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RF pointers for embedded developers

Posted: 27 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:RF? wireless connectivity? embedded system? radio? PCB?

Here are five important tips for developers without much prior experience in dealing with RF now tasked with adding wireless connectivity to their embedded designs.

Consider the trade-offs
One of the first things developers should do is consider the system trade-offs they must make when developing a wireless system. These include:

Range vs data rate: All other factors held frozen, the achievable range of a wireless connection goes up as data rate decreases. You'll have to decide which of them is most important.

Range vs antenna size: Antenna performance (gain) is directly proportional to its size up to half the wavelength for the operating frequency. Given that the antenna is the last stage of transmission and the first stage of reception, its performance has a major impact on the achievable range for the design.

Power consumption vs data rate/throughput, range: If minimising power consumption is your aim, you may have to compromise on data rate or range or both.

Bill of material vs infrastructure cost: An inexpensive radio is unlikely to achieve top performance, so the infrastructure of gateways, receive towers, and mesh nodes will need to make up the difference. This typically requires more sophisticated (and expensive) antennas, closer tower spacing, or the like, increasing the system's infrastructure cost.

Start mechanical design with the antenna
Because the antenna is a critical component in radio performance, a wireless embedded system's mechanical design should cater to the antenna's needs, rather than have the antenna be constrained by the mechanical design. The antenna's size and the size of its ground plane affect antenna efficiency as well as achievable bandwidth, so ensure that adequate space is provided. Further, the antenna's placement is important. Avoid other conductors, absorbers, and dielectrics. Especially stay away from things like batteries, LCD panels, cables, and the like.

Tune antenna in final configuration
Because nearly everything in a physically small system will end up carrying some RF current, it is all effectively part of the antenna. When tuning the antenna for optimal performance, therefore, do so in the final system configuration. Such tuning should use a vector network analyser to measure antenna impedance and match it to the radiation impedance for maximum power transfer.

Design for self-quieting
The system PCB should be designed for reduced EMI, both to minimise received noise (thus improving signal to noise ratio and improving range/throughput) and to reduce radiated emissions that can affect regulatory compliance. Some of the PCB design tips include:

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 20 cm 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.

The tips presented in this article were drawn from a session at the Embedded Systems Conference titled "Introduction to RF for Embedded Designers". It was presented by Kyle Sporre, RF hardware manager, and Dustin Morris, antenna engineer, of Digi Wireless Design Services.

About the author
Rich Quinnell is an engineer, writer (threatening to become novelist) and also editor of EE Times' Industrial Control Designline and EDN's Systems Design Center. In his spare time, he still reviews plays (part time drama critic) and still finds time to dabble in circuit design for fun.

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