Inexpensive SDR-based longwave radio controlled clock for time dissemination in industrial wireless sensor networks

LF and VLF (very low frequency) wireless links have long been used to perform time and frequency dissemination. Indeed, several standard time services exploiting (V)LF signaling are continuously transmitting in many locations around the world, despite they face the competition from the Global Positioning System (GPS), which offers an higher clock synchronization accuracy. In fact, (V)LF signaling ensures an overall accuracy adequate for most of civil and industrial applications, offering indoor reception capability at lower cost and lower power than GPS systems. This work describes a full digital implementation, exploiting the Software Defined Radio paradigm, of a LF receiver, capable to simultaneously perform both the amplitude and phase demodulation of the incoming time reference signal, and of a servo-clock for the time synchronization of wireless sensor network nodes. Despite the DCF77 standard has been considered for the real world implementation, the SDR approach allows to easily migrate to other similar standards worldwide available. The increased system complexity is mitigated by the improved flexibility; in addition, the computational core of the SDR can be sized to be shared among several wireless technologies simultaneously handled. The real-world prototype is implemented by means of a single inexpensive digital signal controller (a member of the dsPIC33 family from Microchip), requiring very few external components. Several experiments have been carried out in a controlled environment, confirming an overall sub-millisecond time synchronization accuracy.