An Energy Autonomous 400 MHz Active Wireless SAW Temperature Sensor Powered by Vibration Energy Harvesting

Abstract

An energy autonomous active wireless surface acoustic wave (SAW) temperature sensor system is presented in this paper. The proposed system adopts direct temperature to frequency conversion using a lithium niobate SAW resonator for both temperature sensing and high-Q resonator core in a cross-coupled RF oscillator. This arrangement simplifies the temperature sensor readout circuit design and reduces the overall system power consumption. A power conditioning circuit based on buck-boost converter is utilized to provide high efficiency power extraction from piezoelectric energy harvester (PEH) and dynamic system power control. The SAW resonator is fabricated in-house using a two-step lithography procedure while the RF oscillator as well as the PEH power conditioning circuit are implemented in standard 65-nm and 0.18- μm CMOS processes respectively. The measured RF transmitter output power is -15 dBm with a phase noise of -99.4 dBc/Hz at 1 kHz offset, achieving a figure of merit (FOM) of -217.6 dB. The measured temperature sensing accuracy is ±0.6 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C in -40 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C to 120 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C range. Fully powered by a vibration PEH, the proposed energy autonomous system has a self-startup voltage of 0.7 V and consumes an average power of 61.5 μW.