Method of detecting a variation in stray capacitance by use of a pll

Abstract

Disclosed is a method of detecting a variation in stray capacitance by use of a PLL circuit. This method involves the steps of: setting respectively a freerunning frequency and a capture range of a phase-locked loop circuit so that a range in which an output frequency of a pre-oscillator changes extends to an interior of the capture range of the phase-locked loop circuit, the output frequency being determined by a synthetic time constant obtained by adding up a variation value of an electric capacitance or the like that is detected by a sensor and a reserve time constant preset in a time constant determining module when detecting the stray capacitance by employing the sensor for detecting the variation in the stray capacitance in contact or non-contact with a body to be detected and a capacitance detecting means including the pre-oscillator having the time constant determining module connected to the sensor and the phase-locked loop circuit connected to the pre-oscillator; detecting a change in a freerunning frequency control voltage in a locked state of the phase-locked loop circuit, the change being caused with the variation in the electric capacitance or the like that is detected by the sensor; and detecting a state variation of the detected body.