Frequency synthesizer

1. A circuit for receiving an input signal and generating an output signal, the input signal having a first frequency, the output signal having a second frequency, said circuit comprising:

a forward branch for generating the output signal; and

a return branch for feeding back the output signal,

wherein the return branch comprises:

a frequency divider for receiving the output signal, for dividing the frequency of the output signal by a factor, and for outputting a modified output signal;

wherein the forward branch comprises:

a detector for comparing the input signal and the modified output signal and outputting a comparison signal indicative of said comparison;

a word-length reduction circuit for reducing the number of bits of said comparison signal, thereby generating a reduced-length comparison signal;

a digital-to-analog converter for converting said reduced-length comparison signal to analog, thereby generating an analog signal; and

an oscillator, controlled by said analog signal.

2. The circuit as claimed in claim 1 , wherein the word-length reduction circuit comprises:

a truncation block for truncating the comparison signal.

3. The circuit as claimed in claim 2 , wherein the word-length reduction circuit further comprises:

a dither block for adding dither to the comparison signal prior to truncation.

4. The circuit as claimed in claim 1 , wherein the word-length reduction circuit comprises:

a sigma-delta modulator.

5. The circuit as claimed in claim 1 , wherein the word-length reduction circuit comprises:

an input, for receiving the comparison signal;

a first word length reduction block, for receiving a modified input signal having a respective first number of bits, for generating an output signal having a respective second number of bits, the respective second number of bits being smaller than the respective first number of bits, and for generating an error signal; and

a second word length reduction block, for receiving the error signal of the first word length reduction block as a respective input signal having a respective first number of bits, and for generating an output signal having a respective second number of bits, the respective second number of bits being smaller than the respective first number of bits, the second word length reduction block comprising an unstable feedback loop such that the output signal is randomized; and

a first adder, connected to receive the output signal of the second word length reduction block and connected to receive the input signal, in order to generate the modified input signal.

6. The circuit as claimed in claim 1 , further comprising an integrator for receiving the comparison signal and outputting an integrated comparison signal to the word length reduction circuit.

7. The circuit as claimed in claim 6 , wherein the integrator is a loop filter.

8. The circuit as claimed in claim 1 , further comprising: a low pass filter for receiving the analog signal from the digital-to-analog converter and outputting a filtered analog signal to the oscillator.

9. The circuit as claimed in claim 8 , wherein the low-pass filter comprises a capacitor connected at one terminal between the digital-to-analog converter and the oscillator, and at the other terminal to ground.

10. The circuit as claimed in claim 1 , wherein said detector is adapted to detect and compare the respective phases of the input signal and the modified output signal.

11. The circuit as claimed in claim 1 , wherein said detector is adapted to detect and compare the respective frequencies of the input signal and the modified output signal.

12. An integrated circuit, comprising the circuit as claimed in claim 1 .

13. An audio system, comprising the integrated circuit as claimed in claim 12 .

14. The audio system as claimed in claim 13 , wherein the audio system is a portable device.

15. The audio system as claimed in claim 13 , wherein the audio system is a mains-powered device.

16. The audio system as claimed in claim 13 , wherein the audio system is an in-car, in-train, or in-plane entertainment system.

17. A video system, comprising the integrated circuit as claimed in claim 12 .

18. The video system as claimed in claim 17 , wherein the video system is a portable device.

19. The video system as claimed in claim 17 , wherein the video system is a mains-powered device.

20. The video system as claimed in claim 17 , wherein the video system is an in-car, in-train, or in-plane entertainment system.

21. A method for receiving an input signal having a first frequency and generating an output signal having a second frequency, said method comprising:

receiving the input signal having the first frequency;

comparing the input signal and a modified output signal, and outputting a comparison signal indicative of said comparison;

reducing the number of bits of said comparison signal, thereby generating a reduced-length comparison signal;

converting said reduced-length comparison signal to an analog signal;

controlling a voltage-controlled oscillator on the basis of said analog signal, thereby generating the output signal having the second frequency; and

dividing the second frequency by a factor, thereby generating said modified output signal.

22. The method as claimed in claim 21 , wherein said reducing step comprises the substep of truncating the comparison signal.

23. The method as claimed in claim 22 , wherein said reducing step further comprises the substep of adding dither to the comparison signal prior to truncation.

24. The method as claimed in claim 21 , wherein said reducing step comprises the substep of using a sigma-delta modulator.

25. The method as claimed in claim 21 , further comprising:

integrating said comparison signal.

26. The method as claimed in claim 21 , further comprising:

low-pass filtering the analog signal.