Fractional N frequency synthesizer
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.
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.