FreeLists / si-list / [SI-LIST] Printed wiring board to chassis coupling

["Douglas C. Smith" <doug@xxxxxxxxxx>]

Hi All,

Coupling from a printed wiring board to a nearby metal chassis can have
significant effects on circuit performance (immunity to EMI) and
emissions performance as well. Recently I made some measurements on a
simple experimental setup to show just how strong such coupling can be.
the Technical Tidbit article on my site at:

http://emcesd.com or http://www.dsmith.org (a pointer to emcesd.com)

gives some of the important results of the experiment (the rest is in my
courses).

The coupling measured was between the ground plane of a small home-built
oscillator and a nearby metal plate. The metal plate was about 2 cm away
from the oscillator ground plane and the coupling between the two was
able to drain all of the available peak current from an HC240 gate at an
oscillator frequency of about 26 MHz (about 40 mA peak). The output of
the oscillator was connected to the metal plate and the signal current
then flowed back to the oscillator ground plane through the mutual
coupling between the planes. The metal plate was "isolated" from all
other connections except for the output of the oscillator. When the
oscillator was moved off the metal plane, the current reduced very
significantly to a small fraction of the 40 mA that occurred when the
oscillator was on the plane. The pictures in the article show the
configuration clearly.

The results of this experiment show that it is impossible to isolate a
board from a nearby metal plane or implement a "single point ground" of
the board except at much lower frequencies than the already low
frequency of 26 MHz. The coupling between board and metal plane is just
too strong.

Doug
-- 
-------------------------------------------------------
    ___          _       Doug Smith
     \          / )      P.O. Box 1457
      =========          Los Gatos, CA 95031-1457
   _ / \     / \ _       TEL/FAX: 408-356-4186/358-3799
 /  /\  \ ] /  /\  \     Mobile:  408-858-4528
|  q-----( )  |  o  |    Email:   doug@xxxxxxxxxx
 \ _ /    ]    \ _ /     Website: http://www.dsmith.org
-------------------------------------------------------

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