a voltage-controlled oscillator (VCO) which generates first to third oscillation signals according to first to third control signals of an automatic band selection mode, an automatic gain tuning mode, and a phase-locking mode, respectively, with respect to the VCO; and

a mode controller which generates:

the first control signal from a first frequency comparison result between a reference signal and a first feedback signal generated by dividing a frequency of an initial oscillation signal of the VCO by a first number, to control the VCO to generate the first oscillation signal in the automatic band selection mode;

the second control signal from a second frequency comparison result between the reference signal and a second feedback signal generated by dividing a frequency of the first oscillation signal by a second number, to control the VCO to generate the second oscillation signal in the automatic gain tuning mode; and

the third control signal to control the VCO to generate the third oscillation signal which is phase-locked in the phase-locking mode,

wherein the mode controller comprises:

a first switch which provides signs of the first or second frequency comparison result to a first or second path, respectively, according to a mode selection signal selecting one of the automatic band selection mode, the automatic gain tuning mode and the phase-locking mode;

a second switch which outputs one of a first voltage, a second voltage or an oscillation-controlled voltage to the VCO, respectively, according to the mode selection signal;

a band controller which generates the first control signal to select an operating frequency band of the VCO according to the sign of the first frequency comparison result which is output to the first path; and

a gain controller which generates the second control signal to select a target gain range of the VCO according to the sign of the second frequency comparison result which is output to the second path.