Patent ID: 8648625

Claim:
A frequency synthesizer comprising: a voltage controlled oscillating part outputting a frequency signal that is determined according to a control voltage; a set signal output part outputting, as a digital signal, a sinusoidal signal with a frequency set by a frequency setting part; a digital/analog converting part digital/analog-converting the digital signal output from said set signal output part; a differential amplifier amplifying a difference between a sinusoidal signal with a frequency corresponding to an output frequency of said voltage controlled oscillating part and the sinusoidal signal from said digital/analog converting part; an analog/digital converting part analog/digital-converting an output signal of said differential amplifier; a phase detecting part quadrature-detecting a frequency signal obtained by said analog/digital converting part and extracting, from the frequency signal, a phase difference signal corresponding to a phase difference between the both sinusoidal signals input to said differential amplifier; an integrating part integrating the phase difference signal and supplying a voltage corresponding to an integration value to said voltage controlled oscillating part; and an amplitude adjusting part detecting, on a basis of both sinusoidal signals obtained by said differential amplifier, a differential value of amplitudes of said both sinusoidal signals, and on a basis of the differential value of the amplitudes, adjusting an amplitude of the output signal of said digital/analog converting part so as to make amplitudes of the both sinusoidal signals input to said differential amplifier equal to each other.