Patent ID: 8629728

Claim:
A voltage-controlled oscillator (VCO) control circuit, applied to a phase locked loop (PLL), for controlling a VCO to process a phase locking procedure after a frequency locking procedure and receiving a frequency locking signal, the VCO control circuit comprising: a judgment unit, coupled to an input end of the VCO, that generates a phase locking signal according to a voltage control signal inputted to the VCO; and an operating frequency controller, coupled to the VCO and the judgment unit; wherein, when the VCO control circuit receives the frequency locking signal indicating a frequency is already locked, the operating frequency controller only generates a first control code to control the VCO to switch from a first candidate VCO curve to a second candidate VCO curve to reduce a phase locking time, and when the operating frequency controller receives the phase locking signal, the operating frequency controller only generates a second control code to control the VCO to switch from the second candidate VCO curve back to the first candidate VCO curve, and wherein the first candidate VCO curve is determined in the frequency locking procedure.