Frequency reference signal generating system and method for frequency synthesizers
A system for generating a frequency reference signal comprising an oscillator, a direct digital synthesizer coupled to the oscillator and configured to receive a signal output from the oscillator, a digital to analog converter coupled to the direct digital synthesizer and configured to receive a sampled signal from the direct digital synthesizer and to convert the sampled signal to an analog waveform, and a bandpass filter coupled to the digital to analog converter and configured to select an aliased output signal from the digital to analog converter at a Nyquist zone other than a first Nyquist zone and to output the frequency reference signal.
1. A system for generating a frequency reference signal comprising:
a crystal oscillator;
a numerically controlled oscillator coupled to the crystal oscillator and configured to receive a signal output from the crystal oscillator;
a digital to analog converter coupled to the numerically controlled oscillator and configured to receive a sideband signal from the numerically controlled oscillator and to generate a counting sequence according to a frequency control word stored by the numerically controlled oscillator; and
a bandpass filter coupled to the digital to analog converter and configured to select an aliased output signal from the digital to analog converter at a Nyquist zone other than a first Nyquist zone and to output the frequency reference signal.
2. The system of claim 1 , wherein the crystal oscillator is at 10 MHz.
3. The system of claim 1 , wherein the Nyquist zone selected by the bandpass filter is a second Nyquist zone.
4. The system of claim 1 , wherein the Nyquist zone selected by the bandpass filter is a third Nyquist zone.
5. The system of claim 1 , wherein the frequency reference signal is at a frequency of approximately 13 MHz.
6. A method for generating a frequency reference signal, the method comprising:
receiving a signal output from a crystal oscillator by a numerically controlled oscillator;
outputting a sideband signal from the numerically controlled oscillator;
receiving the sideband signal from the numerically controlled oscillator with a digital to analog converter;
generating a counting sequence according to a frequency control word stored by the numerically controlled oscillator with the digital to analog converter;
selecting an aliased output signal received from the digital to analog converter at a Nyquist zone other than a first Nyquist zone with a bandpass filter; and
outputting the frequency reference signal from the bandpass filter.
7. The method of claim 6 , wherein the Nyquist zone selected by the bandpass filter is a second Nyquist zone.
8. The method of claim 6 , wherein the Nyquist zone selected by the bandpass filter is a third Nyquist zone.
9. The method of claim 6 , wherein the frequency reference signal is at a frequency of approximately 13 MHz.
10. A system for generating a frequency reference signal comprising:
a first oscillator;
a second oscillator coupled to the first oscillator and configured to receive a signal output from the first oscillator;
a digital to analog converter coupled to the second oscillator and configured to receive a sideband signal from the second oscillator; and
a bandpass filter coupled to the digital to analog converter and configured to select an aliased output signal from the digital to analog converter at a Nyquist zone other than a first Nyquist zone and to output the frequency reference signal.
11. The system of claim 10 , wherein the digital to analog converter is further configured to generate a counting sequence according to a frequency control word stored by the second oscillator.
12. The system of claim 10 , wherein the first oscillator comprises a crystal oscillator.
13. The system of claim 10 , wherein the second oscillator comprise a numerically controlled oscillator.
14. The system of claim 12 , wherein the crystal oscillator is at a frequency of 10 MHz.
15. The system of claim 13 , wherein the numerically controlled oscillator is at a frequency of 3 MHz.
16. The system of claim 10 , wherein the Nyquist zone selected by the bandpass filter is a second Nyquist zone.
17. The system of claim 10 , wherein the Nyquist zone selected by the bandpass filter is a third Nyquist zone.
18. The system of claim 10 , wherein the frequency reference signal is at a frequency of approximately 13 MHz.
19. A method for generating a frequency reference signal, the method comprising:
receiving a signal output from a first oscillator by a second oscillator;
outputting a sideband signal from the second oscillator;
receiving the sideband signal from second controlled oscillator with a digital to analog converter;
selecting an aliased output signal received from the digital to analog converter at a Nyquist zone other than a first Nyquist zone with a bandpass filter; and
outputting the frequency reference signal from the bandpass filter.
20. The method of claim 19 , further comprising generating a counting sequence according to a frequency control word stored by the second oscillator using the digital to analog converter.
21. The method of claim 19 , wherein the first oscillator comprises a crystal oscillator.
22. The method of claim 19 , wherein the second oscillator comprise a numerically controlled oscillator.
23. The method of claim 21 , wherein the crystal oscillator is at a frequency of 10 MHz.
24. The method of claim 22 , wherein the numerically controlled oscillator is at a frequency of 3 MHz.
25. The method of claim 19 , wherein the Nyquist zone selected by the bandpass filter is a second Nyquist zone.
26. The method of claim 19 , wherein the Nyquist zone selected by the bandpass filter is a third Nyquist zone.
27. The system of claim 19 , wherein the frequency reference signal is at a frequency of approximately 13 MHz.