How to Measure High-Speed Serial Jitter Using Clock Recovery

+ Clock Recovery System

Measure High-Speed Serial Jitter

High-speed serial jitter measurements require a timing reference that is synchronous with the data under test. When a device under test (DUT) does not provide a separate clock, an electrical clock recovery unit can recover timing directly from the serial data stream and supply the reference needed by a sampling oscilloscope. A microwave pickoff directs a portion of the serial signal to the clock recovery input while the remaining signal feeds the oscilloscope channel. Engineers configure the input data rate and clock recovery loop characteristics, then lock the recovered clock to the incoming data. The recovered clock is connected to the oscilloscope trigger input, allowing the instrument to capture and analyze the serial waveform. This measurement setup supports both non-return-to-zero (NRZ) and four-level pulse amplitude modulation (PAM4) signals, providing a practical method for evaluating high-speed serial timing behavior when an appropriate clock signal is unavailable from the DUT.

Accurate jitter measurements using a recovered clock depend on proper loop configuration and maintaining the timing relationship between the clock and data paths. Engineers use sampling oscilloscopes with jitter analysis capabilities to examine timing variations while adjusting clock recovery loop characteristics for the measurement. Differences in clock-to-data path delays can cause jitter to become uncorrelated, particularly at low frequencies, resulting in closed eye diagrams and reduced measurement accuracy. To address these challenges, engineers use phase-matched cables and delay lines to minimize timing differences between the recovered clock and measured data. Adjusting the clock recovery loop bandwidth while observing jitter results helps determine whether additional path matching is necessary. Jitter Spectrum Analysis provides further insight into low-frequency jitter contributions and can help assess the impact of timing impairments on measurement accuracy.

High-Speed Serial Jitter Measurement Solution

To measure high-speed serial jitter using clock recovery, split the DUT output so the same serial data drives both the oscilloscope measurement channel and the electrical clock recovery input, then use the recovered clock as the timing reference. The Keysight electrical clock data recovery systems support electrical NRZ and PAM4 signals and provide adjustable clock recovery loop bandwidth and peaking. Once lock is established at the selected symbol rate, the recovered clock can trigger an oscilloscope while the serial waveform is acquired for serial data jitter analysis. Jitter inside the recovery loop bandwidth is tracked by the recovered clock and is reduced in a measurement made relative to that reference; higher-frequency timing variation outside the loop bandwidth remains observable on the data. Optional jitter spectrum analysis adds frequency-domain visibility into timing components and can help identify contributors to excessive jitter. The setup supports serial-data jitter and eye measurements when the DUT does not provide a separate synchronous clock.

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