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Ways to improve RF embedded design

10 Nov 2015  | Richard Quinnell

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Don't split ground planes: Use a single ground plane rather than splitting analogue and digital ground. Also, avoid slots or other large breaks in the ground plane. Otherwise, the return currents for signals will have to loop around the slot, increasing their tendency to radiate. The ideal is to have a continuous ground plane reference for every layer of the PCB.

Bury clocks: High-speed clock signals on traces are effectively RF signals on antennas. Even if the clock rate is relatively slow, sharp transitions on clock edges have a high harmonic content. So, clock signals need to be treated as antennas you don't want to work. Keep traces short, isolate them from other traces and power, include series resistors near the driver, and bury clock traces between ground planes. It's also a good idea to make the outermost layers of the board be ground layers, with vias all along the edges and liberally applied elsewhere to make the PCB behave as a Faraday Cage for all the board's signals.

Avoid power planes: Power planes carry high-frequency transients due to component switching, and can radiate those frequencies. Instead of using planes, route power as traces wide enough to avoid resistive losses. And use plenty of decoupling with small capacitors having high self-resonant frequencies placed near IC power pins.

Use shields over RF components: Marketing won't like the added cost of metal shielding and manufacturing won't like the added complexity of soldering shields on, but stand your ground. Shielding will prove its worth come compliance test time.

Plan for regulatory testing

Electronic devices must meet certain regulatory requirements with regard to the RF energies they emit. These requirements in the US include FCC Part 15B for unintentional radiation, FCC parts 15C, 22, 24, and 27 for intentional radiation (i.e., the wireless signal), and SAR (specific absorption rate) for any device regularly used within 20cm of the human body. Designs will need to be tested to prove their compliance with these requirements. Beyond designing with these requirements in mind, plan on the time and cost involved in actually conducting the tests. The use of a pre-certified wireless module in your design will help reduce the need for intentional radiation testing, but the other tests will still be required.

Sporre and Morris concluded their presentation with some suggested resources that developers might turn to for more information. These included Henry Ott's Tech Tips on EMC, several books by Ralph Morrison, and a Freescale webinar on PCB design that recently became available.


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