Control circuitry for quartz crystal deposition monitor

Monitoring circuit for monitoring at least one resonant frequency of a piezoelectric crystal on which a film of material is being vacuum deposited to control a source of vapors of said material;

• the monitoring circuit comprising:
• a) controllable frequency generating means for generating a drive signal at a frequency, independent of the crystal resonance frequency, selected within a band of RF frequencies that includes the at least one resonant frequency of said piezoelectric crystal;
• b) a duplexing circuit having an input coupled to said frequency generating means to receive said drive signal therefrom, a crystal terminal coupled to said crystal and supplying the drive signal to said crystal and receiving a response signal reflected back from the crystal, and an output terminal at which said response signal appears, the response signal having the frequency of said drive signal but a phase that depends on the proximity of the drive signal frequency to a resonant frequency of said crystal;
• c) phase detector means supplied with said drive signal and said response signal and having a phase output signal whose value indicates the relative phase of the drive and response signals, from a negative level through zero to a positive level;
• and
• d) controller means coupled to said frequency generating means and said phase detector means for controllably changing the frequency of said drive signal and monitoring the frequency at which said phase detector signal indicates a zero level to identify the frequency of each of said one or more resonances.