Frequency Synthesizing Device and Automatic Calibration Method Thereof

A frequency synthesizing device, comprising:

• a voltage-controlled oscillator receiving an adjusting signal and generating an output signal according to the adjusting signal;
• a feedback frequency divider having a plurality of divisor values, the feedback frequency divider receiving the output signal and generating a feedback signal after performing frequency dividing;
• and
• an automatic frequency calibration circuit comprising:
• a first frequency divider receiving a reference frequency and generating a frequency dividing reference frequency;
• a second frequency divider receiving the feedback signal and generating a frequency dividing feedback signal;
• a comparator receiving and comparing outputs from the first frequency divider and the second frequency divider in a predetermined period to generate a comparing result;
• and
• a state machine outputting the adjusting signal according to the comparing result in a calibration mode;
• wherein the calibration mode comprises a coarse adjusting mode;
• the state machine sets the feedback frequency divider to generate the feedback signal by using a first divisor value selected from the plurality of divisor values in the coarse adjusting mode;
• when the comparing result shows a phase of the frequency dividing feedback signal falling behind a phase of the frequency dividing reference frequency, the state machine sets the feedback frequency divider to generate the feedback signal by using a second divisor value selected from the plurality of divisor values different from the first divisor value.