Fast-locking phase-locked loop and associated fast-locking method thereof

1. A fast-locking phase-locked loop (PLL), comprising:

a gear-shifting loop filter, configured to have a dynamic bandwidth, wherein the gear-shifting loop filter comprising:

a resistor set, configured to have a dynamic resistance;

a capacitor set, coupled to the resistor set, configured to have a dynamic capacitance, wherein the capacitor set comprises a first capacitor and a second capacitor; and

a voltage buffer, configured to copy a voltage level on the first capacitor to the second capacitor;

wherein the dynamic resistance is switched from a first resistance to a second resistance and the dynamic capacitance is switched from a first capacitance to a second capacitance, to make the dynamic bandwidth be switched from a first bandwidth to a second bandwidth.

2. The fast-locking PLL of claim 1 , wherein the gear-shifting loop filter further comprises:

another capacitor set, coupled to the resistor set, configured to have another dynamic capacitance;

wherein when the dynamic bandwidth is switched from the first bandwidth to the second bandwidth, said another dynamic capacitance is switched from another first capacitance to another second capacitance, to make phase margin of the fast-locking PLL unchanged.

3. The fast-locking PLL of claim 2 , wherein the first resistance is γ times the second resistance, the first capacitance is α times the second capacitance, and said another first capacitance is β times another second capacitance, to make the first bandwidth be γ times the second bandwidth and the phase margin of the fast-locking PLL be unchanged, wherein α=β=1/γ 2 , and γ is a positive value greater than one.

4. The fast-locking PLL of claim 1 , wherein the resistor set is coupled between a first common node and a second common node of the gear-shifting loop filter, the first capacitor is coupled to the resistor set via the second common node, and the dynamic capacitance is switched from the first capacitance to the second capacitance by coupling the second capacitor to the second common node, wherein the voltage level on the first capacitor is copied to the second capacitor before coupling the second capacitor to the second common node.

5. The fast-locking PLL of claim 4 , wherein the gear-shifting loop filter further comprises:

another capacitor set, coupled to the resistor set, configured to have another dynamic capacitance, wherein said another capacitor set comprises another first capacitor and another second capacitor, and said another first capacitor is coupled to the first common node;

wherein when the dynamic bandwidth is switched from the first bandwidth to the second bandwidth, said another dynamic capacitance is switched from another first capacitance to another second capacitance by coupling said another second capacitor to the first common node, to make phase margin of the fast-locking PLL unchanged.

6. The fast-locking PLL of claim 5 , wherein with aid of the voltage buffer, the voltage level on the first capacitor is copied to said another second capacitor before coupling said another second capacitor to the first common node.

7. The fast-locking PLL of claim 5 , wherein with aid of another voltage buffer within the gear-shifting loop filter, another voltage level on said another first capacitor is copied to said another second capacitor before coupling said another second capacitor to the first common node.

8. The fast-locking PLL of claim 4 , wherein the voltage buffer comprises:

an auto-zero unit gain buffer (UGB), comprising:

an amplifier circuit, having an offset voltage caused by a finite gain or input mismatch of the amplifier circuit;

a storage capacitor, coupled to the amplifier circuit, configured to store the offset voltage;

wherein in a calibration mode of the auto-zero UGB, the offset voltage is stored on the storage capacitor; and in a buffer mode of the auto-zero UGB, an input voltage level on an input terminal of the auto-zero UGB is copied to an output terminal of the auto-zero UGB.

9. The fast-locking PLL of claim 8 , wherein:

in the calibration mode, the storage capacitor is coupled between a first input terminal and a second input terminal of the amplifier circuit, and an output terminal of the amplifier circuit is coupled to the second input terminal of the amplifier circuit; and

in the buffer mode, the input terminal of the auto-zero UGB is coupled to the first input terminal of the amplifier circuit, and the storage capacitor is coupled between the second input terminal of the amplifier circuit and the output terminal of the auto-zero UGB;

wherein the output terminal of the auto-zero UGB is coupled to the output terminal of the amplifier circuit.

10. The fast-locking PLL of claim 8 , wherein a time point of the dynamic bandwidth being switched from the first bandwidth to the second bandwidth is in a period at which the auto-zero UGB operates in the buffer mode.

