User's Manual

APPENDIX F – Guide to Desense ______________________________ Boomer II User Manual & Integrator’s Guide
Copyright Wavenet Technology © November 2003 136 BM210012WT37
270 Madison Avenue, New York, NY 10016. Any well written text on
EMI control should cover the design of shielded enclosures.
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.
Benefits of the Shielding Approach
Emissions reduction can be achieved using shielding source reduction
techniques, such as decoupling, or PCB layout and grounding, or a
combination of the two. Once a shield is in place, any revisions to
product circuitry have no effect on emissions levels. If a circuit level
approach is used to control the emissions, a change in circuitry can
bring a new unknown to the emissions performance.
Alternate EMI Reduction Methods
Although shielding is the brute-force method of reducing emission
levels, other methods are available, such as:
 PCB layout modification using ground layers adjacent to high
speed layers
 Capacitive or filter decoupling
 Redistribution of module interconnects
 Clock Pulling
Clock Pulling
Clock pulling is effective when the emission sources are narrowband.
To implement clock pulling, a method must be devised for the modem
to tell the host/terminal it is having difficulty receiving. Devising such
a method is admittedly very difficult. The host/terminal provides
“pulling” of its internal emission source, which is identified as a
potential problem.
If this source is the cause of the interference, the pulling or slight shift
of the source frequency moves the harmonic energy out of the receive
channel. This is an inexpensive way of solving the problem, as no
special shielding or decoupling is required.
The limitations of the clock pulling method are:
 Computing devices have many more than one source