  Ultrasonic backscatter communication has gained popularity in recent years
where piezoceramic resonators are used as acoustic antennas. Currently,
backscatter communication revolves around the amplitude modulation of the echo
signal by the sensor, which can be modeled as a variable shunt impedance.
Amplitude based sensing is prone to high levels of noise, motion artifacts, and
has low efficiency with respect to power usage and bandwidth. To overcome these
shortcomings, we present here, a frequency-based sensing method for ultrasonic
backscatter communication. This system exhibits a frequency shift in the
ultrasonic sensor's parallel resonance when its shunt capacitance is varied,
similar to an inductive near field link. The concept is simulated using an
equivalent end-to-end model in SPICE that matches well with the experimental
observation. To monitor the resonance frequency shift in the experiments a
variety of methods are employed including ringdown measurements, chirp
spectroscopy, bode analysis and phase locked loop. All the methods provde
substantial proof for feasibility of frequency-based sensing in ultrasonic
backscatter communication enabling passive sensor motes for sensing
applications.
