A circuit comprises an oscillator, a frequency divider and a comparator. The oscillator generates an oscillating signal (Fvco). The frequency divider is communicatively coupled to the oscillator, divides a frequency of the oscillating signal by a denominator and generates a divided signal. The comparator is communicatively coupled to the oscillator and the frequency divider, and is configured to obtain a first count of the divided signal (Fvco/N) within a predetermined time and a second count of a reference signal within the predetermined time; compare the first count with the second count, and generate a comparison result according to the first count and the second count. The oscillator is further configured to adjust the frequency of the oscillating signal according to the comparison result.