Dual-harmonic oscillator for quartz crystal resonator sensors

An oscillator circuit is proposed that simultaneously excites and tracks two harmonic resonances in a piezoelectric resonant sensor that, in particular, can be a quartz crystal resonator (QCR) sensor. By probing the resonator at two harmonic modes at the same time, enhanced sensing capabilities can be conveniently achieved because a larger set of parameters can be measured with a single sensor, thereby increasing the information content and accuracy. The oscillator was tested with AT-cut crystals simultaneously operated at the fundamental and third harmonic, both in air and in contact with liquid. The results show an excellent agreement between the oscillator readings and reference measurements taken with an impedance analyzer. The principle and circuit architecture can be extended from QCRs to a larger class of piezoelectric microresonator sensors.