PURPOSE:To vary the resonance frequency with an external control voltage by controlling an exciting current level of an oscillator having a piezoelectric element with a gain control signal resulting from the sum of an external control voltage and a DC voltage proportional to the exciting current level. CONSTITUTION:An oscillator 1 is operated at a point of mubeta=1 (mu is an open loop gain of the oscillator 1 and beta is a feedback rate) normally, and the operating point is constant with a constant mu when the beta is constant and the gain control voltage of an amplifier circuit 12 is constant. The gain control voltage is an output of an adder circuit 4 being the sum between an output voltage of a current/voltage conversion circuit 3 proportional to the exciting current and an external control voltage. Thus, the exciting current of the oscillator 1 is controlled by varying the external control voltage.
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