Digital subsampling PLL with DTC-based SAR phase estimation
The present disclosure relates to a digital subsampling phase-locked-loop (PLL) with a digital-to-time converter (DTC) based successive-approximation-register (SAR) phase estimation. This disclosed PLL utilizes a DTC and a one-bit sampler to generate one phase word by calculating multiple one-bit phase measurements with a SAR algorithm. The one phase word, which indicates the phase estimation of a radio frequency (RF) output signal compared to a reference signal, enables the PLL to lock the RF output signal with the reference signal in a short settling time. In addition, utilizing the one-bit sampler instead of a conventional frequency divider is good for linearity and low power consumption of the PLL without introducing significant noise in the RF output signal.
1. A phase-locked-loop (PLL) comprising:
a local oscillator (LO) configured to generate a radio frequency (RF) output signal;
a phase sampling unit, which is configured to receive a reference signal and the RF output signal, and to generate a sub-sampled output by sub-sampling the RF output signal with a sampling clock, wherein,
the sampling clock has a same frequency as the reference signal and is offset by a time delay from the reference signal; and
the time delay of the sampling clock offset from the reference signal is updated in each clock-cycle of the reference signal; and
a successive-approximation-register (SAR) phase estimator configured to generate a phase word every N clock-cycles of the reference signal, wherein,
N is a positive integer number;
the phase word is derived from the sub-sampled output generated in each clock-cycle of the every N clock-cycles of the reference signal, and indicates a phase shift of the RF output signal compared to the reference signal; and
the LO is configured to update the RF output signal based on the phase word.
2. The PLL of claim 1 wherein the phase sampling unit comprises:
a digital to time convertor (DTC) controller configured to provide a DTC total control, wherein,
the DTC total control is updated in each clock-cycle of the reference signal; and
in at least some clock-cycles of the every N clock-cycles of the reference signal, the DTC total control is updated based on the sub-sampled output generated in a corresponding previous clock-cycle of the reference signal;
a DTC configured to receive the reference signal and the DTC total control and to provide the sampling clock, wherein the DTC total control determines the time delay of the sampling clock offset from the reference signal in each clock-cycle of the reference signal; and
a sampler configured to sub-sample the RF output signal with the sampling clock so as to provide the sub-sampled output in each clock-cycle of the reference signal.
3. The PLL of claim 2 wherein
the DTC total control is updated non-linearly in the at least some clock-cycles of the every N clock-cycles of the reference signal; and
each non-linear update of the DTC total control leads to an increment or a decrement in the time delay of the sampling clock offset from the reference signal for each clock-cycle of the reference signal.
4. The PLL of claim 2 wherein in some other clock-cycles of the every N clock-cycles of the reference signal, the DTC total control is updated linearly and is not based on any sub-sampled output.
5. The PLL of claim 2 wherein the sampler is a one-bit sampler.
6. The PLL of claim 2 wherein the DTC total control comprises a DTC mixing control and a DTC SAR control, such that the time delay of the sampling clock offset from the reference signal in each clock-cycle of the reference signal is determined by the DTC mixing control and the DTC SAR control, wherein
the DTC mixing control signal is updated not based on the sub-sampled output; and
in the at least some clock-cycles of the every N clock-cycles of the reference signal, the DTC SAR control is updated based on the sub-sampled output generated in the corresponding previous clock-cycle of the reference signal.
7. The PLL of claim 6 wherein,
in first M clock-cycles of the every N clock-cycles of the reference signal, the DTC SAR control is updated linearly with an equal increment in each clock-cycle of the first M clock-cycles of the reference signal, wherein the equal increment in the DTC SAR control leads to an equal increment in the time delay of the sampling clock offset from the reference signal in each clock-cycle of the first M clock-cycles of the reference signal, wherein M is a positive integer and less than N; and
in remaining clock-cycles of the every N clock-cycles of the reference signal, the DTC SAR control is updated non-linearly in each clock-cycle of the remaining clock-cycles of the reference signal, wherein,
each non-linear update of the DTC SAR control is based on the sub-sampled output generated in the corresponding previous clock-cycle of the reference signal; and
each non-linear update of the DTC SAR control leads to an increment or a decrement in the time delay of the sampling clock offset from the reference signal for each clock-cycle of the remaining clock-cycles of the reference signal.
