How is an eye diagram displayed on an oscilloscope?

By overlaying many bit intervals, triggering with the symbol clock

By displaying the signal spectrum

By plotting one long bit stream across many screens

By triggering with the noise signal

What does a thick zero-crossing spread and steep eye sides indicate?

Timing jitter, and high sensitivity to timing error for steep sides

Where should the signal be sampled according to the eye diagram?

Where the eye opening is greatest

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Short note: Eye diagram

Short answer

An eye diagram overlays many symbol periods of a received baseband signal triggered by the clock. The eye opening height indicates noise margin, the width shows the safe sampling interval, and the slope and zero-crossing thickness reveal timing jitter and ISI.

Write a short note on the eye diagram.

An eye diagram is an oscilloscope display of a received digital baseband signal. It is made by overlaying many bit intervals on top of each other: the scope is triggered by the symbol clock and the sweep is set to one or two symbol periods. The pattern looks like an eye. Its shape shows at a glance how much ISI and noise the signal has.

   ____________     ____________
  /            \   /            \
 /   .------.   \ /   .------.   \
|   (  open  )   X   (  open  )   |
 \   '------'   / \   '------'   /
  \____________/   \____________/
     ^    ^
     |    best sampling instant
     eye opening (noise margin)

What the eye shows:

Eye opening (height): the noise margin. A larger vertical opening means more noise can be tolerated.
Eye width: the time range in which the signal can be sampled without errors.
Best sampling time: where the eye opening is greatest.
Slope of the sides: how sensitive the system is to timing error. Steeper sides mean more sensitive.
Thickness of the crossing (zero-crossing spread): the amount of timing jitter.
Distortion at the sampling time: the spread of the lines at the top and bottom shows the amount of ISI.

Use: If ISI or noise is severe, the eye closes, and errors become likely. Engineers use the eye diagram to adjust equalisers, choose pulse shaping (e.g. raised-cosine roll-off), set the decision threshold and sampling time, and quickly check the quality of a link.

✓ Answer checked by Claude Opus 5.5 (high effort)

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