Recovered Clock Filtering for Accurate Serial Eye Measurements
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing clock recovery techniques, such as constant clock recovery and first-order phase-locked loop methods, fail to accurately extract a wander-tracked clock from serial data streams for performing 'eye' diagram measurements, particularly in high-speed Serial Attached SCSI (SAS) applications, leading to inclusion of low-frequency jitter and instability in 'eye' diagram results.
Innovation Solution
A clock recovery technique that determines initial recovered clock times from transition edges, interpolates time interval errors, and applies lowpass filtering to separate and retain low-frequency jitter, effectively tracking signal wander and correcting the clock to produce a wander-tracked clock for accurate 'eye' diagram measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If constant clock recovery technique is used, then clock can be recovered from serial data stream, but low frequency jitter (wander) is included in the recovered clock causing inaccurate eye diagram measurements
Solution Approach 1:
The patent segments the clock recovery process into two distinct stages: initial clock recovery using constant clock recovery technique, and subsequent refinement using a phase-locked loop to eliminate low-frequency wander. This segmentation allows each stage to address specific aspects of clock recovery without compromising the other.
Solution Approach 2:
The patent introduces a phase-locked loop as an intermediary component between the initial clock recovery and the final eye diagram measurement. The PLL acts as a mediator that filters out low-frequency wander while preserving the essential clock signal, thereby resolving the contradiction between measurement accuracy and specification compliance.
2Measurement precision
If first order phase-locked loop technique is used, then wander-tracked clock can be recovered, but significant digital processing capability and time are required for simulation
Solution Approach 1:
The patent extracts the essential wander-tracking function from the complex time-domain simulation of a phase-locked loop and implements it using a simplified digital filter with a specific transfer function. This extraction maintains the wander-tracking capability while eliminating the computational burden of full PLL simulation.
Solution Approach 2:
The patent replaces the mechanical/physical phase-locked loop system with a digital filter implementation that mathematically models the PLL behavior. This substitution maintains the functional equivalence for wander tracking while dramatically reducing the computational resources and time required for simulation.
3Reliability
If thousands of clock cycles are simulated to obtain meaningful PLL results, then accurate wander tracking is achieved, but processing time and computational resources increase significantly
Solution Approach 1:
The patent applies preliminary action by using the initial clock recovery results to provide starting conditions for the phase-locked loop simulation. This allows the PLL to converge to accurate wander tracking much faster, reducing the number of clock cycles needed from thousands to a manageable number while maintaining simulation accuracy.
Data Source
AI summary
A method for recovering a clock from a serial data stream for use in producing “eye” diagram measurements includes determining an initial recovered clock from the serial data stream based upon transition edges. Time interval error values are then determined as a function of the transition edges and corresponding initial recovered clock times. Time interval error values are interpolated for initial recovered clock times not associated with transition edges to produce a complete set of time interval error values. The complete set of time interval error values are lowpass filtered to produce a filtered set of time interval error values. The clock for the serial data stream is reconstructed from the filtered set of time interval error values.