RF Receiver PLL Frequency Correction for Free-Running Crystals
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Solution Overview
Problem
Existing RF receivers face challenges in automatic frequency correction without adjusting the resonance of crystal resonators, which can disrupt other device functionalities and introduce noise sources like in-band spurs and long-term drift.
Innovation Solution
The system adjusts the phase-locked loop (PLL) to correct frequency errors instead of the crystal resonator, allowing the reference oscillator signal to remain free-running and serving multiple functionalities, while using a leakage canceler and rate adaptor to mitigate noise and drift issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the crystal resonator's resonance is adjusted for frequency correction, then the LO signal frequency error is corrected, but other device functionalities using the reference oscillator are disrupted and noise sources are introduced
Solution Approach 1:
The system separates the frequency correction function from the reference oscillator by introducing a dedicated frequency correction oscillator that operates independently. This allows the reference oscillator to maintain its resonance frequency for multiple device functionalities while the correction oscillator handles frequency error compensation specifically for the LO signal.
Solution Approach 2:
A frequency correction oscillator is introduced as an intermediary component between the reference oscillator and the LO signal generation. This correction oscillator receives the reference signal, applies frequency correction based on error detection, and generates the corrected LO signal, thereby protecting the reference oscillator from direct involvement in frequency adjustments.
2Measurement precision
If the crystal resonator's resonance is adjusted for frequency correction, then the LO signal frequency error is corrected, but in-band spurs and long-term drift noise are introduced
Solution Approach 1:
The frequency correction oscillator serves as an intermediary that isolates the reference oscillator from direct frequency adjustment operations. By detecting frequency errors and generating correction signals through this intermediary component, the system avoids the noise and spurs that would result from directly adjusting the crystal resonator's resonance.
Solution Approach 2:
The system replaces mechanical/resonant adjustment of the crystal resonator with an electronic frequency correction mechanism. Instead of physically or electrically adjusting the resonator's resonance frequency, the system uses electronic oscillation frequency adjustment in the correction oscillator, which does not introduce the same noise and spur problems.
3Adaptability or versatility
If a free-running crystal is used without frequency correction, then other device functionalities are not disrupted, but LO signal frequency error causes carrier down-conversion to incorrect frequency positions
Solution Approach 1:
The system segments the frequency control functions by allowing the reference oscillator to run freely for supporting multiple device functionalities, while a separate frequency correction oscillator handles the precise frequency control needed for accurate carrier down-conversion. This segmentation enables both goals to be achieved simultaneously.
Solution Approach 2:
The system implements a feedback mechanism where frequency errors in the LO signal are detected and used to generate correction signals that adjust the frequency correction oscillator. This closed-loop feedback ensures accurate carrier down-conversion frequency positioning while the reference oscillator remains undisturbed and free-running.
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 approach enables accurate frequency correction without disrupting other device functionalities, reducing noise interference and maintaining performance across various communication standards.
Implementation Method 1
a phase-locked loop (PLL) to provide the LO signal based on a reference oscillator signal
Implementation Method 2
a mixer for mixing the received signal with a local oscillator (LO) signal to down-convert the desired carrier in the received signal to baseband or some non-zero IF
Implementation Method 3
a digitally controlled crystal oscillator (DCXO) and crystal resonator
Data Source
AI summary
Embodiments of a receiver for using a first oscillator signal provided by a crystal resonator to support multiple, different functionalities are provided. The receiver comprises a phase-locked loop (PLL) configured to provide a second oscillator signal based on the first oscillator signal provided by the crystal resonator; a first mixer configured to mix a received signal received over a first input path with the second oscillator signal received over a second input path to provide a first frequency-shifted signal; and an automatic frequency controller (AFC) configured to estimate a frequency offset of the second oscillator signal and adjust the PLL to compensate for the frequency offset. The receiver further can include solutions for mitigating potential sources of noise caused by the frequency of the first oscillator signal not being compensated for by the AFC.