CDR Frequency Acquisition Hold for Loss-of-Lock Recovery
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Solution Overview
Problem
Conventional clock and data recovery (CDR) devices face challenges in automatically acquiring and maintaining frequency lock with input signals of unknown frequency, leading to data loss during temporary interruptions or frequency variations, as they rely on predetermined baud rates and struggle with fractional spurs due to limited resolution in fractional-N frequency synthesizers.
Innovation Solution
A system and method for automatic frequency acquisition and hold in CDR devices, utilizing a phase detector, rotational frequency detector, and phase-frequency detector to dynamically adjust and maintain frequency lock across a broad range of frequencies, enabling continuous operation even with bursty or interrupted signals, by calculating and storing frequency ratios and switching between acquisition and hold modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a VCO sweep method is used for frequency acquisition, then the system can acquire signals across a wide frequency range, but the acquisition time increases and productivity decreases
Solution Approach 1:
The frequency acquisition process is divided into two distinct stages: coarse frequency acquisition that rapidly sweeps through wide frequency ranges to locate the signal band, and fine frequency acquisition that precisely locks to the exact signal frequency. This segmentation allows the system to achieve both wide frequency coverage and fast acquisition by optimizing each stage separately.
Solution Approach 2:
The coarse frequency acquisition performs preliminary action by quickly identifying the approximate frequency range where the signal exists before the fine acquisition begins. This preliminary localization eliminates the need to sweep the entire frequency range during fine acquisition, significantly reducing total acquisition time.
2Adaptability or versatility
If an auxiliary frequency acquisition system is added to extend VCO tuning range, then frequency adaptability improves, but device complexity increases
Solution Approach 1:
The coarse and fine frequency acquisition functions are merged into a single integrated circuit block, sharing common components such as the VCO, phase detector, and feedback dividers. This merging achieves wide frequency tuning range while minimizing additional complexity by reusing existing circuit elements rather than adding completely separate acquisition systems.
3Measurement precision
If fractional-N frequency synthesizer is used to achieve fine frequency resolution, then frequency precision improves, but fractional spurs are generated
Solution Approach 1:
The fine frequency adjustment function is extracted from the fractional-N synthesizer and implemented separately using a small tuning range VCO that operates only within a narrow band around the coarse frequency. This extraction eliminates the fractional spurs generated by fractional-N division while maintaining fine frequency resolution through direct digital synthesis or precise analog tuning of the narrowband VCO.
Data Source
AI summary
A system and method are provided for automatic frequency acquisition maintenance in a clock and data recovery (CDR) device. In an automatic frequency acquisition (AFA) mode, the method uses a phase detector (PHD) to acquire the phase of a non-synchronous input communication signal having an initial first frequency. In the event of a loss of lock/loss of signal (LOL/LOS) signal being asserted, a frequency ratio value is retrieved from memory. Using a phase-frequency detector (PFD), the reference signal, and the frequency ratio value, a synthesized signal is generated. In response to using the PFD to generate the synthesized signal and the LOL/LOS signal being deasserted, a rotational frequency detector (RFD) is used to generate a synthesized signal having a frequency equal to the frequency of the input communication signal. With the continued deassertion of the LOL/LOS signal, the PHD is enabled and the phase of the input signal is acquired.