Re: [SI-LIST] : 20-H Rule for Power Planes

Bill Dempsey (bdempsey@dnaent.com)
Thu, 27 May 1999 18:44:51 -0500

I must be dumb....

To understand this 20H problem better let me break down my
understandings
and see where I went wrong.

EMI = Electro Magnetic Interference

Hmmmm... Okay.

I've got the electric field part down (i.e. the +++ plane with
reference to the --- plane makes an E field).
Kinda reminds me of a PP capacitor.

Gotta have the magnetic part to get an EM wave. Of course if I'm
just interested in E field disturbance then I guess I don't care. But
I'm searching for EM disturbances....so I'll look elsewhere.

The "M" (or H) portion as I recall is caused by a change in current.
(Quick
check to my brain recalls a name of Gauss so I'll use that to impress
you
all out there). Okay, granted. A noise source is present on one plane
with reference to
the other. This can cause a change in my E field.... What exactly is
the
strength of the H field in a PP capacitor with a time varying noise
source.
(I honestly can't say I know that). Current is flowing in the planes
which
generates what size H field in the presence of my E field?

How do I "trick" my E/M waves (where's the M again?) from jumping off my
board by cutting back the +++ plane? That one's got me fooled. I think
Mr. Maxwell needs a seance session with me and I'll ask him.

If anyone can explain this in basic terms I'd love to hear it. Or
better
yet if you've got some near field and far field measurements on boards
with
and without the 20H rule in effect I'd love to see the results.

Help!

Bill Dempsey
DNA Enterprises, Inc.

p.s. is Cisco hiring?

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