Phase-locked loop and method for the same

1. A phase-locked loop, comprising:

a first loop comprising a controlled oscillator and a phase detector, wherein the controlled oscillator is configured to generate an oscillation signal, and wherein the phase detector is configured to generate a first signal indicative of a timing difference between a reference signal and the oscillation signal;

a second loop configured to generate, based on the oscillation signal, a second signal indicative of a timing error of the oscillation signal's cycle time, and to generate a correction signal based on the second signal; and

a combiner configured to generate a control signal for the controlled oscillator by combining the correction signal and a third signal derived from the first signal,

wherein the second loop comprises:

a time-to-digital converter configured to generate, based on the oscillation signal and a delayed replica of the oscillation signal, a fourth signal indicative of the oscillation signal's actual cycle time.

2. The phase-locked loop of claim 1 , wherein the controlled oscillator is configured to generate the oscillation signal based on the control signal.

3. The phase locked loop of claim 1 , wherein a sampling frequency of the time-to-digital converter is at least 20 times lower than an oscillation frequency of the oscillation signal.

4. The phase-locked loop of claim 1 , wherein the second loop further comprises:

a delay element configured to generate the delayed replica of the oscillation signal by delaying the oscillation signal.

5. The phase-locked loop of claim 1 , wherein the second loop further comprises:

a second combiner configured to generate the second signal by combining the fourth signal and a fifth signal indicative of a nominal cycle time of the oscillation signal.

6. The phase-locked loop of claim 5 , wherein the second loop further comprises:

an integrator configured to generate the correction signal by continuously integrating the second signal.

7. The phase-locked loop of claim 5 , wherein the second loop further comprises:

a third combiner configured to generate a sample of the correction signal by combining the second signal with a prior sample of the correction signal.

8. The phase-locked loop of claim 7 , wherein the second loop further comprises:

a second delay element coupled between an output and an input of the third combiner, wherein the delay element is configured to delay the prior sample of the correction signal.

9. The phase-locked loop of claim 6 , wherein the second loop further comprises:

a filter configured to band-pass filter the correction signal.

10. The phase-locked loop of claim 1 , wherein the first loop further comprises a loop filter configured to generate the third signal by filtering the first signal.

11. The phase-locked loop of claim 1 , wherein the phase detector is a time-to-digital-converter.

12. The phase-locked loop of claim 1 , wherein the controlled oscillator is a digitally controlled oscillator.

13. The phase-locked loop of claim 1 , wherein the controlled oscillator is a voltage controlled oscillator.

14. A transmitter comprising a phase-locked loop according to claim 1 .

15. The transmitter of claim 14 , further comprising:

a mixing circuit configured to up-convert a baseband transmit signal using a signal derived from the oscillation signal.

16. A receiver comprising a phase-locked loop according to claim 1 .

17. The receiver of claim 16 , further comprising:

a mixing circuit configured to down-convert a radio frequency receive signal using a signal derived from the oscillation signal.

18. A method for a phase-locked loop comprising a first loop and a second loop, wherein the first loop comprises a controlled oscillator and a phase detector, the method comprising:

generating, using the controlled oscillator, an oscillation signal;

generating, using the phase detector, a first signal indicative of a timing difference between a reference signal and the oscillation signal;

generating, using the second loop, a second signal indicative of a timing error of the oscillation signal's cycle time;

generating, using the second loop, a correction signal based on the second signal; and

generating a control signal for the controlled oscillator by combining the correction signal and a third signal derived from the first signal

wherein generating the second signal comprises generating, based on the oscillation signal and a delayed replica of the oscillation signal, a fourth signal indicative of the oscillation signal's actual cycle time.

19. The method of claim 18 , wherein generating the oscillation signal is based on the control signal.

20. The method of claim 18 , wherein generating the second signal further comprises generating the delayed replica of the oscillation signal by delaying the oscillation signal.

21. The method of claim 18 , wherein generating the second signal further comprises combining the fourth signal and a fifth signal indicative of a nominal cycle time of the oscillation signal.

22. The method of claim 21 , wherein generating the correction signal further comprises continuously integrating the second signal.

23. The method of claim 21 , wherein generating the correction signal further comprises:

generating a sample of the correction signal by combining the second signal with a prior sample of the correction signal.