PLL with beat-frequency operation
A PLL has a controlled oscillator with a limited frequency range. It has a phase accumulator and a phase predictor whose ranges are limited to a value K related to their bit width. K is less than the ratio of the maximum output frequency and the minimum reference frequency. The PLL locks the output frequency to a value higher than the FCW times the reference frequency. The PLL includes a means for setting the output frequency to a target frequency before achieving final lock. The PLL may have a lock detector. After acquiring lock, the PLL may reduce the bit width and K value, for example by cutting power to or switching off some of the bits, or by switching off slow counters in a multi-counter system.
1. A first beat-frequency PLL, comprising:
a controlled oscillator with an output clock frequency whose range is limited;
a phase accumulator comprising a modulo-K counter with a first K value related to a first bit width; and
a phase predictor with the first K value, the first bit width, and a first FCW;
wherein a maximum K value is less than a ratio of a maximum output clock frequency divided by a minimum reference clock frequency, and wherein the first beat-frequency PLL is configured to lock the output clock frequency to a value higher than the first FCW times a frequency of a reference clock signal.
2. The first beat-frequency PLL of claim 1 , wherein the output clock frequency range is fully included in a single frequency lock range (FLR).
3. The first beat-frequency PLL of claim 1 , further comprising a means for setting the output clock frequency in a target FLR prior to the first beat-frequency PLL achieving final lock.
4. The first beat-frequency PLL of claim 3 , wherein the means for setting the output clock frequency includes a frequency-lock loop, the frequency-lock loop comprising counters to measure the output clock frequency and a frequency of the reference clock signal, and a feedback loop to set the output clock frequency within a fraction of a size of the FLR to a selected integer times the first K value, plus the first FCW, times the frequency of the reference clock signal, wherein the selected integer is larger than one.
5. The first beat-frequency PLL of claim 3 , wherein the means for setting the output clock frequency includes a frequency-lock loop, the frequency-lock loop comprising counters to measure the output clock frequency and a frequency of the reference clock signal, and wherein the first beat-frequency PLL performs a search for an oscillator control code that yields a required multiplication factor between the output clock frequency and the reference clock signal frequency.
6. The first beat-frequency PLL of claim 5 , wherein the means for setting the output clock frequency includes a search controller performing one of a binary search, a linear search, a jump search, an interpolation search, an exponential search, a Fibonacci search, and a Newton-Raphson search.
7. The first beat-frequency PLL of claim 3 , wherein the means for setting the output clock frequency includes a loop filter with programmable filter parameters.
8. A second beat-frequency PLL, comprising:
a controlled oscillator with an output clock frequency;
a modulo-K counter with a first K value related to a first bit width and a second K value related to a second bit width, wherein a modulo-K counter input is coupled with a controlled oscillator output;
a phase predictor with the first K value, the first bit width, a first frequency control word (first FCW), the second K value, the second bit width, and a second FCW; and
a lock detector configured to provide a phase lock indicator signal;
wherein, upon the phase lock indicator signal indicating that the second beat-frequency PLL is in lock, at a start of a modulo-K counter modulo-K cycle, the modulo-K counter switches from the first K value and the first bit width to the second K value and the second bit width, and the phase predictor simultaneously switches from the first K value, the first bit width and the first FCW to the second K value, the second bit width, and the second FCW.
9. The second beat-frequency PLL of claim 8 , wherein, upon the phase lock indicator signal indicating that the second beat-frequency PLL is not in lock, at the start of a modulo-K counter modulo-K cycle, the modulo-K counter switches from the second K value and the second bit width to the first K value and the first bit width, and the phase predictor simultaneously switches from the second K value, the second bit width, and the second FCW to the first K value, the first bit width and the first FCW.
10. The second beat-frequency PLL of claim 8 , wherein the first K value and the second K value are programmable.
11. The second beat-frequency PLL of claim 10 , wherein bit widths of a register and a phase comparator change simultaneously with the modulo-K counter and the phase predictor.
12. The second beat-frequency PLL of claim 10 , wherein a K-value effectively changes by one of cutting and restoring power to part of the most significant modulo-K counter bits.
13. The second beat-frequency PLL of claim 10 , wherein a K-value effectively changes by one of interrupting and restoring an input signal to part of the most significant modulo-K counter bits.
14. The second beat-frequency PLL of claim 10 , wherein the modulo-K counter comprises a fast counter and a slow counter, and a K-value changes by switching off the slow counter.
15. The second beat-frequency PLL of claim 8 , wherein the lock detector receives a phase difference and determines if successive values of the phase difference are within a narrow band around zero to indicate that the second beat-frequency PLL is in phase lock, and upon determining that the second beat-frequency PLL is in phase lock, the lock detector asserts the phase lock indicator signal.
16. The second beat-frequency PLL of claim 15 , wherein the phase lock indicator signal includes a measure of phase lock tightness and wherein the second K value, the second bit width, and the second FCW depend on the measure of phase lock tightness.
17. A method for operating a beat-frequency PLL, comprising the following steps:
determining multiple frequency lock ranges (multiple FLRs) covering a controlled oscillator frequency range;
based on a required frequency, determining a target FLR, wherein the target FLR is one of the multiple FLRs and the target FLR includes the required frequency;
operating the controlled oscillator at a frequency in the target FLR;
determining a final K-value related to the multiple FLRs and a final FCW to generate the required frequency in the target FLR; and
simultaneously setting the final K-value and the final FCW at a start of a modulo-K counter modulo-K cycle and operating the beat-frequency PLL to generate the required frequency in the target FLR.
18. The method of claim 17 , wherein operating the controlled oscillator at a frequency in the target FLR comprises presetting the controlled oscillator at one of a highest frequency, a middle frequency, and a lowest frequency.
19. The method of claim 17 , wherein operating the controlled oscillator at a frequency in the target FLR comprises:
determining a single FLR covering the controlled oscillator frequency range;
determining an initial K-value related to the single FLR and an initial FCW to generate the required frequency in the single FLR; and
setting the initial K-value and the initial FCW and operating the beat-frequency PLL to generate the required frequency in the single FLR.
20. The method of claim 17 , wherein operating the controlled oscillator at a frequency in the target FLR comprises comparing a ratio of an output clock frequency and a reference frequency and using a search algorithm to determine an oscillator control code that yields the frequency in the target FLR, and wherein the search algorithm includes one of a binary search, a linear search, a jump search, an interpolation search, an exponential search, a Fibonacci search, and a Newton-Raphson search.
21. The method of claim 17 , wherein determining the final K-value related to the multiple FLRs and the final FCW comprises using the formula f DCO /f REF =FCW+(li−1)*K, with liϵ{1, 2, . . . }.