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.
An eye diagram shows the spread of the signal. The more the jitter or the longer the distance the thicker the signal width. Eventually the receiver is unable to detect the edges of the signal.
