Michael Rigby-Jones wrote:
> The last place I worked used some very strange decoupling caps.  They were
> fitted underneath the IC, without using a socket.  They were very thin with
> the same outline as the IC and thin pins that could be pushed through the
> same holes as the IC.  This got around the problem of long leads with the
> socket type decouplers.
>
> Regards
>
> Mike Rigby-Jones
> mrjones@nortelnetworks.com


The company that does those is CCI: http://www.cci-msc.com/  They also
make some nice bus bars for board stiffening and power distribution.
Unfortunately the last time I looked at their Micro-Q caps (several
years ago), they were prohibitively expensive.  If you were to use more
than a few of them per board, it was more cost effective to add a
power/ground plane and a good bypassing structure to the pcb.  However
they are very good if you've already got a noisy design that has to be
fixed; less expensive (in the short term) than a redesign.


In response to other topics in this thread:

Its also important to remember that the clock frequency is usually not
the frequency that determines the design point for your power
distribution system.  The rule-of-thumb is:

design frequency = .3 / rise time

Where the frequency is in Hz, and "rise time" is the fastest switching
speed in use in your circuit, in seconds.  So at a rise time of 2.5ns
(not uncommon) the frequency you should be designing your bypassing for
is 120 MHz (even if your PIC is just ticking along at 4 MHz).  (Note
that the ".3" ranges between .3 and .5, depending upon whose thumb you
are using.)



The positioning of bypass capacitors is a popular subject, and a good
place for information on it is at Howard Johnson's web site:
http://signalintegrity.com , where the subject has been addressed
several times in his newsletter:

http://signalintegrity.com/news/1_6.htm
http://signalintegrity.com/news/2_3.htm
http://signalintegrity.com/news/2_1.htm
http://signalintegrity.com/news/2_26.htm

This reprint from PCB Design Mag is good too:
http://signalintegrity.com/articles/straight/bypass.htm


Note however, that I believe in all instances these discussions regard
pcb's with full power and ground planes; the recommendations there may
not be applicable to 2-layer, wire-wrap, or proto-board layouts.


This is also a popular subject on the signal integrity mail-list,
(subscription info at http://www.qsl.net/wb6tpu/si-list/info.html ), and
in its archives at http://www.qsl.net/wb6tpu/si-list/  (Threads on this
list are often addressed in Johnson's newsletter.)



Gary Crowell