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16.
Methods of Controlling Emissions
Preferred methods of controlling emissions observe that the emissions must be contained to a level 40 dB
less than the FCC Part 15 requirements. For WAN products, the accepted method of achieving this is to
shield.
Through past experience, it has become evident that standard techniques used to achieve FCC certification
are not enough to satisfy wireless communications. Engineering teams logically attempt an array of
decoupling, partial shielding, and PCB layout methods which produce incremental improvements, but do
not achieve the emission control requirements. Hybrid methods of shielding and source reduction are often a
good approach.
IMPORTANT : Unless the host platform is already close to the goals set out in this document, source
reduction efforts may only drive up the direct materials cost of the product and not increase return on that
investment.
If a compromise is chosen where the target levels are not the goal, standard EMI techniques can be of value.
For narrowband emissions, some form of clock frequency "pulling" or control can be implemented.
16.1 Shielding Approach
The mechanical design of the host product must allow the EMC engineers to create a Faraday Box
shield design. This is an electrically-continuous shielded enclosure. If designed properly, such an
enclosure easily attenuates radiated signals from the host device.
The shield approach appears to be a big step at first. The advantage is that the shield will minimize
the possible redesign required of the host PCB platform and circuitry.
For a thorough discussion of shielded enclosure design, an excellent reference is Electromagnetic
Compatibility: Principles and Applications by David A Weston. The
publisher is Marcel Dekker, Inc. 270 Madison Avenue, New York, NY 10016. Any well-written
text on EMI control probably covers the design of shielded enclosures.
16.2 Components of the Shield Design
To be effective, the shield design must incorporate:
•
A highly conductive shielded enclosure that encapsulates all of the active circuitry. This can
be constructed of sheet metal or plated/sprayed plastic.
•
Decoupling on all signals exiting the enclosure
•
Control of aperture sizes in the shield to less than l/10 of the frequency of interest. This
would apply to keyboard and display apertures in the enclosure. Testing of aperture radiation
at the frequencies of interest may prove larger apertures are acceptable to the particular
scenario.
d15 Modem Integration and Application Developers Guide version draft 0.53
Page 178 of 181
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