Improving the accuracy and operating range of quartz microbalance sensors by a purposely designed oscillator circuit

A method is presented to measure accurately the resonant frequency and to increase the functioning range of quartz microbalance sensors subject to heavy acoustic loading, such as encountered in in-liquid operation and with viscoelastic coating films. The method is based on an electronic circuit for the active cancellation of the crystal electrical capacitance, inserted in a phase-locked loop oscillator which enable the tracking of the quartz series resonance with virtually zero error irrespective of the load. The principle is discussed and experimental results confirming its successful application are presented.