
> travels at the speed of light in the medium.
>
> Larry Miller
>
>
>
> -------------------------------------------
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>
>
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