[SI-LIST] Re: Radiated Emissions
- From: "Curt McNamara" <CurtM@xxxxxxxxxxx>
- To: <Americomrh@xxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 17 Aug 2005 09:32:16 -0500
Somewhere in your product is an emission source that is radiating
an electric field. If there were no enclosure, this field would =20
have a field strength intensity pattern characteristic of the=20
radiating antenna -- in emissions we usually consider the radiating=20
antenna to be either a dipole (electric field) or a loop (magnetic
field).
When a cover is placed on a product and a slot is present the energy
from
the internal field source bounces around inside the enclosure, and can
only=20
escape through the slot (which then becomes an antenna). The slot
"focuses"=20
the energy, increasing field strength in front of the slot. The overall
energy=20
from your radiating antenna did not increase, however you have
concentrated it=20
in one direction (or area).
Our local EMI gurus (Kimmel and Gerke, www.emiguru.com) recommend=20
finding your highest frequency (clock rate or edge rate), calculate=20
the 10th harmonic, and then establishing what a 1/4 (or 1/8) wavelength
would be. This number gives you the maximum distance that a slot could
have.=20
In your case at 688MHz the wavelength is .436m, and a 1/4 wavelength
slot would be about 4".
If you need the enclosure you can bridge the slot with gasketing
(conductive foam or beryllium copper are popular). It is possible
that when you removed the door the energy spread out wide enough
for all peaks to be below the limit. However, it often occurs that
the energy shows up somewhere else -- once you do a thorough scan
with the door removed you may find peaks over the limit (probably
at a lower frequency). If you do seal the slot the energy will still
be present and probably show up somewhere else. So overall the best
plan is to find the source and reduce its magnitude. After all, this
will also improve the signal integrity of the product!
Curt
Curt McNamara. P.E.=20
Senior Electrical Engineer=20
Logic Product Development=20
411 Washington Ave. N Suite 101=20
Minneapolis, MN 55401=20
Tel 612-436-5178=20
Fax 612-672-0443=20
curtm@xxxxxxxxxxx=20
www.logicpd.com=20
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Americomrh@xxxxxxx
Sent: Tuesday, August 16, 2005 9:49 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Radiated Emissions
Our hardware is experiencing an emission problem at 688 MHz. The
device is=20
roughly 5" x 4" x 1.5" with a grounded, metal front panel. When we
removed=20
the front panel (leaving a gaping hotel), the overall emission is
reduced by=20
20dB. How is this possible? We are looking at isolating the panel
from=20
ground as our current solution.
=20
Robert Hanson
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=20
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