Frequency synthesizer and method for controlling same

A frequency synthesizer, comprising:

• a phase comparator circuit that compares the phases of a reference clock and feedback clock;
• a low-pass filter that smoothes an output signal of said phase comparator circuit and generates a control voltage signal;
• a voltage-controlled oscillator that includes a first variable capacitance diode whose DC bias voltage is controlled by said control voltage signal and a plurality of compensation variable capacitance diodes;
• a feedback circuit that generates said feedback clock from an output signal of said voltage-controlled oscillator;
• a monitor circuit that monitors said control voltage signal, outputs a plurality of control signals for said plurality of compensation variable capacitance diodes, and changes the voltage levels of at least one of said plurality of control signals when the voltage level of said control voltage signal is out of a predetermined voltage range;
• and
• a plurality of time constant circuits that, when the voltage level of any of said plurality of control signals change, supply a DC bias voltage to corresponding one of said plurality of compensation variable capacitance diodes, slowing down the voltage change so that a locked state that said frequency synthesizer is in is not canceled.

A method for controlling a frequency synthesizer, comprising:

• providing a frequency synthesizer including
• a phase comparator circuit that compares the phases of a reference clock and feedback clock;
• a low-pass filter that smoothes an output signal of said phase comparator circuit and generates a control voltage signal;
• a voltage-controlled oscillator that comprises a first variable capacitance diode whose DC bias voltage is controlled by said control voltage signal, a compensation variable capacitance diode, and another capacitance;
• and
• a feedback circuit that generates said feedback clock from a clock generated by said voltage-controlled oscillator;
• the method further comprising:
• in case where said frequency synthesizer is in a locked state, and
• said control voltage signal is out of a predetermined voltage range, and
• when it is possible to bring said control voltage signal back within a predetermined voltage range via said phase comparator circuit and said low-pass filter circuit, changing a DC bias voltage of said compensation variable capacitance diode thereby changing an oscillation frequency of said voltage-controlled oscillator, changing the DC bias voltage of said compensation variable capacitance diode slowly so that said locked state is not canceled and said control voltage signal is brought back within a predetermined voltage range;
• and
• when it is impossible to change the DC bias voltage of said compensation variable capacitance diode so as to bring said control voltage signal back within a predetermined voltage range, changing a capacitance value of said another capacitance and restarting the digital tuning of said voltage-controlled oscillator.