Control circuitry for quartz crystal deposition monitor

Abstract

A monitoring circuit monitors a piezoelectric crystal for changes in its resonant frequency as a film of material is vacuum deposited on it, e.g. to control a source of vapors of the material. A controllable frequency generator generates a drive signal of a accurately known frequency and phase within a band of RF frequencies that includes the resonant frequency of the crystal. The RF signal is applied through a duplexing circuit to the crystal, and a response signal is applied through the duplexing circuit to a signal amplifier and limiter circuit to a phase detector. The phase detector is also supplied with the RF drive signal, and has a phase output signal which is zero when the crystal is at resonance, and is negative or positive when the applied RF signal is below or above resonance, respectively. A micro-controller can be employed for controllably sweeping the frequency of the drive signal and then monitoring the frequencies at which the phase detector signal indicates zero, to identify the frequency of each of the one or more resonances. A cable compensation control circuit can also be coupled to the duplexing circuit for balancing the reactance of the conductive paths associated with the crystal. This can include a voltage controlled capacitance and a portion of a transformer within the duplexing circuit.