[snip]
VI. Conclusion.
Unlike boards without internal power and ground planes, multilayer
boards have a built-in capacitance that is a more effective source of
current than surface decoupling capacitors at high frequencies. In the
time-domain, this means that most of the initial current supplied to a
fast switching device is provided by the interplane capacitance.
[snip]
And you don't get a heckuva lot of interplane capacitance
from an island that is only 1" by 1" with a 6 mil dielectric in
between as in one designed for a clock oscillator. This supports
the idea that at frequencies where the discrete capacitors must
deliver charge through inductive reactance (i.e. the discrete circuit
is decoupling at frequencies above resonance), the interplane
capacitance does the work. Could this suggest that the island
approach will be less useful at extremely high clock rates where
the size of the island becomes dimensionally interesting (2+ GHz)
and the island interplane capacitance is small?
Regards,
[email protected]
----------
From: Barry Ma
To: emc-pstc; B.Gleason
Subject: RFI Problems with Certified Computers
List-Post: [email protected]
Date: Thursday, April 18, 1996 8:33AM
Hi Ladies/Gentlemen,
I have been reading with interest the discussion articles on this
subject. Since Mike Violette 04/15/96 presented his opinion on VCC/GND
plane layout in multilayer board, the discussion seems to be focused
on PCB EMC design. Max Kelson 04/16/96 wrote:
[snip]
What this ferrite/cap configuration would do is to force the
oscillator to draw all transient current from the capacitor. Or, in
otherwords, the rest of the caps on the board would be unable to help
provide fast-transient current because of the ferrite. This would
keep the current loop (power AND GROUND) small and prevent it from
infecting the rest of the board. The path for the transient current
surges would be from the capacitor to the IC's power pin, out the
IC's ground pin and back to the negative side of the capacitor (a
relatively small loop).
[snip]
It might be worthwhile to pay attention to research work done by
professors at the Univ. of Missouri-Rolla. In the article "Power Bus
Decoupling on Multilayer Printed Circuit Board", IEEE Trans. on EMC,
vol. 37, pp. 155-166, May 995, they wrote:
Regards,
Barry Ma
