Patent ID: 8467758

Claim:
A frequency tuning circuit which supports a first operation mode, a second operation mode, and a third operation mode, the circuit comprising: a voltage controlled oscillator which includes a first control terminal and outputs an oscillation signal which is controllable based on a first control voltage provided to the first control terminal; a digital frequency detector which detects a frequency error between a reference signal and a target signal obtained by dividing a frequency of the oscillation signal at a desired rate; a controller which adjusts again of the frequency error to generate a first digital control code; a register which outputs the first digital control code as a second digital control code in the first operation mode and the second operation mode, saves the first digital control code as a third digital control code at an end of the first operation mode, and outputs the third digital control code as the second digital control code at a beginning of the third operation mode; a digital-to-analog converter which converts the second digital control code into an analog control signal and outputs the analog control signal; a bandlimiting filter which filters the analog control signal to generate the first control voltage when connected to the first control terminal of the voltage controlled oscillator; a buffer amplifier which drives the bandlimiting filter when connected to the digital-to-analog converter and the bandlimiting filter; and a switcher which switches a state of connections among the digital-to-analog converter, the first control terminal of the voltage controlled oscillator, the bandlimiting filter, and the buffer amplifier, and wherein in the first operation mode, the digital-to-analog converter supplies the analog control signal with the first control voltage to the first control terminal of the voltage controlled oscillator, in the second operation mode, the digital-to-analog converter supplies the analog control signal to the buffer amplifier, the buffer amplifier drives the bandlimiting filter, and the bandlimiting filter generates the first control voltage, and in the third operation mode, the digital-to-analog converter supplies the analog control signal to the bandlimiting filter, and the bandlimiting filter generates the first control voltage.