Study of oscillator-circuit behavior for QCM gas sensor

A quartz resonator with its electrode coated with a sensing film is used as a gas sensor because of its resonance frequency shift due to the mass loading effect. Although resonance frequency shift can be measured by an impedance analyzer, the method of measuring the shift using an oscillation circuit is widely used for simplicity. The oscillation frequency shift deviates from that of the resonance frequency mainly due to the dependency of active impedance upon its load change when the large viscoelastic effect accompanied by film coating or vapor sorption occurs. Thus, we established an analysis method of the oscillation frequency shift, taking active impedance change into account. The values predicted by the analysis agree well with the experimental ones when a quartz resonator is coated with an organic film.