A careful look at RF for your IoT PCBs

IoT devices remain in their infancy, with a lot of questions being asked and experimenting going on. Radio frequency, or RF, is one technology that IoT companies and startups are dialing into to advance state-of-the-art IoT and, in doing so, increasing profit margins for their IoT products. In some cases, there are a lot of guesstimations going on, largely based on assumptions gleaned from smartphone technology.

However, with IoT devices and unlike smartphones, RF is a brand new ballgame, something akin to “black magic” as some tech pundits put it.

The prudent thing to do is take a close look at RF and the specific challenges it poses to IoT devices. There are some key design considerations you have to factor in. Those deal with the right RF antenna to use, anticipated RF interference, impedance matching and testing.

But even before those considerations, you first have to look at RF and what you expect from it in your IoT product. Remember what I said earlier: Growing numbers of IoT devices are based on rather small rigid-flex and flex circuits. There’s very little real estate on these tiny boards for the micro devices populating them.

The type of RF antenna and precisely designing it in your IoT device should be at the top of your considerations. Chip, proprietary and wire antennas are typically used for printed circuit board (PCB) designs. For IoT and RF applications, chip antennas are mostly used for ultra-crammed designs, mostly in low frequency ranges. Chip antennas are easy to implement in a design, but they are somewhat expensive.

An antenna is central to the performance of an RF device, especially when you’re doing verification and certification testing and calibration. There are certain devices that operate at different RF frequencies, sometimes called “multiple bands of frequencies.” The antennas for these must be very precise. For example, the antenna must be designed properly and follow the rules of physics to have good access to the multiple frequencies on the RF band. An example is a large antenna, larger than Wi-Fi running at speeds of 2.5-5 GHz.

Also keep in mind that RF is a very sensitive circuit — and even more so in a small IoT device. Noise is the troublemaker in this instance as it creates RF interference, which translates into poor signal integrity. You have to take into account the types of components you’re using on the rigid-flex or flex circuit and where they’re placed. Analog circuits are usually the culprits causing the most noise. The basic rule of thumb is to separate digital and analog circuitry at a safe distance to avoid problematic analog-created noise from disrupting digital circuit operations.

The third major design consideration is impedance matching. It is important because RF signals are extremely noise sensitive. A small noise ripple can alter RF signal performance to a great extent.

Therefore, impedance matching is extremely critical for RF. Digital signals — even if they are very high-speed — have a certain tolerance. But for RF, the higher the frequency, the smaller the tolerance becomes. For example, the PCB designer must keep it at 50 ohms — 50 ohms out from the driver, 50 ohm during transmission and 50 ohms in to the receiver.

Lastly, RF design must be done so that you can test it. Cost associated with testing and certification is significant. You have to consider that space is a limiting factor. You also have to think outside the box when doing an IoT RF design so that it can be satisfactorily tested.

Keep in mind that you don’t have the latitude or flexibility to test an IoT PCB like a rigid conventional PCB. In a conventional PCB, you have plenty of real estate to put test points for the oscilloscopes, multimeters, all of these. You don’t have that luxury with IoT PCBs due to real estate limitations, and testing in an IoT device should be modular, but precise.

Even the IoT PCB testbeds you design are very tight in terms of size and in terms of the tolerance. Sometimes, you have to design a segment of the board where you can test in a batch format.

Here, for example, you check function levels 1, 2 and 3 in one set of tests because you don’t have the luxury of creating a test program for individual parts of the circuitry because space is limited. Without precise modular testing, your IoT device performance would be jeopardized and field failures would result.

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