Fractional N frequency synthesizer

• US 20030062959A1
• Filed: 10/01/2002
• Published: 04/03/2003
• Est. Priority Date: 10/02/2001
• Status: Active Grant

• Current Assignee: Renesas Electronics Corporation
• Original Assignee: NEC Compound Semiconductor Devices, Ltd. (NEC Corporation)
• Inventors: Takahashi, Yutaka, Tsuda, Kazutoshi
• Granted Patent: US 6,873,213 B2
• Time in Patent Office: Days
• US Class Current: 331/17
• CPC Class Codes: H03L 7/1976 using a phase accumulator for controlling the counter or frequency divider

Abstract

A frequency synthesizer (100) that may have reduced spurious noise caused by a voltage controlled oscillator (VCO) (4) output sneaking into an input side of a phase comparison circuit (1) has been disclosed. A beat frequency component that may be generated by mixing of a portion of a VCO output sneaking into an input side of a phase comparison circuit (1) through a reference signal (REF) or a comparison signal (SIG) may be shifted to a high frequency region by providing a modulator circuit (7) on a reference signal side or a comparison signal side. Thus, a low pass filter circuit (3) may provide attenuation to the spurious noise. In this way, spurious noise in the VCO output may be reduced.

Claims

1. A fractional N frequency synthesizer controlling an oscillation frequency of a voltage controlled oscillator output of a voltage controlled oscillator essentially based on a phase difference between a comparison signal and a modulated reference signal, comprising:

a first frequency dividing circuit coupled to receive the voltage controlled oscillator output and providing the comparison signal by dividing the voltage controlled oscillator output by a first frequency dividing number;

a frequency dividing number control circuit for controlling the first frequency dividing number of the first frequency dividing circuit to a non-integer value to provide the comparison signal;

a modulating circuit coupled to receive a reference signal and providing the modulated reference signal having a first predetermined modulation width every T periods where T is an integer greater than or equal to 2;

a phase comparison circuit coupled to receive the modulated reference signal and the comparison signal and providing a phase difference signal based on a phase difference between the modulated reference signal and the comparison signal; and

a low pass filter coupled to receive the phase error signal and providing a low frequency component of the phase difference signal to the voltage controlled oscillator.

2. The fractional N frequency synthesizer according to claim 1, wherein the modulated reference signal is modulated by the first predetermined modulation width with respect to the reference signal every T periods where T is an integer greater than or equal to 2 and the modulating circuit includes:

a delay circuit coupled to receive the reference signal and providing a delayed reference signal essentially delayed by the first predetermined modulation width;

a selector control circuit providing a selector control signal; and

a selector circuit coupled to receive the selector control signal and selecting the reference signal or the delayed reference signal to provide the modulated reference signal.

3. The fractional N frequency synthesizer according to claim 2, wherein:

the selector control circuit includes a counter coupled to receive the delayed reference signal and providing the selector control signal by essentially dividing the frequency of the delayed reference signal by 2.

4. The fractional N frequency synthesizer according to claim 1, wherein the modulating circuit includes:

a second frequency dividing circuit coupled to receive the reference signal and providing the modulated reference signal by providing a frequency division of the reference signal in accordance with a second frequency dividing number; and

a modulation control circuit providing a different value for the second frequency dividing number once per T periods.

5. The fractional N frequency synthesizer according to claim 4, wherein the modulation control circuit includes:

a counter coupled to receive the modulated reference signal and providing a selector control signal by essentially dividing the frequency of the modulated reference signal by 2; and

a selector for selecting between a first number and a second number based on the selector control signal to provide the second frequency dividing number.

6. The fractional N frequency synthesizer according to claim 5, wherein the modulation control circuit includes:

an adder adding a selector output and a base frequency dividing number to provide the second frequency dividing number; and

the selector providing the selector output.

7. The fractional N frequency synthesizer according to claim 1, wherein:

the modulating circuit selectively providing the modulated reference signal as the reference signal or the reference signal modulated by essentially the first predetermined modulation width every T periods.

8. The fractional N frequency synthesizer according to claim 1, wherein:

the modulating circuit selectively providing the modulated reference signal as the reference signal modulated by a second predetermined modulation width or the reference signal modulated by essentially the first predetermined modulation width every T periods.

