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In telecommunication, an eye pattern, also known as an eye diagram is an oscilloscope display in which a digital data signal from a receiver is repetitively sampled and applied to the vertical input, while the data rate is used to trigger the horizontal sweep. It is so called because, for several types of coding, the pattern looks like a series of eyes between a pair of rails.
Several system performance measures can be derived by analyzing the display. If the signals are too long, too short, poorly synchronized with the system clock, too high, too low, too noisy too slow to change, or have too much undershoot or overshoot, this can be observed from the eye diagram. An open eye pattern corresponds to minimal signal distortion. Distortion of the signal waveform due to intersymbol interference and noise appears as closure of the eye pattern. In summary the following features of the eye-diagram define
1. Eye opening (height, peak to peak), measure of the additive noise in the signal
2. Eye overshoot/undershoot, measure of the peak distortion
3. Eye width, measure of timing synchronization & jitter effects.
An eye diagram is a simple and convenient tool for studying the effects of intersymbol interference (ISI) and other channel impairments in digital transmission. To construct an eye diagram, plot the received signal against time on a fixed-interval axis. At the end of the fixed time interval, wrap around to the beginning of the time axis. Thus the diagram consists of many overlapping curves. One way to use an eye diagram is to look for the place where the "eye" is most widely opened, and use that point as the decision point when demapping a demodulated signal to recover a digital message.
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