PAM4 Transmitter Transition Time Measurement Using Linear-Fitted Waveforms
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Solution Overview
Problem
High-speed signaling communication standards, such as 100 GBASE-KP4, face challenges in accurately measuring signal transition times for PAM4 transmitters due to noise and jitter, which are exacerbated by the lossy printed circuit board and the difficulty in translating existing PAM2 transition time measurement methods to higher-order PAM schemes like PAM4.
Innovation Solution
A novel transition time measurement technique using a linear-fitted waveform to measure rise and fall times of PAM4 signals, where transmitter equalization is disabled, and a test pattern is transmitted to capture the waveform, allowing for the calculation of transition times from the 20% to 80% point of the peak-to-peak voltage difference, effectively combining jitter and noise specifications into a single figure of merit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If existing PAM2 transition time measurement methods are used for PAM4 transmitters, then measurement process is simple, but measurement accuracy deteriorates due to noise and jitter in PAM4 signals
Solution Approach 1:
The patent changes the measurement approach by disabling transmitter equalization during measurement to eliminate its distorting effect on transition times. It also modifies the measurement calculation to use specific voltage thresholds (20% and 80% of peak-to-peak voltage difference) rather than traditional methods, thereby achieving accurate PAM4 transition time measurements without being affected by noise and jitter
Solution Approach 2:
The patent extracts and removes the equalization function from the measurement process by disabling it specifically during transition time measurements. This allows the measurement to capture the true transmitter characteristics without the complicating effect of equalization, while the equalization remains functional during normal operation
2Reliability
If transmitter equalization is enabled during measurement, then signal quality is improved, but transition time measurement accuracy deteriorates due to equalization distortion
Solution Approach 1:
The patent segments the operational modes by separating measurement operations from normal transmission operations. During measurement, equalization is disabled to obtain accurate transition time data. During normal transmission, equalization is enabled to maintain signal quality. This segmentation allows each operation to optimize for its specific purpose without compromise
Data Source
AI summary
Methods, apparatus and systems for measuring signal transition times for a four-level pulse modulated amplitude (PAM4) transmitter. During a test procedure, a PAM4 transmitter is configured to repetitively transmitting a four-level test pattern, which is captured and digitized. The digitized data is processed to generate a linear-fitted waveform. A voltage modulation amplitude (VMA) level for each of a −1 and +1 PAM4 signal level is measured and used to derive 20% and 80% VMA levels in an eye diagram. The rise transition time is then determined by measuring the time interval between when a rising signal crosses the 20% and 80% VMA levels, and the fall transition time is determined by measuring the time interval between when a falling signal crosses the 80% and 20% VMA levels.