Clock generating apparatus and frequency calibrating method of the clock generating apparatus

A clock generating apparatus, comprising:

• a time-to-digital converter (TDC), arranged to convert a timing difference between a reference clock and a variable clock to generate a digital value;
• a calibrating device, arranged to generate a control signal according to the digital value and the reference clock;
• a controllable oscillator, arranged to generate an oscillating signal according to the control signal and the digital value;
• a feedback device, arranged to generate the variable clock to the TDC according to the oscillating signal;
• a digital loop filter, coupled between the time-to-digital converter and the controllable oscillator;
• and
• a sigma-delta modulator, coupled to the feedback device;
• wherein the calibrating device calibrates the controllable oscillator to make the oscillating signal have a target oscillating frequency, and the digital loop filter and the sigma-delta modulator are disabled while the calibrating device is calibrating the controllable oscillator.