PLL-Tuned Crystal Filter for Oscillator Phase Noise Reduction
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Solution Overview
Problem
Current high-speed communication and RADAR systems face limitations in achieving low phase noise in oscillating signals due to the constraints of resonator quality factors, power handling, and noise generated in oscillator circuitry, particularly in reference signal generators using Quartz crystals, SAW, MEMS, NEMS, BAW, or FBAR resonators.
Innovation Solution
A device comprising a mechanical resonator signal generator, a series-resonant crystal filter, and a phase-locked loop (PLL) that produces a filtered signal approximately 0 or 180 degrees out-of-phase with the oscillating signal, with the PLL generating a tuning control signal to adjust the filter's frequency, ensuring the filter's center frequency aligns with and tracks the signal generator's frequency, reducing phase noise through self-tuning and feedback mechanisms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a mechanical resonator is used to generate oscillating signals, then the signal generation is achieved, but phase noise is increased due to resonator quality factor limitations, power handling constraints, and oscillator circuitry noise
Solution Approach 1:
A series-resonant crystal filter is introduced as an intermediary component between the mechanical resonator and the output. This filter selectively attenuates phase noise components while passing the fundamental oscillating signal, thereby reducing phase noise without compromising signal generation reliability
Solution Approach 2:
A phase-locked loop (PLL) is implemented to provide feedback control. The PLL compares the phase of the oscillating signal with a reference and generates a tuning control signal that adjusts the filter's center frequency to track the signal generator frequency, maintaining optimal noise reduction while preserving signal integrity
2Object-generated harmful factors
If a fixed-frequency filter is used to reduce phase noise, then phase noise reduction is achieved at a specific frequency, but the filter cannot track frequency variations of the signal generator
Solution Approach 1:
The filter is designed with frequency tuning capability through a varactor diode or similar tuning mechanism. The PLL dynamically adjusts the filter's center frequency by varying the resonant frequency of the series-resonant crystal, enabling the filter to track frequency variations of the signal generator and maintain effective phase noise reduction across a range of frequencies
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively reduces both far-out and close-in phase noise by maintaining a consistent phase difference and frequency alignment, enhancing the signal quality and reducing spurious signals across a wide frequency spectrum.
Implementation Method 1
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
Implementation Method 2
comparing a phase of the oscillating signal to a phase of the filtered signal using a phase-locked loop (PLL)
Implementation Method 3
generating a tuning control signal with the PLL to adjust a filtering frequency of the filter
Data Source
AI summary
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.