Re: [SI-LIST] : deCoupling caps and there placement

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From: Christian Schuster (schuster@ise.ch)
Date: Mon Nov 13 2000 - 22:49:23 PST


I wonder where these numbers for closely/widely spaced planes
come from (10 and 30 mils, respectively).... Is there anybody
on the list with some detailed information on that? Is there
no dependence on board permittivity, risetime/clock frequency,
capacitance (and inductance) of the bypass caps etc.?

I studied the influence of bypass cap placement
a while ago using the FDTD method. It gives you a good imagination
of what is "physically" happening: a parasitic plane wave mode
is excited at e.g. a via location, propagating radially away
between two given reference planes (resulting in what is
commonly referred to as ground bounce). Placing some bypass
caps nearby will decrease the wave front amplitude considerably
(see attached figures). Anybody interested can contact me, and
I can point you to corresponding references in the literature.

Best regards,

Ch. Schuster

Doug Brooks wrote:
>
> I interpret it just as it says and as Todd says in the interview.
> The study that said "the location of the decoupling ...unimportant" (which
> study is referenced in footnote 4 and which study is the oft quoted Tod
> Hubing, et al study of 1995) applied only to narrowly spaced planes (10
> mils), and not to other configurations. Therefore, the "unqualified"
> extension to the idea that placement in unimportant (in general) is
> projecting beyond the validity of the study. The paper in 1995 simply does
> not show that placement is unimportant in the general case --- only in the
> case where the planes are closely spaced (within 10 mils.)
>
> Doug


nocaps.gif

withcaps.gif

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