A 1.25 µJ per Measurement Ultrasound Rangefinder System in 65nm CMOS for Explorations With a Swarm of Sensor Nodes

This paper presents an ultrasound rangefinder system able to find relative distances among energy-constrained sensor nodes. The nodes build a swarm that is operated in collision and multipath rich environments. A new distance measurement technique combining Wake-up and Frequency Modulated Continuous Wave (FMCW) is proposed to enable the ranging while neglecting the echoes from passive reflectors in the environment. The building blocks of the sensor nodes comprise a transmitter, a wake-up receiver, and a ranging receiver, all implemented in a 65 nm CMOS technology. The transmitter includes two switched-capacitor converters and an output multiplexer to generate a four-level driving signal and broadcast either a wake-up sequence or a digitally synthesized ultrasound Chirp. The transmitter dissipates 0.43μ J and 0.82μ J to broadcast the wake-up signal and the Chirp, respectively. A mixer first architecture is exploited in the wake-up receiver to reduce the always-on power consumption of the nodes. The ranging receiver uses a heterodyne architecture suited for the FMCW. The power consumption of the wake-up receiver and ranging receiver is 23.6 nW and 0.56μ W, respectively. The proposed rangefinder is experimentally characterized up to a 1 m distance in air and dissipates 1.25μ J per measurement, achieving a resolution of 18.7 mm at 0.55 m.