Apparatus comprising a sigma-delta modulator and method of generating a quantized signal-delta modulator
A method of generating a quantized signal in a Sigma-Delta modulator ( 25 ) comprises the steps of feeding a modulator input signal to a quantizer ( 15 ) via at least one integrator ( 12, 13 ); generating in the quantizer ( 15 ) a quantized signal; feeding back the quantized signal to be subtracted from the modulator input signal; and generating a dither signal to be applied to a point in the Sigma-Delta modulator. The dither signal is applied to a selected one of a number of different points ( 11, 14 ) in the Sigma-Delta modulator ( 25 ) in dependence of a control signal. In this way a method of generating a quantized signal in a Sigma-Delta modulator is provided which provides optimal results for different modes of the application, such as phase modulation mode and frequency modulation mode in a Bluetooth receiver.
1. A method of generating a quantized signal in a Sigma-Delta modulator arranged to operate together with a fractional divider in a frequency synthesizer for use in a receiver for modulated signals, the method comprising the steps of: feeding a modulator input signal to a quantizer via at least one integrator; generating in said quantizer a quantized signal; feeding back said quantized signal to be subtracted from said modulator input signal; generating a dither signal to be applied to a point in the Sigma-Delta modulator; and applying said dither signal to a selected one of a number of different points in the Sigma-Delta modulator in dependence of a control signal indicating a modulation format said modulated signals.
2. The method according to claim 1 , wherein one of said number of different points in the Sigma-Delta modulator is an input to said quantizer.
3. The method according to claim 1 , wherein one of said number of different points in the Sigma-Delta modulator is a terminal for said modulator input signal.
4. A method according to claim 3 , wherein the dither signal is applied to a first point in the Sigma-Delta modulator when said modulated signals are modulated according to a first modulation format and applied to a second point in the Sigma-Delta modulator when said modulated signals are modulated according to a second modulation format.
5. The method according to claim 2 , wherein the dither signal is applied to a first point in the Sigma-Delta modulator when said modulated signals are modulated according to a first modulation format and applied to a second point in the Sigma-Delta modulator when said modulated signals are modulated according to a second modulation format.
6. The method according to claim 1 , wherein the dither signal is applied to a first point in the Sigma-Delta modulator when said modulated signals are modulated according to a first modulation format and applied to a second point in the Sigma-Delta modulator when said modulated signals are modulated according to a second modulation format.
7. The method according to claim 1 wherein the frequency synthesizer is used in a receiver for Bluetooth signals.
8. The method according to claim 7 , wherein the dither signal is applied to a first point in the Sigma-Delta modulator when said Bluetooth signals are frequency modulated and applied to a second point in the Sigma-Delta modulator when said Bluetooth signals are phase modulated.
9. The method according to claim 1 wherein the method further comprises the step of applying an additional dither signal to at least one other point in the Sigma-Delta modulator.
10. An apparatus comprising a receiver for modulated signals, said receiver comprising a frequency synthesizer in which a Sigma-Delta modulator is arranged to operate together with a fractional divider, said Sigma-Delta modulator comprising: at least one integrator having an input connected to a modulator input signal; a quantizer arranged to generate a quantized signal and having an input connected to an output of said integrator; means for feeding back said quantized signal to be subtracted from said modulator input signal; a dither signal generator arranged to generate a dither signal to be applied to a point in the Sigma-Delta modulator; and means for applying said dither signal to a selected one of a number of different points in the Sigma-Delta modulator in dependence of a control signal indicative of a modulation format for said modulated signals.
11. The apparatus according to claim 10 , wherein one of said number of different points in the Sigma-Delta modulator is an input to said quantizer.
12. The apparatus according to claim 10 , wherein one of said number of different points in the Sigma-Delta modulator is a terminal for said modulator input signal.
13. The apparatus according to claim 10 , wherein the means for applying the dither signal is arranged to apply the dither signal to a first point in the Sigma-Delta modulator when said modulated signals are modulated according to a first modulation format and to apply the dither signal to a second point in the Sigma-Delta modulator when said modulated signals are modulated according to a second modulation format.
14. The apparatus according to claim 10 wherein the frequency synthesizer is adapted for use in a receiver for Bluetooth signals.
15. The apparatus according to claim 14 , wherein the means for applying the dither signal is arranged to apply the dither signal to a first point in the Sigma-Delta modulator when said Bluetooth signals are frequency modulated and to apply the dither signal to a second point in the Sigma-Delta modulator when the Bluetooth signals are phase modulated.
16. The apparatus according to claim 10 , wherein the apparatus is further arranged to apply an additional dither signal to at least one other point in the Sigma-Delta modulator.