Transport Delay Measurement Using Cross-Correlation
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Solution Overview
Problem
Current methods, such as spectrum analysis, Time Domain Transmissometry, and Vector Network Analyzer, are inadequate for directly measuring transport delays and jitter in high-speed clock driven systems, particularly on printed circuit boards, due to the complexity of high-speed serial data standards like PCI Express and FB-DIMM.
Innovation Solution
A method utilizing a realtime oscilloscope for cross-correlation analysis between two test points to compute transport delay, involving clock recovery, filtering, and interpolation based on LMS error, to determine transport delay and jitter, providing an 'idealized' reference clock for edge comparison.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional measurement techniques (spectrum analysis, TDT, VNA) are used to measure transport delays, then component-level delays can be obtained, but direct system-level transport delay measurement is impossible
Solution Approach 1:
The patent introduces an intermediary approach by using cross-correlation analysis as a mediator between the two measurement points. Instead of directly measuring delay with conventional instruments, the method uses the correlation between waveforms at different points to indirectly determine the transport delay, achieving direct system-level measurement where none existed before
Solution Approach 2:
The patent replaces the mechanical/instrument-based measurement system (spectrum analyzers, TDT, VNA) with a computational approach using cross-correlation algorithms. This substitution enables direct measurement of system-level transport delays that were previously inaccessible to conventional instrumentation methods
2Loss of information
If component transport delays are summed to obtain overall delay, then theoretical delay can be calculated, but actual system performance cannot be verified
Solution Approach 1:
The patent implements feedback by measuring the actual transport delay directly in the system and using this measurement to verify and adjust the theoretical calculations. The cross-correlation method provides feedback on the true system behavior, allowing comparison with component-summed delays and identification of discrepancies due to interactions or unmodeled effects
Solution Approach 2:
The system performs self-measurement by using its own internal signals and cross-correlation capability to determine its actual transport delay. This self-service approach allows the system to verify its own performance without external measurement equipment, ensuring the reliability of the measured delay values
3Ease of manufacture
If high speed backplane is treated as simple PC board, then manufacturing is simplified, but signal integrity and transport delay control deteriorate
Solution Approach 1:
The patent replaces complex physical design iterations with a computational measurement and adjustment approach. Instead of repeatedly modifying the backplane physical layout to control delays, the method uses cross-correlation measurement to directly assess and compensate for actual transport delays, achieving precision without sacrificing manufacturing simplicity
Data Source
AI summary
A method of measuring transport delay and jitter with a realtime oscilloscope using cross-correlation acquires waveforms from two test points in a system under test. Clock recovery is run on both waveforms to obtain respective rates and offsets. A time offset between the two waveforms is computed. The jitter from the two test points is filtered and a mean-removed cross-correlation coefficient is computed from the filtered jitters. A fractional delay is computed using interpolation based on LMS error, and the respective computational components are summed to compute a transport delay between the two test points. The transport delay may be used to adjust clock edges from one waveform for comparison with data transition edges of the other waveform to measure jitter.