PURPOSE: To improve the quality of a waveform by applying full wave rectification to an output of a sinusoidal wave output voltage controlled oscillator, converting the result into a rectangular wave, comparing the wave with a reference signal having a frequency twice that of the signal from the sinusoidal wave output voltage controlled oscillator and controlling the sinusoidal wave output voltage controlled oscillator depending on the result of comparison.

CONSTITUTION: An output from a sinusoidal wave output voltage controlled oscillator VCO 1 is subjected to full wave rectification by a waveform conversion circuit 2, the result is converted into a rectangular wave and the wave is phase-compared with a reference signal from a reference signal source 3 having a frequency fR twice that of the VCO 1. That is, the phase comparison is implemented twice for one period of the sinusoidal wave output having a frequency of fO. Since a control signal to the VCO 1 is outputted via a loop filter 5 in response to the result of comparison, the correction to obtain the desired frequency fO is applied to the sinusoidal wave output from the VCO 1 and the correction to make 50% duty ratio is applied. Thus, the sinusoidal wave output with high stability whose duty ratio is 50% is obtained in this way and the quality of the waveform is considerably improved.