---
title: "Short note: Eye diagram"
subject: "Communication Systems"
chapter: 6
key: "semester_6/communication_systems:short-note-eye-diagram"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-eye-diagram
site: BCT Engineering
---

# Short note: Eye diagram

Communication Systems · Chapter 6: Baseband Digital Data Transmission

- Asked in: 2081 Bhadra (CS II), 4 marks; 2076 Chaitra (CS II), 5 marks; 2072 Kartik (CS II), 4 marks; 2072 Chaitra (CS II), 2.5 marks; 2069 Chaitra (CS II), 4 marks (asked 5 times)
- Video: [What is an Eye Diagram?](https://www.youtube.com/watch?v=ROhgIWBteQQ) — Iain Explains Signals, Systems, and Digital Comms
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-eye-diagram
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-6

## Question

Write a short note on the 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.

## Answer

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.

```text
   ____________     ____________
  /            \   /            \
 /   .------.   \ /   .------.   \
|   (  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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is an eye diagram displayed on an oscilloscope?
   A. By overlaying many bit intervals, triggering with the symbol clock
   B. By displaying the signal spectrum
   C. By plotting one long bit stream across many screens
   D. By triggering with the noise signal
   Answer: A. The scope is triggered by the symbol clock with sweep set to one or two symbol periods, so many intervals overlap. (Key point: How an eye diagram is formed)

2. What does the vertical eye opening indicate?
   A. The timing jitter
   B. The noise margin
   C. The bit rate
   D. The carrier frequency
   Answer: B. A wider vertical opening means more noise can be tolerated before a wrong decision. (Key point: Eye opening)

3. What does a thick zero-crossing spread and steep eye sides indicate?
   A. Low ISI and high noise margin
   B. Timing jitter, and high sensitivity to timing error for steep sides
   C. Perfect sampling
   D. No noise
   Answer: B. The spread at the crossings measures timing jitter; steeper sides mean the system is more sensitive to sampling-time errors. (Key point: Jitter and timing sensitivity)

4. Where should the signal be sampled according to the eye diagram?
   A. At the zero crossing
   B. Where the eye opening is greatest
   C. At the edge of the eye
   D. Anywhere in the bit
   Answer: B. The best sampling time is where the eye opening is largest, giving the best margin against ISI and noise. (Key point: Best sampling instant)