Apparatus and methods for reducing noise in oscillating signals
Methods and apparatus are described for reducing noise, such as phase noise, in an oscillating signal. The oscillating signal may be generated by a signal generator having a mechanical resonator, such as a crystal oscillator. A filter may be coupled to the output of the mechanical resonator and may have its center frequency adjusted using a phase-locked loop (PLL). A feedback signal from the filter to the signal generator may also be used.
1. A device, comprising:
a signal generator comprising a mechanical resonator and configured to produce an oscillating signal;
a filter comprising a series-resonant crystal, the filter having an input configured to receive the oscillating signal and an output configured to produce a filtered signal;
a phase locked loop (PLL) having a first input configured to receive the oscillating signal and a second input configured to receive the filtered signal, and having an output configured to provide a control signal to the filter to tune the filter; and
a feedback path from the output of the filter to an input of the signal generator.
2. The device of claim 1 , wherein the PLL is a digital PLL.
3. The device of claim 2 , wherein the PLL does not include a frequency mixer.
4. The device of claim 1 , wherein the signal generator further comprises oscillator circuitry coupled to the mechanical resonator to drive the mechanical resonator.
5. The device of claim 1 , wherein the PLL is configured to generate the control signal to tune the filter to maintain a phase difference of approximately 180 degrees between the oscillating signal and the filtered signal.
6. A method, comprising:
generating an oscillating signal using a mechanical resonator;
filtering the oscillating signal with a filter to produce a filtered signal that is approximately zero degrees or approximately 180 degrees out-of-phase with the oscillating signal;
comparing a phase of the oscillating signal to a phase of the filtered signal using a phase-locked loop (PLL);
generating a tuning control signal with the PLL to adjust a filtering frequency of the filter; and
providing the filtered signal to the mechanical resonator as a feedback signal.
7. The method of claim 6 , wherein generating a tuning control signal comprises generating a tuning control signal to maintain a phase difference of approximately 180 degrees between the oscillating signal and the filtered signal.
8. The method of claim 6 , wherein the filter is a bandpass filter, and wherein generating a tuning control signal with the PLL to adjust a filtering frequency of the filter comprises generating a tuning control signal to adjust a center frequency of the filter.
9. The method of claim 6 , wherein the PLL is a digital PLL.