Also known as: eye diagram, eye pattern
An eye diagram overlays many short segments of a demodulated signal, each one symbol period long (or two), so they stack on the same time axis into the characteristic “eye” shapes between the symbol levels.1 It is the time-domain companion to the constellation diagram: where the constellation shows symbol quality on the IQ plane, the eye shows how cleanly the signal transitions between levels and how much margin the decoder has at the sampling instant.
How it works
Imagine slicing the demodulated waveform into pieces exactly one symbol long and laying them all on top of one another, triggered on the symbol clock — like a repeatedly retriggered oscilloscope. Wherever the signal could be at that phase, a trace passes; the region no trace enters is the open “eye.” The decoder samples once per symbol, ideally at the instant the eye is widest, and slices the amplitude to the nearest level. Reading the eye:
- Vertical opening is the amplitude margin — the gap between the highest a low symbol reaches and the lowest a high symbol reaches. Noise eats into it from both sides; when it closes, low and high symbols become indistinguishable and errors appear.
- Horizontal opening is the timing margin — the span of sampling instants that still fall inside the eye. Timing jitter and intersymbol interference narrow it. A wide horizontal opening means the clock-recovery loop can be slightly off and still sample correctly.
- Thickness of the traces reflects noise and residual ISI; a fuzzy crossing point at the eye edges signals timing jitter.
The best sampling phase is the vertical line through the widest part of the eye, which is exactly what timing recovery tries to find and hold.
Variants
An M-level signal shows M−1 stacked eyes: a two-level scheme has a single eye, while a four-level C4FM or 4FSK signal shows three eyes, one between each adjacent pair of the four levels. Eye diagrams are most natural for real (one-dimensional) baseband signals — amplitude or frequency modulations — and are drawn separately for the I and Q rails of a complex signal. A well-designed pulse-shaping filter, such as a root-raised-cosine, is built specifically to force the ISI to zero at the sampling instant, which is what keeps the eye open in a bandlimited channel.
In practice
Because it exposes timing and amplitude margin at once, the eye diagram is the standard bench and software test for whether a link has usable margin or is riding the edge of failure. A signal that decodes but shows a nearly closed eye is fragile — a little more noise or multipath will tip it into errors — whereas a wide, clean eye has headroom. Watching the eye while adjusting gain, tuning, or filter settings gives immediate feedback that a constellation alone (which hides timing information) does not.
Relevance to SDR
For frequency-modulated trunking signals the eye diagram is often more diagnostic than the constellation, since the information lives in instantaneous frequency and forms clean stacked eyes after FM demodulation. GopherTrunk’s eye-diagram panel shows the three-eye pattern of the 4800-baud C4FM family, letting an operator see timing and noise margin at a glance and confirm that symbol timing is locked to the widest opening before trusting a marginal decode.
Sources
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Eye pattern — Wikipedia, for the overlaid-symbol-period display and what an open eye indicates. ↩