New-Tech Europe Magazine | July 2017

IoT Special Edition

are generally even less stable with tolerances of ±100ppm. Classical RF analog transceivers and receivers are typically built with phase-coincidence demodulators. They will generally employ either an external discriminator circuit or an integrated FSK demodulator. As a result of the analog demodulation principle, these products offer a carrier frequency acceptance range of up to ±100kHz. If we consider the carrier frequency of an IoT transmitter node of 868.3MHz based upon a low-cost crystal reference that has a tolerance of ±50ppm, the center frequency of the node can have a spread of about ±43kHz. This value may already exceed the FSK deviation, which is an essential modulation parameter. Typical allowable FSK deviation values for IoT sensor node applications are between ±10kHz to ±50kHz. Nevertheless, RF products with analog demodulators can cope with carrier frequency spreads that are larger than the FSK deviation due to their wide carrier frequency acceptance. Modern highly integrated RF products perform the demodulation, and many other necessary signal conditioning operations, in the digital domain. This is possible due to modern semiconductor processes that are based on small geometries, thus leading to very compact IC designs. However, due to their digital nature, most modern RF transceivers exhibit relatively small carrier frequency acceptance ranges, compared to their legacy analog counterparts. Therefore, receiving a signal from an IoT sensor node can be a challenge for a digital RF receiver if the sensor node exhibits a poor frequency accuracy because of a wide tolerance crystal.

Fig. 2: Sub-GHz applications space

25kHz). Furthermore, as Sub-GHz devices typically run on proprietary protocols, it is relatively easy to optimize them for power efficiency and long battery life; both essential for battery powered or energy- harvesting IoT remote sensors. Fig. 2 illustrates a number of IoT applications that benefit significantly from Sub-GHz technology. Carrier frequency acceptance matters An important element when using wireless links for any application, but especially IoT applications, is the ability for the receiving nodes to track carrier frequency deviations of the transmitting nodes. Modern integrated RF receivers and transmitters use quartz crystal technology for generating a local reference frequency within each device. Affordable crystals typically exhibit frequency stabilities in the range of ±10ppm to ±50ppm approximately. Less integrated RF products, which are often based on devices such as SAW resonators,

GHz often refers to one of the ISM bands, for example at 433.92MHz or 868.3MHz. A 2.4GHz based system offers a relatively high data throughput, often it is in the order of several Megabits-per-second (Mbps) for Wi- Fi and substantially less at around 260 kbps for BLE. Obviously, Wi-Fi is compatible to WLAN infrastructure such as routers and can therefore directly connect to the IoT. The various incarnations of Bluetooth can connect directly to a mobile device, which can then, in turn, provide a connection to the IoT / Internet. One downside of a 2.4GHz wireless link is its relatively short range (<10m) due to the high propagation losses that occur in comparison to Sub-GHz based systems. Sub-GHz is an ideal choice for use if long range (up to 1km outdoors) is important to the application or installation. Sub-GHz provides high levels of robustness as well as excellent immunity against disturbing signals through the use of narrow- band radio channels (often around

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