All-digital phase locked loop (ADPLL) system

1. An all-digital phase locked loop system for generating an oscillator output signal under control of a digital reference input, comprising:

a digitally controlled oscillator;

a digital loop filter for generating a multiple bit digital control signal for the digitally controlled oscillator;

a sigma-delta modulator for generating an additional 1-bit digital control signal for the digitally controlled oscillator;

a digital divider for dividing the oscillator output signal and providing a digital divided signal; and

a digital adder for subtracting the digital divided signal from the digital reference input, and providing a digital output, the most significant bits of which are applied to an input of the digital loop filter and the least significant bits of which are applied to an input of the sigma-delta modulator.

2. The system of claim 1 , wherein the sigma-delta modulator comprises a two-stage MASH configuration.

3. The system of claim 2 , wherein each stage of the sigma-delta modulator includes a quantizer with an input to which a dithering value is added.

4. The system of claim 3 , wherein each stage of the sigma-delta modulator comprises two parallel channels each with a subtractor, a channel register and an adder.

5. The system of claim 4 , further comprising a multiplexer, a stage register, an adder and a quantizer.

6. The system of claim 5 , wherein the subtractor of the first channel has a first n-bit wide input and a second n-bit wide input; the subtractor of the second channel has a first n-bit wide input and a second n-bit wide input; the first input of the subtractor in the first channel receives an n-bit wide digital input signal; the first input of the subtractor in the second channel receives the same n-bit wide digital input signal as the subtractor in the first channel; the second input of the subtractor in the first channel receives a digital signal equivalent to +2 n ; the second input of the subtractor in the second channel receives a digital signal equivalent to −2 n ; the output of the subtractor in the first channel is connected to an input of the channel register in the same channel; the output of the subtractor in the second channel is connected to an input of the channel register in the same channel; the channel register in the first channel has an (n+2)-bit wide output connected to a first input of the adder of the first channel; the channel register in the second channel has a (n+2)-bit wide output connected to a first input of the adder of the second channel; the adder of the first channel has an output connected to a first input of the multiplexer; the adder of the second channel has an output connected to a second input of the multiplexer; the multiplexer has an output connected to an input of the stage register; the quantizer receiving an input from the stage register and providing a digital 1-bit output applied to a control input of the multiplexer.

7. The all-digital phase locked loop system according to claim 6 , wherein each stage has an adder with a first input connected to the output of the stage register, a second input to which a dithering value is applied and an output connected to the input of the quantizer.

8. The all-digital phase locked loop system according to claim 2 , wherein each stage of the sigma-delta modulator comprises two parallel channels each with a subtractor, a channel register and an adder; and further comprises a multiplexer, a stage register, an adder and a quantizer; and wherein the subtractor of the first channel has a first n-bit wide input and a second n-bit wide input, the subtractor of the second channel has a first n-bit wide input and a second n-bit wide input, the first input of the subtractor in the first channel receives an n-bit wide digital input signal, the first input of the subtractor in the second channel receives the same n-bit wide digital input signal as the subtractor in the first channel, the second input of the subtractor in the first channel receives a digital signal equivalent to +2 n , the second input of the subtractor in the second channel receives a digital signal equivalent to −2 n , the output of the subtractor in the first channel is connected to an input of the channel register in the same channel, the output of the subtractor in the second channel is connected to an input of the channel register in the same channel, the channel register in the first channel has a (n+2)-bit wide output connected to a first input of the adder of the first channel, the channel register in the second channel has a (n+2)-bit wide output connected to a first input of the adder of the second channel, the adder of the first channel has an output connected to a first input of the multiplexer, the adder of the second channel has an output connected to a second input of the multiplexer, the multiplexer has an output connected to an input of the stage register, the quantizer receiving an input from the stage register and providing a digital 1-bit output applied to a control input of the multiplexer.

9. The system of claim 1 , wherein the sigma-delta modulator comprises multiple stages; and wherein each stage of the sigma-delta modulator includes a quantizer with an input to which a dithering value is added.