User's Manual

July 23, 2002 B-9
CreataLink2 XT Hardware Integrator’s Guide Appendix B
Methods of Controlling Emissions
Methods of Controlling Emissions
The preferred means of control is to contain emissions to a level 40 dB less than the
FCC Part 15 requirements. For WAN products, shielding achieves this control.
Standard techniques to achieve FCC certification are insufficient for wireless
communications. Decoupling, partial shielding, and PCB layout methods produce
only incremental improvements. Hybrid methods of shielding and source reduction
are more effective approaches.
Source reduction efforts can increase the product direct materials cost unless the
host platform is close to the emission goals. Standard EMI techniques are valuable
when the target levels are not the goal.
Shielding Approach
The mechanical design of the host product must allow for EMC engineers to create
a Faraday Box shield design. This box is an electrically continuous shielded
enclosure that easily attenuates radiated signals from the host device. See
Electromagnetic Compatibility: Principles and Applications by David A.
Weston for information on shielded enclosure design. The shield minimizes the
possible redesign required of the host PCB platform and circuitry.
Components of the Shield Design
Effective shield design incorporates:
• A highly conductive shielded enclosure that encapsulates all of the active
circuitry, and is 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 λ/10 of the frequency of
interest. This size applies to keyboard and display apertures in the enclosure.
Aperture radiation testing at the frequencies of interest might prove that
larger apertures are acceptable to the particular scenario.
Benefits of the Shielding Approach
Emissions reduction uses shielding source reduction techniques, such as
decoupling, PCB layout and grounding, or a combination of the two. With a shield
in place, any changes to product circuitry have no effect on emissions levels. If a
circuit-level approach controls the emissions, a change in the circuitry can change
emissions performance.
Alternate EMI Reduction Methods
Methods other than shielding to reduce emission levels are as follows:
• PCB layout modification using ground layers adjacent to high speed layers
• Capacitive or filter decoupling
• Redistribution of module interconnects