.SIGMA..DELTA. modulator for PLL circuit

Claims

What is claimed is:

1. A .SIGMA..DELTA. modulator for producing a modulation signal for modulating a frequency division ratio of a comparison frequency divider of a PLL circuit, the .SIGMA..DELTA. modulator comprising: an adder for producing random numbers for modulating the frequency division ratio of the comparison frequency divider as the modulation signal, wherein the adder produces the random numbers to decrease the modulation width of the frequency division ratio.

2. A .SIGMA..DELTA. modulator for producing a modulation signal for modulating a frequency division ratio of a comparison frequency divider of a PLL circuit, the .SIGMA..DELTA. modulator comprising: an adder for producing random numbers for modulating the frequency division ratio of the comparison frequency divider by addition processing of input signals according to a predetermined operational logic, wherein the operational logic is set to decrease the modulation width of the frequency division ratio.

3. A .SIGMA..DELTA. modulator for producing a modulation signal for modulating a frequency division ratio of a comparison frequency divider of a PLL circuit, the .SIGMA..DELTA. modulator comprising: a plurality of integrators connected in series, each of which integrates an input signal and outputs an overflow signal when its integrated value has exceeded a predetermined value; a plurality of differentiators connected selectively to the plurality of integrators, each of the differentiators transferring the overflow signal of its associated integrator; and an adder for producing the modulation signal by multiplying a predetermined coefficient by the overflow signals transferred from the plurality of differentiators and adding the multiplied values thus obtained, wherein the predetermined coefficient is set to decrease the modulation width of the frequency division ratio.

4. The .SIGMA..DELTA. modulator according to claim 3, wherein the absolute value of the predetermined coefficient is set to be less than a predetermined value.

5. The .SIGMA..DELTA. modulator according to claim 4, wherein the .SIGMA..DELTA. modulator is a third-order .SIGMA..DELTA. modulator having three integrators, and the predetermined value is 2.

6. The .SIGMA..DELTA. modulator according to claim 4, wherein the .SIGMA..DELTA. modulator is a fourth-order .SIGMA..DELTA. modulator having four integrators, and the predetermined value is 4.

7. A .SIGMA..DELTA. modulator for producing a modulation signal for modulating a frequency division ratio of a comparison frequency divider of a PLL circuit, the .SIGMA..DELTA. modulator comprising: a plurality of integrators connected in series, each of which integrates an input signal and outputs an overflow signal when its integrated value has exceeded a predetermined value; a plurality of differentiators selectively connected to the plurality of integrators, each of the differentiators transferring the overflow signal of its associated integrator; and a plurality of adders each of which produces the modulation signal by multiplying a predetermined coefficient by the overflow signal transferred from its associated differentiator and adding the multiplied values thus obtained, wherein the predetermined coefficient is set such that the modulation width of the frequency division ratio decreases.

8. The .SIGMA..DELTA. modulator according to claim 7, wherein the absolute value of the predetermined coefficient is set to be less than a predetermined value.

9. The .SIGMA..DELTA. modulator according to claim 8, wherein the .SIGMA..DELTA. modulator is a third-order .SIGMA..DELTA. modulator having three integrators and the predetermined value is 2.

10. The .SIGMA..DELTA. modulator according to claim 8, wherein the .SIGMA..DELTA. modulator is a fourth-order .SIGMA..DELTA. modulator having four integrators and the predetermined value is 4.

11. A .SIGMA..DELTA. modulator for producing a modulation signal for modulating a frequency division ratio of a comparison frequency divider of a PLL circuit, the .SIGMA..DELTA. modulator comprising: N, where N.gtoreq., integrators connected in series, each of which integrates an input signal and outputs an overflow signal when its integrated value has exceeded a predetermined value; N adders connected in series for producing a computation signal; and a plurality of differentiators each of which transfers the overflow signal from its associated integrator or the computation signal from its associated adder, wherein the N adders include: a first stage adder for adding an overflow signal from the Nth-stage integrator, an overflow signal from the differentiator associated with the Nth stage integrator, and an overflow signal from the differentiator associated with the (N-1)th stage integrator; second to (N-2)th stage adders for adding a computation signal from the previous stage adder, a computation signal transferred from the previous stage adder via a differentiator, and overflow signals transferred from differentiators associated with the (N-2)th to second stage integrators; an (N-1)th stage adder for adding a computation signal from the previous stage adder and an overflow signal transferred from the differentiator associated with the first stage integrator, and subtracting a computation signal transferred from the previous stage adder via a differentiator; and an Nth stage adder for adding a computation signal from the (N-1)th stage adder and a fixed frequency division ratio to produce the modulation signal.

12. A .SIGMA..DELTA. modulator for producing a modulation signal for modulating a frequency division ratio of a comparison frequency divider of a PLL circuit, the .SIGMA..DELTA. modulator comprising: three integrators connected in series, each of which integrates an input signal and outputs an overflow signal when its integrated value has exceeded a predetermined value; three adders connected in series for producing a computation signal; and a plurality of differentiators connected to the three integrators and two adders, each of which transfers an overflow signal from its associated integrator or a computation signal from its associated adder, wherein the three adders include: a first adder for adding an overflow signal from the first integrator, an overflow signal transferred from the differentiator associated with the first integrator, and an overflow signal transferred from the differentiator associated with a second integrator; a second adder for adding a computation signal from the first adder and an overflow signal transferred from the differentiator associated with a third integrator, and subtracting a computation signal transferred from the first adder via a differentiator; and a third adder for adding a computation signal from the second adder and a fixed frequency division ratio to produce the modulation signal.

