Precise beat frequency evaluation circuit for multi-oscillators QCM gas detectors

Presented work deals with crystal oscillators that can be used as a quartz crystal microbalance (QCM) gas sensor. There are presented two basic oscillators that are very common in literature. The oscillators were put into practice and tested in the gas chamber with different crystals of basic frequency 10 MHz. There were tested clean crystal resonators and resonators coated by a sensitive layer. Frequency response of the oscillators is usually very small and it is comparable with the effects such as temperature change or crosstalk in the evaluation circuit. The evaluation circuit thus must be designed carefully. Main goal of this paper is comparison of two basic designs of the oscillators and presentation of the precise evaluation circuit with minimal crosstalk and reverse transmission in signal ways.