Sensitivity Enhancement of Quartz Crystal Capacitive Sensor Using Series Inductive Reactance

In this paper, resonant frequency expression for equivalent circuit of quartz crystal capacitive sensor in series with a tuning inductor was derived. The expression indicates that cascading inductance can significantly increase the load capacitance sensitivity. Experiments using nominal capacitors in series with a calculated inductance validated the theoretical derivation and showed that the frequency pulling sensitivity versus capacitance variations was considerably improved in different levels depending on the quartz crystal fundamental frequency. A quartz crystal capacitive proximity sensor with a calculated inductance was also investigated. Compared with conventional quartz crystal capacitive proximity sensor, the displacement sensitivity of the new sensor with an inductor can be improved two times or more. The new circuit can be used to develop high-sensitivity capacitive sensor.