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the name given to the "picture" or pattern produced when multiple traces are recorded on an oscilloscope by sending multiple 1's and 0's down a cable assembly and "looking" at the far end of the cable. The 1's and 0's are randomly generated so that many different sequences of highs and lows are examined by the eye pattern.
This test is an excellent way to look at how the cable assembly will perform in the "real world". It encompasses attenuation and risetime degradation (reduction in amplitude and risetime as a function of frequency components of the pulse and bit pattern sequence), dispersion (jitter due to differing speeds of signal transmission as a function of frequency) and within-pair skew (offset of differential signal due to differing electrical lengths of the wires within the differential pair). The "eye" of the input signal is compared to the "eye" of the signal at the far end . The height and width of the "eye" (referred to as the "eye opening") show how the signal has been distorted and can be used to analyze if the user's receiving circuit can reliably distinguish the logic 1's and 0's emerging from the cable assembly. Ideally, the eye pattern would be a square that is the same height and width as the input signal. The X-axis plots time and the Y-axis plots amplitude. Referring to the illustration, 1,2,.,8 are "snapshots" of the signal at equal lengths of time apart (X number of clock cycles) and these "snapshots" are superimposed one on top of another yielding the eye-pattern in the lower left of the illustration. (illustration courtesy of Agilent Technologies and Electronic Design )