9. The fractional N frequency synthesizer according to claim 1, wherein:

the modulating circuit providing the modulated reference signal having a different predetermined modulation width every T periods where T is an integer greater than or equal to 2.

10. The fractional N frequency synthesizer according to claim 9, wherein the modulating circuit is variable modulating circuit including:

a delay circuit coupled to receive the reference signal and providing the different predetermined modulation width in accordance with the non-integer value.

11. The fractional N frequency synthesizer according to claim 1, wherein:

the frequency dividing number control circuit controlling the first frequency dividing number on a time basis.

12. The fractional N frequency synthesizer according to claim 1, wherein:

the reference signal is a periodic clock signal; and

the phase comparison signal includes a pulse width indicating a phase difference magnitude and a logic level indicating a phase direction.

13. A fractional N frequency synthesizer controlling an oscillation frequency of a voltage controlled oscillator output of a voltage controlled oscillator essentially based on a phase difference between a modulated comparison signal and a reference signal, comprising:

a first frequency dividing circuit coupled to receive the voltage controlled oscillator output and providing a comparison signal by dividing the voltage controlled oscillator output by a first frequency dividing number;

a frequency dividing number control circuit for controlling the first frequency dividing number of the first frequency dividing circuit to a non-integer value to provide the comparison signal;

a modulating circuit coupled to receive the comparison signal and providing the modulated comparison signal having a first predetermined modulation width every T periods where T is an integer greater than or equal to 2;

a phase comparison circuit coupled to receive the reference signal and the modulated comparison signal and providing a phase difference signal based on a phase difference between the reference signal and the modulated comparison signal; and

a low pass filter coupled to receive the phase error signal and providing a low frequency component of the phase difference signal to the voltage controlled oscillator.

14. The fractional N frequency synthesizer according to claim 13, wherein the modulated comparison signal is modulated by the first predetermined modulation width with respect to the comparison signal every T periods where T is an integer greater than or equal to 2 and the modulating circuit includes:

a delay circuit coupled to receive the comparison signal and providing a delayed comparison signal essentially delayed by the first predetermined modulation width;

a selector control circuit providing a selector control signal; and

a selector circuit coupled to receive the selector control signal and selecting the comparison signal or the delayed comparison signal to provide the modulated comparison signal.

15. The fractional N frequency synthesizer according to claim 14, wherein:

the selector control circuit includes a counter coupled to receive the delayed comparison signal and providing the selector control signal by essentially dividing the frequency of the delayed comparison signal by 2.

16. The fractional N frequency synthesizer according to claim 13, wherein:

the modulating circuit selectively providing the modulated comparison signal as the comparison signal or the comparison signal modulated by essentially the first predetermined modulation width every T periods.

17. A fractional N frequency synthesizer controlling an oscillation frequency of a voltage controlled oscillator output of a voltage controlled oscillator essentially based on a phase difference between a modulated comparison signal and a reference signal, comprising:

a first frequency dividing circuit coupled to receive the voltage controlled oscillator output and providing the modulated comparison signal by dividing the voltage controlled oscillator output by a first frequency dividing number that is a non-integer;

a modulation control circuit for changing the first frequency dividing number of the first frequency dividing circuit every T periods where T is an integer greater than or equal to 2;

a phase comparison circuit coupled to receive the reference signal and the modulated comparison signal and providing a phase difference signal based on a phase difference between the reference signal and the modulated comparison signal; and

a low pass filter coupled to receive the phase error signal and providing a low frequency component of the phase difference signal to the voltage controlled oscillator.

18. The fractional N frequency synthesizer according to claim 17, further including:

a frequency dividing number control circuit providing a base frequency dividing number to the modulation control circuit.

19. The fractional N frequency synthesizer according to claim 18, wherein the modulation control circuit includes:

a selector control circuit providing a selector control signal;

a selector circuit coupled to receive the selector control signal and selecting a first number or a second number to provide a selector output; and

an adder adding the selector output and the base frequency dividing number to provide the first frequency dividing number.

20. The fractional N frequency synthesizer according to claim 19, wherein:

the selector control circuit includes a counter coupled to receive the modulated comparison signal and providing the selector control signal by essentially dividing the frequency of the modulated comparison signal by 2.