User's Manual

Boomer II User Manual & Integrator’s Guide __________________________________________________ Desense
BM210012WT27 77 Wavenet Technology
Components of the Shield Design
To be effective, the shield design must incorporate:
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A highly conductive shielded enclosure that encapsulates all of
the active circuitry. This can be constructed of sheet metal or
plated/sprayed plastic.
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Decoupling on all signals exiting the enclosure
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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:
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PCB layout modification using ground layers adjacent to high
speed layers
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Capacitive or filter decoupling
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Redistribution of module interconnects
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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 it is having difficulty receiving. Devising such a method
is admittedly very difficult. The host 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:
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Computing devices have many more than one source
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Each source must be identified and controlled. This
identification is at times difficult.