Dual-harmonic oscillator with frequency and resistance outputs for quartz resonator sensors

An oscillator circuit is proposed that simultaneously excites and tracks two harmonic resonances in a quartz crystal resonator 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. The oscillator outputs two couples of signals, respectively related to the sensor series resonant frequency and quality factor, inversely proportional to the series motional resistance, for the fundamental and third harmonic. The circuit also provides compensation of the sensor parallel capacitance for increased accuracy. Experimental tests were run with 5-MHz AT-cut crystals exposed to solutions of different density-viscosity products, obtaining results in good agreement with the theory. Evidences of different dynamic responses at the fundamental and third harmonic were also obtained, that could be possibly related to the difference in acoustic wavelength and penetration depth into liquid.