VCO control circuit and method thereof, fast locking PLL and method for fast locking PLL

1. 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.

2. The VCO control circuit as recited in claim 1 , wherein the judgment unit generates the phase locking signal when the judgment unit determines that the voltage control signal is greater than a first threshold voltage.

3. The VCO control circuit as recited in claim 2 , wherein an output frequency of the VCO corresponding to the second candidate VCO curve is smaller than an output frequency of the VCO corresponding to the first candidate VCO curve.

4. The VCO control circuit as recited in claim 3 , wherein, after the operating frequency controller generates the second control code to control the VCO to switch from the second candidate VCO curve to the first candidate VCO curve, if the judgment unit determines that the voltage control signal is greater than a second threshold voltage, the judgment unit provides a frequency adjustment determining signal to the operating frequency controller to generate a third control code to control the VCO to switch from the first candidate VCO curve to a third candidate VCO curve, the second threshold voltage is larger than the first threshold voltage, and an output frequency of the VCO corresponding to the third candidate VCO curve is larger than the output frequency of the VCO corresponding to the first candidate VCO curve.

5. The VCO control circuit as recited in claim 3 , wherein, after the operating frequency controller generates the second control code to control the VCO to switch from the second candidate VCO curve to the first candidate VCO curve, if the judgment unit determines that the voltage control signal is smaller than the first threshold voltage, the judgment unit provides a frequency adjustment determining signal to the operating frequency controller to generate a fourth control code to control the VCO to switch from the first candidate VCO curve to a fourth candidate VCO curve, and an output frequency of the VCO corresponding to the fourth candidate VCO curve is larger than the output frequency of the VCO corresponding to the first candidate VCO curve.

6. A fast locking phase locked loop (PLL) circuit, for accelerating convergence of a phase when a frequency is locked, the fast locking PLL circuit comprising:

a detector, that generates a voltage control signal according to a phase difference between a reference signal and a feedback signal;

a filter, coupled to the detector, that filters the voltage control signal to generate a filtered voltage control signal;

a voltage-controlled oscillator (VCO), coupled to the filter, that generates an output signal according to the filtered voltage control signal;

a frequency divider, coupled to the VCO and the detector, that generates the feedback signal according to the output signal; and

a VCO control circuit, coupled to the filter and the VCO;

wherein, when the VCO control circuit receives a frequency locking signal, the VCO control circuit 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 VCO control circuit determines that the filtered voltage control signal is greater than a first threshold voltage, the VCO control circuit 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 when the frequency is locked.

7. The fast locking PLL circuit as recited in claim 6 , wherein the VCO control circuit comprises:

a judgment unit, coupled to the filter, that generates a phase locking signal according to the filtered voltage control signal inputted to the VCO; and

an operating frequency controller, coupled to the VCO and the judgment unit, that provides one of the first control code and the second control code to the VCO;

wherein, when the operating frequency controller receives a frequency locking signal, the operating frequency controller generates the first control code to control the VCO to switch from the first candidate VCO curve to the second candidate VCO curve, and when the operating frequency controller receives the phase locking signal, the operating frequency controller generates the second control code to control the VCO to switch from the second candidate VCO curve to the first candidate VCO curve.

8. The fast locking PLL circuit as recited in claim 6 , wherein an output frequency of the VCO corresponding to the second candidate VCO curve is smaller than an output frequency of the VCO corresponding to the first candidate VCO curve.

9. The fast locking PLL circuit as recited in claim 8 , wherein, after the operating frequency controller generates the second control code to control the VCO to switch from the second candidate VCO curve to the first candidate VCO curve, if the judgment unit determines that the voltage control signal is greater than a second threshold voltage, the judgment unit provides a frequency adjustment determining signal to the operating frequency controller to generate a third control code to control the VCO to switch from the first candidate VCO curve to a third candidate VCO curve, the second threshold voltage is larger than the first threshold voltage, and an output frequency of the VCO corresponding to the third candidate VCO curve is larger than the output frequency of the VCO corresponding to the first candidate VCO curve.

10. The fast locking PLL circuit as recited in claim 8 , wherein, after the operating frequency controller generates the second control code to control the VCO to switch from the second candidate VCO curve to the first candidate VCO curve, if the judgment unit determines that the voltage control signal is smaller than the first threshold voltage, the judgment unit provides a frequency adjustment determining signal to the operating frequency controller to generate a fourth control code to control the VCO to switch from the first candidate VCO curve to a fourth candidate VCO curve, and an output frequency of the VCO corresponding to the fourth candidate VCO curve is larger than the output frequency of the VCO corresponding to the first candidate VCO curve.

11. A voltage-controlled oscillator (VCO) control method, 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 method comprising:

generating only 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 when the frequency locking signal is received; and

generating only a second control code to control the VCO to switch from the second candidate VCO curve back to the first candidate VCO curve when a voltage control signal inputted by the VCO is larger than a first threshold voltage,

wherein the first candidate VCO curve is determined when the frequency is locked.

12. The control method as recited in claim 11 , wherein an output frequency of the VCO corresponding to the second candidate VCO curve is smaller than an output frequency of the VCO corresponding to the first candidate VCO curve.

13. The VCO control method as recited in claim 12 , wherein, after the second control code is generated to control the VCO to switch from the second candidate VCO curve to the first candidate VCO curve, if the voltage control signal is greater than a second threshold voltage, the method further comprises providing a frequency adjustment determining signal to an operating frequency controller to generate a third control code to control the VCO to switch from the first candidate VCO curve to a third candidate VCO curve, the second threshold voltage is larger than the first threshold voltage, and an output frequency of the VCO corresponding to the third candidate VCO curve is larger than the output frequency of the VCO corresponding to the first candidate VCO curve.

14. The VCO control method as recited in claim 12 , wherein, after the second control code is generated to control the VCO to switch from the second candidate VCO curve to the first candidate VCO curve, if the voltage control signal is smaller than the first threshold voltage, the method further comprises providing an adjustment signal to an operating frequency controller to generate a fourth control code to control the VCO to switch from the first candidate VCO curve to a fourth candidate VCO curve, and an output frequency of the VCO corresponding to the fourth candidate VCO curve is larger than the output frequency of the VCO corresponding to the first candidate VCO curve.