Powering autonomous sensors by RF harvesting

RF energy can be harvested in order to power autonomous sensors either from the surrounding environment or from dedicated sources. A conventional RF harvester is mainly composed by an antenna, a matching network and a rectifier. At low power level, e.g. -10 dBm and below, the corresponding voltage amplitude at the antenna is low and comparable to the voltage drop of the diodes used in the rectifier. Two matching network configuration are compared in terms of efficiency, a shunt-inductor and an L-network. Then, in order to boost the voltage at the rectifier input and thus the rectifier efficiency, an L-network optimized for an input power of -10 dBm at 868 MHz is proposed in this work. As for the rectifier, a half-wave rectifier with a single zero-bias Schottky diode HSMS2820 was selected for high powers (≥5 dBm) and HSMS2850 for low powers (<5 dBm). First, a theoretical analysis was performed that takes into account the parasitic capacitance of the diode. Then, simulations with ADS (Harmonic Balance) were carried out taking into account the circuit layout. Both in the analysis and simulations, the antenna was substituted by an AC generator with an output resistance of both 50 and 300Ω. Finally, a printed circuit board (PCB) implementation was performed using a 0.5 pF capacitor and a 27 nH inductor for the L-network. First, the input power was generated by an RF generator and directly connected to the circuit. The RF-to-DC efficiency was of 45 % at 868 MHz with an optimum load of 2.5 k. Efficiencies of 34.5 % and 22.5 % were achieved at -15 dBm and -20 dBm, respectively. Ultimately, two antennas have been manufactured at the resonant frequency of the system in order to implement a real system. The RF-to-DC efficiency was of 41.85 % at 868 MHz with an optimum load of 4 k and efficiencies of 29.42 % and 19.73 % were achieved at -15 dBm and -20 dBm, respectively.