Direct digital synthesizer for reference frequency generation
A direct digital frequency synthesizer having a multi-modulus divider, a numerically controlled oscillator and a programmable delay generator. The multi-modulus divider receives an input clock having an input pulse frequency f osc and outputs some integer fraction of those pulses at an instantaneous frequency f Vp that is some integer fraction (1/P) of the input frequency. The multi-modulus divider selects between at least two ratios of P (1/P or 1/P+1) in response to a signal from the numerically controlled oscillator. The numerically controlled oscillator receives an accumulator increment (i.e., the number of divided pulse edges) required before an overflow occurs that causes the multi-modulus divider to change divider ratios in response to an overflow. The numerically controlled oscillator also outputs both the overflow signal and a delay signal to the delay generator. The delay signal contains phase-dithering noise that is induced by input from a pseudo-random noise generator.
1. A frequency synthesizer comprising:
an input for receiving a clock reference signal at a first input frequency;
a multi-modulus divider for dividing the input frequency to provide an intermediate signal with an adjusted frequency;
a numerically controlled oscillator comprising an accumulator receiving an accumulator increment value for providing a phase value and a latch circuit that receives and is clocked by the intermediate signal with the adjusted frequency wherein the numerically controlled oscillator outputs a delay value and an overflow signal each comprised of one or more bits wherein said overflow signal is provided to said multi-modulus divider to select between at least a first divider ratio and a second divider ratio;
a pseudo-random noise generator that generates a pseudo-random sequence for input into the accumulator wherein the pseudo-random noise generator is a zero-mean pseudo-random noise generator or a non-zero-mean pseudo-random noise generator; and
a programmable delay generator comprising a delay control circuit that receives at least said overflow value and said delay value and calculates a delay time for application to each pulse in the intermediate signal with the adjusted frequency to provide an output signal with a final adjusted frequency (f out ).
2. The frequency synthesizer of claim 1 wherein said multi-modulus divider selects between a first divider ratio, a second divider ratio and a third divider ratio.
3. The frequency synthesizer of claim 1 wherein the accumulator increment is in the range of 0 to 2 N A −1 where N A is the number of bits in an accumulator value A output from the accumulator.
4. The frequency synthesizer of claim 3 wherein the numerically controlled oscillator latch circuit comprises at least N A switching elements to latch the accumulator value.
5. The frequency synthesizer of claim 4 wherein the value of A is latched at an average frequency of f OUT or at the adjusted frequency.
6. The frequency synthesizer of claim 4 wherein the latch circuit comprises at least one additional switching element for latching at least one overflow bit from the accumulator.
7. The frequency synthesizer of claim 3 wherein the delay value is at least some portion of the accumulator value.
8. The frequency synthesizer of claim 7 wherein the delay value has a number of bits N D and wherein N A ≧N D .
9. The frequency synthesizer of claim 8 further comprising an arithmetic circuit that provides an accumulator increment K to the numerically controlled oscillator.
10. The frequency synthesizer of claim 9 wherein the accumulator increment is the sum of the output from the non-zero-mean pseudo-random number generator and the most significant bits of a tuning increment.
11. The frequency synthesizer of claim 1 wherein the pseudo-random noise generator is a non-zero-mean pseudo-random noise generator and wherein the non-zero-mean pseudo-random noise generator induces the numerically controlled oscillator to output a phase-dithered delay value.
12. The frequency synthesizer of claim 11 wherein the non-zero-mean pseudo-random noise generator further comprises a digital ΔΣ modulator.
13. The frequency synthesizer of claim 12 , wherein the non-zero-mean pseudo-random noise generator outputs a binary pseudo-random sequence.
14. The frequency synthesizer of claim 13 further comprising a dither control circuit that enables and disables the non-zero-mean pseudo-random noise generator.
15. The frequency synthesizer of claim 14 wherein the dither control circuit controls a multiplexer to select between a tuning word dithered by the non-zero-mean pseudo-random noise generator and a tuning word not dithered by the non-zero-mean pseudo-random noise generator.
16. The frequency synthesizer of claim 1 wherein the non-zero-mean pseudo-random noise generator is integrated with the numerically controlled oscillator.
17. The frequency synthesizer of claim 1 wherein a spectral shape of the pseudo-random noise output by the pseudo-random noise generator is selected to reduce the magnitude of spurious tones of the output signal at specific frequencies.
18. A frequency synthesizer comprising:
an input means for receiving a clock reference signal at a first input frequency;
a multi-modulus divider means for dividing the input frequency to provide an intermediate signal with an adjusted frequency;
a numerically controlled oscillator means comprising an accumulator receiving an accumulator increment value for providing a phase value and a latch circuit that receives and is clocked by the intermediate signal with the adjusted frequency wherein the numerically controlled oscillator outputs a delay value and an overflow signal comprised of one or more bits wherein said overflow signal is provided to said multi-modulus divider to select between at least a first divider ratio and a second divider ratio;
a pseudo-random noise generator that generates a pseudo-random sequence for input into the accumulator wherein the pseudo-random noise generator is a zero-mean pseudo-random noise generator or a non-zero-mean pseudo-random noise generator; and
a programmable delay generator means comprising a delay control which receives at least said overflow value and said delay value and calculates a delay time for application to each pulse in the intermediate signal to provide an output signal with a final adjusted frequency (f out ).
19. The frequency synthesizer of claim 18 wherein said multi-modulus divider selects between a first divider ratio, a second divider ratio and a third divider ratio.
20. The frequency synthesizer of claim 18 wherein the pseudo-random noise generator is a non-zero-mean pseudo-random noise generator and wherein the frequency synthesizer further comprises a dither control circuit that enables and disables the non-zero-mean pseudo-random noise generator.
21. The frequency synthesizer of claim of claim 20 wherein the non-zero-mean pseudo-random noise generator outputs a binary non-zero-mean binary sequence.