Eye pattern
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In telecommunication, an eye pattern is an oscilloscope display in which a pseudorandom 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, when the signal is in NRZ code, the pattern looks like a series of eyes between a pair of rails.
Note: System performance information can be derived by analyzing the display. 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.
Only when the horizontal range is interger multiply of the period of the input signal, steady view can be obtained. Only when the input is sinc like, the pattern of eyes can be seen.
Source: from Federal Standard 1037C and from MIL-STD-188