Adjusting phase of a digital phase-locked loop

1. A frequency synthesis system comprising:

a first digital phase-locked loop configured to output a first clock signal;

a second digital phase-locked loop configured to output a second clock signal having substantially the same frequency as the first clock signal; and

a phase adjustment circuit configured to measure a relative phase difference between the first clock signal and the second clock signal and output an indication of the relative phase difference,

wherein the second digital phase-locked loop is configured to adjust a phase of the second clock signal based on the indication of the relative phase difference to cause the relative phase difference to be reduced.

2. The frequency synthesis system of claim 1 , further comprising a combining circuit configured to generate an output clock signal based on the first clock signal and the second clock signal.

3. The frequency synthesis system of claim 1 , wherein the second digital phase-locked loop comprises a digitally controlled oscillator and a time-to-digital converter, wherein the phase adjustment signal is applied in a signal path from an output of the time-to-digital converter to an input to the digitally controlled oscillator.

4. The frequency synthesis system of claim 1 , wherein the second digital phase-locked loop comprises a digital loop filter configured to receive a signal that is based on an output of a phase detector of the second digital phase-locked loop and the indication of the relative phase difference.

5. The frequency synthesis system of claim 1 , wherein the phase adjustment circuit is configured to measure the relative phase difference between the first clock signal and the second clock signal based on a first signal from a feedback path of the first digital phase-locked loop and a second signal from a feedback path of the second digital phase-locked loop.

6. The frequency synthesis system of claim 1 , wherein the phase adjustment circuit is configured to measure the relative phase difference between the first clock signal and the second clock signal based on an output of a first accumulator associated with the first digital phase-locked loop and an output of a second accumulator associated with the second digital phase-locked loop, the first accumulator and the second accumulator configured to reset at different times.

7. The frequency synthesis system of claim 1 , wherein the first digital phase-locked loop is configured to adjust a phase of the first clock signal based on the indication of the relative phase difference.

8. The frequency synthesis system of claim 1 , wherein the first clock signal and the second clock signal have phases that are within 1° of each other.

9. A phase-locked loop comprising:

a digitally controlled oscillator configured to generate a clock signal; and

a time-to-digital converter configured to provide an output signal,

wherein the phase-locked loop is configured to receive a phase adjustment signal in a signal path from an output of the time-to-digital converter to an input to the digitally controlled oscillator, and wherein a phase of the clock signal is based on the phase adjustment signal and the output signal of the time-to-digital converter.

10. The phase-locked loop of claim 9 , further comprising a digital loop filter coupled in the signal path from the output of the time-to-digital converter to the input to the digitally controlled oscillator, the digital loop filter being configured to receive a signal that is based on an output of a phase detector of the phase-locked loop and the phase adjustment signal.

11. The phase-locked loop of claim 9 , further comprising a phase detector, a summer configured to receive an output of the phase detector and the phase adjustment signal, and a digital loop filter configured to receive an output of the summer.

12. The phase-locked loop of claim 9 , further comprising a summer configured to receive a frequency command word and the phase adjustment signal and an accumulator configured to receive an output of the summer.

13. The phase-locked loop of claim 9 , wherein the phase is adjustable over a range of 360°.

14. The phase-locked loop of claim 9 , wherein the phase adjustment signal is a digital word.

15. A method of synchronizing phases of clock signals provided by digital phase-locked loops, the method comprising:

detecting a relative phase difference between a first clock signal provided by a first digital phase-locked loop and a second clock signal provided by a second digital phase-locked loop, the first clock signal and the second clock signal having substantially the same frequency;

providing an indication of the relative phase difference between the first clock signal and the second clock signal to the first digital phase-locked loop; and

adjusting a phase of the first clock signal based on the indication of the relative phase difference between the first clock signal and the second clock signal to cause the relative phase difference to be reduced.

16. The method of claim 15 , wherein said adjusting comprises applying a phase adjustment signal to a signal path between an output of a time-to-digital converter and an input of a digitally controller oscillator in the first digital phase-locked loop.

17. The method of claim 15 , wherein said detecting comprises comparing an output of a first accumulator associated with the first digital phase-locked loop with an output of a second accumulator associated with the second digital phase-locked loop, and wherein the first accumulator and the second accumulator are configured to reset at different times.

18. The method of claim 15 , further comprising generating an output clock signal based on the first clock signal and the second clock signal.

19. The phase-locked loop of claim 9 , wherein the phase-locked loop comprises a phase adjustment input configured to receive the phase adjustment signal from outside of a feedback path of the phase-locked loop.

20. The phase-locked loop of claim 9 , further comprising a summer in the signal path, the summer being configured to receive the phase adjustment signal from outside of a feedback path of the phase-locked loop and to adjust the phase of the clock signal based on the phase adjustment signal.

21. The phase-locked loop of claim 9 further comprising a summer in the signal path from the output of the time-to-digital converter to the input to the digitally controlled oscillator, wherein the summer comprises three inputs and one of the three inputs is configured to receive the phase adjustment signal.

22. The frequency synthesis system of claim 1 , wherein the second digital phase-locked loop is configured to adjust the phase of the second clock signal in response to a phase re-synchronization signal being asserted.