User's Manual

Boomer II User Manual & Integrator’s Guide ______________________________ APPENDIX F – Guide to Desense
BM210012WT37 135 Copyright Wavenet Technology © November 2003
The indirect method cannot account for characteristics of the data
protocol and is less effective. Also, the bandwidth of the noise source is
important. If the source is narrow-band, it has less effect than one
occupying the entire channel bandwidth. The method is not effective in
determining desensitisation at IF frequencies or from less obvious
sources such as mixed products. The method provides information on
how much effort, if any, needs to occur to resolve desense problems.
This method is useful when connection of the wireless card is not yet
facilitated by the platform. This measurement could be performed
without the wireless card present. This method determines the
magnitude of the emissions, without extensive test facility
requirements.
Methods of Controlling Emissions
Preferred methods of controlling emissions observe that the emissions
must be contained to a level 40dB less than the FCC Part 15
requirements. For WAN (Wide Area Network) 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/terminal 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.
Shielding Approach
The mechanical design of the host/terminal 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/terminal.
The shield approach appears to be a big step at first. The advantage is
that the shield will minimise the possible redesign required of the
host/terminal 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.