Fractional frequency synthesizer and phase locked loop utilizing fractional frequency synthesizer and method thereof

摘要

A fractional frequency synthesizer, applied to a phase-locked loop, includes a phase detector, a loop filter, a controllable oscillator, a first frequency divider, and a sigma-delta modulator (SDM). The phase detector generates a phase difference signal according to a reference signal and a feedback signal. The loop filter filters the phase difference signal to generate a filtered signal. The controllable oscillator generates the frequency signal according to the filtered signal. The first frequency divider generates the feedback signal by dividing a frequency of the frequency signal according to a dividing factor. The SDM determines the dividing factor according to a control signal.