An oscillator circuit for dual-harmonic tracking of frequency and resistance in quartz resonator sensors

An oscillator circuit is proposed that simultaneously excites and tracks two harmonic resonances in a quartz crystal resonator sensor. The oscillator outputs two pairs of signals, related to the sensor series resonant frequency and motional resistance for the fundamental and the third harmonic, respectively. The circuit also provides compensation of the sensor parallel capacitance for increased accuracy. By probing the resonator with the superposition of two harmonic modes simultaneously, enhanced sensing capabilities can be advantageously achieved because a larger set of parameters can be measured with a single sensor and its response is tracked in real time. Experimental tests were first run with the developed oscillator connected to 5 MHz AT-cut crystals exposed to different liquid solutions, obtaining results in good agreement with the theory. Evidence of different dynamic responses at the fundamental and the third harmonic was obtained, possibly related to differences in acoustic penetration depth into the liquid. The oscillator was then tested with the sensor loaded by microdroplets of liquid solutions deposited by a piezoelectric microdispenser. The oscillator could detect and track the resulting time response of the sensor, outperforming measurement methods based on impedance analysis in terms of speed and resolution, and evidencing a complex combination of effects in the sensor transient response.