A signal generating circuit includes a detecting circuit, a charge pump, a first level shifter, a filtering circuit, a second level shifter and a controllable oscillator. The detecting circuit outputs a detecting signal according to a reference signal and an oscillating signal. The charge pump outputs a first output signal by performing a charging or discharging operation according to the detecting signal. The first level shifter adjusts a voltage level of the first output signal to thereby output a second output signal. The filtering circuit generates a first filtered control signal according to the second output signal. The second level shifter adjusts a voltage level of the first filtered controlling signal to output a second filtered control signal. The controllable oscillator outputs the oscillating signal according to the second filtered control signal.