Oscillator and a PLL circuit using the oscillator

A PLL circuit comprising:

• an oscillator comprising a variable inductor unit of variable inductance, and a variable capacitance device connected to the variable inductor unit, the oscillator oscillating and generating a VCO signal of an oscillation frequency determined by the inductance of the variable inductor unit and the capacitance of the variable capacitance device;
• a phase generator operable to generate a phase signal denoting the phase of the VCO signal;
• and
• a loop filter operable to filter the phase signal and to output a capacitance control signal;
• wherein the variable inductor unit comprises
• a first inductor,
• a current signal generator operable to detect an electric signal denoting current flowing to the first inductor or the voltage at both ends of the first inductor, and to generate a current signal based on the electric signal, and
• a second inductor that receives the current signal,
• wherein the first inductor and second inductor are disposed to a predetermined magnetically coupled position, and the inductance of the first inductor is set desirably;
• and
• the oscillator controls both end voltages of the variable capacitance device based on the capacitance control signal, and sets the capacitance of the variable capacitance device to a desired level.