Radio Emissions of an IoT Device and How to Measure Them

The device under test (DUT): Anisca Bird consists of a wooden perch that can be mounted in front of the entrance of barn owl nest boxes. It is smart because it contains an integrated weight sensor as well as a low frequency RFID reader which detects a passive identification ring attached to the observed owl's leg. Every time an owl visits its nest box, our device detects the owl's identity, measures its weight, and transmits this data using a low-power wireless standard called SIGFOX.

When they are active, the RFID reader and the SIGFOX transceiver emit electromagnetic signals at 125 kHz and 868 MHz, respectively. Because of imperfections, they also radiate unwanted, so-called spurious emissions, at other frequencies. Because these radio waves could potentially interfere with other electronic devices, norms and standards were established to limit the power of both main and spurious emissions.

The most relevant norms for our tests are ETSI EN 300 330 for the RFID reader and ETSI EN 300 220-2 for SIGFOX.

The actual measurements started in our partner lab's anechoic chamber – a room completely shielded from any radio signals and with walls covered with non-reflective structures. The prototype and an RFID bird ring were placed on a rotating turn table. Using the USB service port, we turned on the RFID reader in continuous wave (CW) mode, transmitting a constant 125 kHz signal. This signal was picked up by a wide-band H-field antenna, and using a spectrum analyzer, the magnetic field strength could be plotted across the entire frequency range from 10 kHz to 30 MHz.

For frequencies above 30 MHz until 1 GHz, the receiving antenna was replaced with a far-field antenna. In both frequency ranges, measurements were also performed with the DUT in standby mode. Lastly, the same types of measurements were performed with the SIGFOX module active for 30 MHz up to 6 GHz.

In the figure above, the magnetic field strength of the RFID reader in active mode is plotted in green. A distinct peak is visible close to its intended frequency of operation: 125 kHz. The pink line is an envelope describing the maximum allowed field strength, which the DUT respects.

Looking at spurious emissions between 30 MHz and 1 GHz, the limits – in red – are much lower, as these are unwanted emissions. A slight peak at 100 MHz comes from the internal voltage converter providing a regulated 5V supply to the RFID reader. However, as the peak is still below the required limit, this is not a concern.

An additional result was produced for the SIGFOX signal: the radiation pattern of the antenna. The antenna doesn't radiate equally strongly towards all directions. Even though the antenna itself is isotropic by design, it no longer performs as such when mounted in vicinity of cables and the aluminum case. Most importantly, in none of the directions did the signal strength cross the maximum allowed power limits (14 dBm or 25 mW e.r.p.).

With all emission tests passed, we are now confident to launch production of the first batch of owl weight trackers for the Swiss Ornithological Institute (Vogelwarte Sempach)!