Eye diagram

An eye diagram is the single most important picture in high-speed design: take the wiggly signal, chop it into one-bit-wide slices, and overlay thousands of them on top of each other. All the 1-to-0, 0-to-1, 1-to-1 transitions pile up and leave a clear open space in the middle that looks like an eye. The size of that opening tells you, at a glance, how much margin the receiver has — a wide-open eye means easy, error-free sampling; a half-closed eye means trouble; a fully closed eye means the link is dead.

The eye has two crucial dimensions. Its vertical opening is the voltage margin — how far above noise the '1' sits and below it the '0' — squeezed shut by attenuation, crosstalk and noise. Its horizontal opening is the timing margin — how wide the safe sampling window is — squeezed shut by jitter and ISI. The receiver's job is to place its sampling point dead-centre in this opening; equalization exists precisely to pry a smeared, closed eye back open. A PAM4 signal has three stacked eyes instead of one, each only a third the height — which is why PAM4 is so much harder.