11. A fast-locking method of a fast-locking phase-locked loop (PLL), comprising:

utilizing a resistor set of a gear-shifting loop filter within the fast-locking PLL to control a dynamic resistance and utilizing a capacitor set of the gear-shifting loop filter to control a dynamic capacitance, to make the gear-shifting loop filter have a dynamic bandwidth, wherein the capacitor set comprises a first capacitor and a second capacitor;

with aid of a voltage buffer within the gear-shifting loop filter, copying a voltage level on the first capacitor to the second capacitor; and

switching the dynamic resistance from a first resistance to a second resistance, and switching the dynamic capacitance from a first capacitance to a second capacitance, to make the dynamic bandwidth be switched from a first bandwidth to a second bandwidth.

12. The fast-locking method of claim 11 , wherein the gear-shifting loop filter further comprises another capacitor set having another dynamic capacitance, and the fast-locking method further comprising:

in response to the dynamic bandwidth being switched from the first bandwidth to the second bandwidth, switching said another dynamic capacitance from another first capacitance to another second capacitance, to make phase margin of the fast-locking PLL unchanged.

13. The fast-locking method of claim 12 , wherein the first resistance is γ times the second resistance, the first capacitance is α times the second capacitance, and said another first capacitance is β times another second capacitance, to make the first bandwidth be γ times the second bandwidth and the phase margin of the fast-locking PLL be unchanged, wherein α=β=1/γ 2 , and γ is a positive value greater than one.

14. The fast-locking method of claim 11 , wherein the resistor set is coupled between a first common node and a second common node of the gear-shifting loop filter, the first capacitor is coupled to the resistor set via the second common node, and the step of switching the dynamic capacitance from the first capacitance to the second capacitance comprises:

after the voltage level on the first capacitor is copied to the second capacitor, coupling the second capacitor to the second common node.

15. The fast-locking method of claim 14 , further comprising:

utilizing another capacitor set of the gear-shifting loop filter to control another dynamic capacitance, wherein said another capacitor set comprises another first capacitor and another second capacitor, and said another first capacitor is coupled to the first common node; and

in response to the dynamic bandwidth being switched from the first bandwidth to the second bandwidth, switching said another dynamic capacitance from another first capacitance to another second capacitance by coupling said another second capacitor to the first common node, to make phase margin of the fast-locking PLL unchanged.

16. The fast-locking method of claim 15 , wherein the step of switching said another dynamic capacitance from said another first capacitance to said another second capacitance further comprises:

with aid of the voltage buffer, copying the voltage level on the first capacitor to said another second capacitor before coupling said another second capacitor to the first common node.

17. The fast-locking method of claim 15 , wherein the step of switching said another dynamic capacitance from said another first capacitance to said another second capacitance further comprises:

with aid of another voltage buffer within the gear-shifting loop filter, copying another voltage level on said another first capacitor to said another second capacitor before coupling said another second capacitor to the first common node.

18. The fast-locking method of claim 14 , wherein the voltage buffer comprises an auto-zero unit gain buffer (UGB), the auto-zero UGB comprises an amplifier circuit and a storage capacitor coupled to the amplifier circuit, and the step of copying the voltage level on the first capacitor to the second capacitor comprises:

in a calibration mode of the auto-zero UGB, storing an offset voltage of the amplifier circuit on the storage capacitor, wherein the offset voltage is caused by a finite gain or input mismatch of the amplifier circuit; and

in a buffer mode of the auto-zero UGB, copying an input voltage level on an input terminal of the auto-zero UGB to an output terminal of the auto-zero UGB, wherein the input voltage level represents the voltage level on the first capacitor.

19. The fast-locking method of claim 18 , wherein:

in the calibration mode, the storage capacitor is coupled between a first input terminal and a second input terminal of the amplifier circuit, and an output terminal of the amplifier circuit is coupled to the second input terminal of the amplifier circuit; and

in the buffer mode, the input terminal of the auto-zero UGB is coupled to the first input terminal of the amplifier circuit, and the storage capacitor is coupled between the second input terminal of the amplifier circuit and the output terminal of the auto-zero UGB;

wherein the output terminal of the auto-zero UGB is coupled to the output terminal of the amplifier circuit.

20. The fast-locking method of claim 18 , wherein a time point of the dynamic bandwidth being switched from the first bandwidth to the second bandwidth is in a period at which the auto-zero UGB operates in the buffer mode.