I/O Interfaces & Buses

Which statement about the eye diagram used to assess high-speed serial signal quality is correct?

  1. AIt overlays the waveforms of many bits; the wider and taller the central "eye" opening, the more margin there is
  2. BThe more closed the eye, the better the signal
  3. CIt is a chart for checking the health of human eyes
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Answer: A. It overlays the waveforms of many bits; the wider and taller the central "eye" opening, the more margin there is

An eye diagram slices the waveform at each bit period and overlays the pieces. An eye-shaped gap appears between the 0 and 1 traces; its height represents the margin against noise and its width the margin against timing variation (jitter). Channel loss, reflections and jitter close the eye. The receiver decides bits at the center of the eye, so its opening is a measure of quality.

  • A. Correct. It shows margins against noise and timing jitter.
  • B. A closed eye makes 0s and 1s hard to tell apart.
  • C. It is a chart for evaluating electrical signals.
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