13. A .SIGMA..DELTA. modulator for producing a modulation signal for modulating a frequency division ratio of a comparison frequency divider of a PLL circuit, the .SIGMA..DELTA. modulator comprising: four integrators connected in series, each of which integrates an input signal and outputs an overflow signal when its integrated value has exceeded a predetermined value; four adders connected in series for producing a computation signal; and a plurality of differentiators each of which transfers an overflow signal from its associated integrator or a computation signal from its associated adder; wherein the four adders include: a first adder for adding an overflow signal from the first integrator, an overflow signal transferred from the differentiator associated with a first integrator, and an overflow signal transferred from the differentiator associated with a second integrator; a second adder for adding a computation signal from the first adder, a computation signal transferred from the first adder via a differentiator, and an overflow signal transferred from the differentiator associated with a third integrator; a third adder for adding the computation signal from the second adder and an overflow signal transferred from the differentiator associated with a fourth integrator, and subtracting a computation signal transferred from the second adder via a differentiator; and a fourth adder for adding a computation signal from the third adder and a fixed frequency division ratio to produce the modulation signal.

14. A PLL circuit comprising: a comparison frequency divider for performing a fractional-N operation according to a modulation signal; a .SIGMA..DELTA. modulator connected to the comparison frequency divider, for producing the modulation signal, wherein the .SIGMA..DELTA. modulator includes an adder for producing random numbers for performing a fractional-N operation of the comparison frequency divider as the modulation signal, wherein the adder produces the random numbers to decrease the modulation width of the frequency division ratio.

15. A fractional-N PLL frequency synthesizer comprising: a comparison frequency divider for performing a fractional-N operation according to a modulation signal; a .SIGMA..DELTA. modulator connected to the comparison frequency divider, for producing the modulation signal, the .SIGMA..DELTA. modulator including an adder for producing random numbers for performing a fractional-N operation of the comparison frequency divider as the modulation signal, the adder producing the random numbers to decrease the modulation width of the frequency division ratio; a reference frequency divider for producing a reference signal; and a phase comparator connected to the comparison frequency divider and the reference frequency divider for comparing the reference signal and the comparison signal to produce a phase comparison signal.

16. A base station for a mobile communication system for performing wireless communication with a mobile station, the base station comprising: a transmitter circuit for modulating a signal to be transmitted to the mobile station using a wireless carrier wave signal to produce a modulated transmission signal; a receiver circuit for demodulating a modulated signal received from the mobile station using the wireless carrier wave signal to produce a reception signal; and a PLL circuit connected to the transmitter circuit and the receiver circuit for producing the wireless carrier wave signal, the PLL circuit including a fractional-N PLL frequency synthesizer, the frequency synthesizer including: a comparison frequency divider for receiving the wireless carrier wave signal and performing a fractional-N operation according to a modulation signal to produce a comparison signal; a .SIGMA..DELTA. modulator connected to the comparison frequency divider, for producing the modulation signal, the .SIGMA..DELTA. modulator including an adder for producing random numbers for performing a fractional-N operation of the comparison frequency divider as the modulation signal, the adder producing the random numbers to decrease the modulation width of the frequency division ratio; a reference frequency divider for producing a reference signal; and a phase comparator connected to the comparison frequency divider and the reference frequency divider for comparing the reference signal and the comparison signal to produce a phase comparison signal.

17. A mobile station for a mobile communication system for performing wireless communication with a base station, the mobile station comprising: a transmitter circuit for modulating a signal to be transmitted to the base station using a wireless carrier wave signal to produce a modulated transmission signal; a receiver circuit for demodulating the modulated signal received from the base station to produce a reception signal; and a PLL circuit connected to the transmitter circuit and the receiver circuit for producing the wireless carrier wave signal, the PLL circuit including a fractional-N PLL frequency synthesizer, the frequency synthesizer including: a comparison frequency divider for receiving the wireless carrier wave signal and performing a fractional-N operation according to a modulation signal to produce a comparison signal; a .SIGMA..DELTA. modulator connected to the comparison frequency divider, for producing the modulation signal, the .SIGMA..DELTA. modulator including an adder for producing random numbers for performing a fractional-N operation of the comparison frequency divider as the modulation signal, the adder producing the random numbers to decrease the modulation width of the frequency division ratio; a reference frequency divider for producing a reference signal; and a phase comparator connected to the comparison frequency divider and the reference frequency divider for comparing the reference signal and the comparison signal to produce a phase comparison signal.

18. A mobile communication system comprising: a base station; and a mobile station capable of performing wireless communication with the base station, at least one of the base station and mobile station including a fractional-N PLL frequency synthesizer, the frequency synthesizer including; a comparison frequency divider for performing a fractional-N operation according to a modulation signal to produce a comparison signal; a .SIGMA..DELTA. modulator connected to the comparison frequency divider, for producing the modulation signal, the .SIGMA..DELTA. modulator including an adder for producing random numbers for performing a fractional-N operation of the comparison frequency divider as the modulation signal, the adder producing the random numbers to decrease the modulation width of the frequency division ratio; a reference frequency divider for producing a reference signal; and a phase comparator connected to the comparison frequency divider and the reference frequency divider for comparing the reference signal and the comparison signal to produce a phase comparison signal.