Eye diagrams: The tool for serial data analysis

By Arthur Pini – The eye diagram is a general-purpose tool for analyzing serial digital signals. It shows the effects of vertical noise, horizontal jitter, duty cycle distortion, inter-symbol interference, and crosstalk, all of which can close the “eye.” While engineers have used eye diagrams for decades, oscilloscopes continually get new features that increase its value.

Oscilloscopes form eye diagrams—the separation between the two binary data states “1” and “0”—by overlaying multiple single clock periods on a persistence display. The accumulation shows the history of multiple acquisitions.

Additive noise tends to close the eye vertically while timing jitter and uncertainty closes the eye horizontally. Duty cycle distortion (DCD) and inter-symbol interference (ISI) change the shape of the eye. The channel will fail if the eye closes to the point where the receiver can no longer recognize “0” and “1” states.

In the days of analog oscilloscopes, the eye diagram was formed by triggering the oscilloscope with the serial data clock and acquiring multiple bits over time using a persistence or storage display. This technique adds the trigger uncertainty or trigger jitter to the eye diagram for each acquisition. Digital oscilloscopes form the eye by acquiring very long record with many serial bits.

The clock period is determined, and the waveform is broken up or “sliced” into multiple single-bit acquisitions overlaid in the persistence display. In this way, all the data is acquired with a single value of trigger jitter that’s eliminated by using differential time measurements within the eye. more>

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