Circuit for continuous motional series resonant frequency and motional resistance monitoring of quartz crystal resonators by parallel capacitance compensation

A deep analysis of the problem associated with oscillators as interface circuits for quartz-crystal-microbalance sensors, reveals that the so-called static capacitance of the sensor is one of the elements that makes the use of oscillators more critical for sensors applications. A phase-locked-loop based circuit specifically designed for compensating the parallel capacitance effects in quartz crystal resonator sensors is presented. This circuit permits the calibration of the external circuitry to the sensor and an accurate determination of the effective capacitive compensation. The system provides a continuous measurement of the motional series resonant frequency and motional resistance. An extension and automation of the proposed system for multiple sensor characterization is introduced. The theoretical analysis of the circuit along with the experimental results presented prove that the proposed system is a good alternative for quartz sensors characterization.