8. The PLL of claim 6 wherein the DTC mixing control is updated linearly with an equal increment in each clock-cycle of the every N clock-cycles of the reference signal, wherein the equal increment in the DTC mixing control leads to an equal increment in the time delay of the sampling clock offset from the reference signal for each clock-cycle of the reference signal.
9. The PLL of claim 8 wherein,
in first M clock-cycles of the every N clock-cycles of the reference signal, the DTC SAR control is updated linearly with an equal increment in each clock-cycle of the first M clock-cycles of the reference signal, wherein the equal increment in the DTC SAR control leads to an equal increment in the time delay of the sampling clock offset from the reference signal in each clock-cycle of the first M clock-cycles of the reference signal, wherein
M is a positive integer and less than N; and
the equal increment in the DTC SAR control and the equal increment in the DTC mixing control are different; and
in remaining clock-cycles of the every N clock-cycles of the reference signal, the DTC SAR control is updated non-linearly in each clock-cycle of the remaining clock-cycles of the reference signal, wherein,
each non-linear update of the DTC SAR control is based on the sub-sampled output generated in the corresponding previous clock-cycle of the reference signal; and
each non-linear update of the DTC SAR control leads to an increment or a decrement in the time delay of the sampling clock offset from the reference signal for each clock-cycle of the remaining clock-cycles of the reference signal.
10. The PLL of claim 2 wherein the DTC total control comprises a DTC mixing control and a DTC SAR control, such that the time delay of the sampling clock offset from the reference signal in each clock-cycle of the reference signal is determined by the DTC mixing control and the DTC SAR control, wherein
the DTC mixing control signal remains unchanged in each clock-cycle of the every N clock-cycles of the reference signal; and
in the at least some clock-cycles of the every N clock-cycles of the reference signal, the DTC SAR control is updated based on the sub-sampled output generated in the corresponding previous clock-cycle of the reference signal.
11. The PLL of claim 2 wherein,
N is 8;
in first 4 clock-cycles of the every 8 clock-cycles of the reference signal, the DTC total control is updated linearly with an equal increment in each clock-cycle of the first 4 clock-cycles of the reference signal, wherein the equal increment in the DTC total control leads to an equal increment in the time delay of the sampling clock offset from the reference signal in each clock-cycle of the first 4 clock-cycles of the reference signal; and
in remaining 4 clock-cycles of the every 8 clock-cycles of the reference signal, the DTC total control is updated non-linearly in each clock-cycle of the remaining 4 clock-cycles of the reference signal, wherein,
each non-linear update of the DTC total control is based on the sub-sampled output generated in the corresponding previous clock-cycle of the reference signal; and
each non-linear update of the DTC total control leads to an increment or a decrement in the time delay of the sampling clock offset from the reference signal for each clock-cycle of the remaining 4 clock-cycles of the reference signal.
12. The PLL of claim 11 wherein a duty cycle of the RF output signal is between 25% and 75%.
13. The PLL of claim 1 wherein the LO is a digitally controlled oscillator (DCC)).
14. The PLL of claim 13 further comprising a loop filter between the SAR phase estimator and the LO, wherein the loop filter is configured to provide a digital control signal to the LO based on the phase word.
15. The PLL of claim 14 wherein the loop filter is a low-pass filter.
16. The PLL of claim 14 further comprising:
a differentiator following the SAR phase estimator, wherein the differentiator is configured to differentiate the phase word into a frequency parameter; and
a proportional integral (PI) control unit between the differentiator and the loop filter, wherein,
the PI control unit is configured to receive the frequency parameter from the differentiator and provide a difference output signal to the loop filter; and
the loop filter is configured to provide the digital control signal by filtering the difference output signal.
17. The PLL of claim 16 wherein both the differentiator and the PI control unit operate at a frequency as 1/N of a frequency of the reference signal.
18. The PLL of claim 13 further comprising a loop filter between the SAR phase estimator and the LO, wherein the loop filter is configured to receive the phase word from the SAR phase estimator and to provide a digital control signal to the LO.
19. The PLL of claim 1 wherein N is greater than 4.