Before this:Clock & symbol recoverySNR, EVM & BER
The eye diagram
Key takeaways An eye diagram overlays many single-symbol slices of a demodulated signal on top of each other. The clear region that emerges — the eye — shows link quality at a glance: its height is the amplitude margin against noise, its width is the timing margin against jitter, and the best sampling instant is where the eye is most open. A wide, open eye decodes cleanly; a closing eye warns of noise, jitter, or ISI.
The last lesson gave numeric grades (SNR/EVM/BER); the eye diagram is their picture. It is the single most useful visual for symbol quality and ties directly to clock recovery, which decides where in the eye to sample.
Building the eye
Take the demodulated waveform — the signal stepping between symbol levels — and cut it into pieces exactly one (or two) symbol periods wide. Now draw every piece on the same axes, all starting at the same left edge. The random data means each slice takes a different path, but they all pass through the same transition zones and avoid the same open centre. That open centre is the eye.
Reading the two margins
The eye’s opening is a direct picture of decoding headroom:
- Height (vertical opening) — the gap between symbol levels at the decision instant. This is the amplitude margin: how much noise can push a sample before it crosses into the wrong level. Noise (low SNR) thickens the traces and shrinks the height.
- Width (horizontal opening) — how long the eye stays open. This is the timing margin: how far the sampling instant can drift and still land in the clear. Timing jitter narrows it; inter-symbol interference pulls the crossings inward from both sides.
The decoder wants to sample at the widest, tallest point — the centre of the eye — which is exactly the instant clock recovery hunts for. A well-locked timing loop parks the sampler dead-centre in the eye.
A closing eye names the problem
Because each impairment attacks the eye in its own way, a glance often diagnoses the fault:
| Symptom in the eye | Likely cause |
|---|---|
| Traces thick, eye short | noise / low SNR |
| Crossings blurred left–right | timing jitter |
| Opening squeezed, levels pulled in | ISI / multipath |
| Eye fully closed | any of the above, too severe to decode |
More symbol levels means more, smaller eyes stacked vertically (four-level C4FM shows three eyes), and each must stay open. When the eye is shut, no sampling instant is safe and the decode fails — the visual companion to a high BER.
Quick check: the vertical opening (height) of the eye represents…
Recap
- An eye diagram overlays single-symbol slices; the clear eye shows quality at a glance.
- Height = amplitude margin against noise; width = timing margin against jitter.
- The ideal sampling instant is the eye’s centre — where clock recovery aims.
- A closing eye names the fault: thick traces (noise), blurred crossings (jitter), squeezed opening (ISI).
Next up: turning raw, error-prone symbols into trustworthy data — framing and error correction.
Frequently asked questions
What is an eye diagram?
An eye diagram is made by chopping a demodulated signal into single-symbol-wide slices and drawing them all on top of each other. Where the traces avoid each other an open space appears — the eye. A wide-open eye means clean, easy-to-decide symbols; a closing eye means noise, timing jitter, or inter-symbol interference is eating away the margin the decoder relies on.
How do you read an eye diagram?
Look at the open space in the middle. Its height is the voltage margin between symbol levels — how much noise the decoder can tolerate before mistaking one level for another. Its width is the timing margin — how much the sampling instant can drift and still land in the clear. The best moment to sample is the widest, tallest point of the opening, right at the eye’s centre.