PLL Clock Recovery with PDV Compensation
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Solution Overview
Problem
Packet delay variation (PDV) significantly affects the accuracy and stability of slave clocks in IEEE 1588 Precision Time Protocol (PTP) systems, leading to clock noise that can render recovered clocks unusable for end-system applications due to jitter, wander, and other imperfections.
Innovation Solution
A method and apparatus for synchronizing the frequency of a slave clock with a master clock, which includes a phase-locked loop (PLL) with PDV compensation, using accumulated residence times from network devices to directly measure and remove PDV effects, allowing the slave clock to recover the master clock as if the communication path were free of PDV, and employing filtering to mitigate residual delays and clock noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If packet-based clock synchronization is used in networked systems, then scalability and flexibility are improved, but packet delay variation introduces clock noise that degrades synchronization accuracy
Solution Approach 1:
The patent segments the clock synchronization process into distinct functional blocks: packet reception, timestamp extraction, PDV measurement, error signal generation, filtering, and oscillator control. This segmentation allows each component to be optimized independently, particularly enabling the filtering stage to address PDV-induced noise while maintaining the overall scalability of packet-based synchronization
Solution Approach 2:
The patent introduces an intermediary filtering mechanism between the PDV measurement and the oscillator control. This filter acts as a mediator that processes the error signal to remove PDV-induced noise components before the signal reaches the oscillator, thereby maintaining both scalability and synchronization accuracy
2Device complexity
If traditional clock recovery mechanisms are used without PDV compensation, then device complexity is reduced, but clock noise from packet delay variation renders recovered clocks unusable for high-precision applications
Solution Approach 1:
The patent implements a feedback mechanism where the measured PDV is fed back into the error signal generation process. The PDV measurement is subtracted from the raw error signal to create a corrected error signal that is free from PDV-induced noise, which then drives the oscillator control. This feedback loop maintains clock quality without requiring overly complex mechanisms
Solution Approach 2:
The patent replaces traditional mechanical or hardware-based clock recovery mechanisms with a software/digital processing approach. By using digital filtering and PDV measurement algorithms, the system achieves high-precision clock recovery without complex mechanical components, thereby maintaining simplicity while improving reliability
3Measurement precision
If filtering is applied to mitigate residual delays and clock noise, then synchronization quality is improved, but processing time and computational resources increase
Solution Approach 1:
The patent applies partial filtering by selectively processing only the necessary components of the error signal. Rather than applying heavy filtering to the entire signal chain, the filter is applied only to the PDV-corrected error signal, and only to the extent necessary to remove residual noise. This partial action approach maintains synchronization quality while minimizing processing time and computational overhead
Data Source
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AI summary
This invention relates to methods and devices for frequency distribution based on, for example, the IEEE 1588 Precision Time Protocol (PTP). Packet delay variation (PDV) is a direct contributor to the noise in the recovered clock and various techniques have been proposed to mitigate its effects. Embodiments of the invention provide a mechanism to directly measure and remove PDV effects in the clock recovery mechanism at a slave clock. One particular embodiment provides a clock recovery mechanism including a phase-locked loop (PLL) with a PDV compensation feature built-in. An aim of the invention is to enable a slave clock to recover the master clock to a higher quality as if the communication path between master and slave is free of PDV. This technique may allow a packet network to provide clock synchronization services to the same level as time division multiplexing (TDM) networks and Global Positioning System (GPS).