A digital phase-locked loop having a phase frequency detector (PFD), a 3-state phase frequency detection converter (3-state PFD converter), a loop filter and a digital voltage-controlled oscillator is provided. The PFD receives an input frequency and a reference frequency and outputs a first signal and a second signal based on the phase difference between the input frequency and the reference frequency. The 3-state PFD converter outputs a 3-state signal according to the first and second signals, wherein the 3-state signal is presented in 1, 0 and −1. The loop filter outputs at least one control bit based on only the 3-state signal. The DCO adjusts the outputted oscillation frequency according to the control bit.