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Dealing with platform interference

04 Jan 2016  | Kenneth Wyatt

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Remediation checklist
As I mentioned, the product design must be developed with EMC in mind and no corners should be cut. This will consist of:

 • A near perfect PC board layout
 • Filtering of DC-DC converters
 • Filtering of any high frequency device
 • Filtering of the radio module
 • Local shielding around high noise areas
 • Possibly shielding the entire product
 • Proper antenna placement

The PC board layout is critical and is where most of your effort should reside. An eight or ten layer stack-up will provide the most flexibility in segregating the power supply, analogue, digital, and radio sections and provide multiple ground return planes, which may be stitched together around the board edge to form a Faraday cage. Care must be taken to avoid return current contamination between sections. The power plane for the radio section should be isolated (except via a narrow bridge) from the digital power plane. This can provide up to 40 dB of isolation between the digital circuitry and radio.

It is vital that the power and ground return planes be nonadjacent layers and 3-4 mils apart at the most. This will provide the best high frequency bypassing. Clock, or other high speed traces, should avoid passing through too many vias and should not change reference planes.

Power supply sections should be well isolated from sensitive analogue or radio circuitry (including antennas). Be aware of primary and secondary current loops and their return currents. These return currents should not share the same return plane paths as digital, analogue, or radio circuits. remember that return currents want to return directly under the source trace.

The book, Platform Interference in Wireless Systems, by retired Intel engineers, Kevin Slattery and Harry Skinner, is very useful in providing ideas for measurement and remediation. For general product design guidelines, the book, EMI Troubleshooting Cookbook for Product Designers, by Patrick André and Kenneth Wyatt, describes several basic design concepts to reduce EMI.


References
Slattery and Skinner, Platform Interference in Wireless Systems—Models, Measurement, and Mitigation, Newness Press, 2008.

André and Wyatt, EMI Troubleshooting Cookbook for Product Designers, SciTech Publishing, 2014.


About the author
Kenneth Wyatt is an independent consultant and specialist in electromagnetic compatibility (EMC) design, test and troubleshooting. One speciality is his use of do-it-yourself probes and use of low-cost test equipment and measurement techniques for evaluating EMC issues on the workbench.


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