Eye diagrams provide an intuitive graphical representation of electrical or optical digital communication signals. The quality of the signal, that is, the rise and fall times, the amount of intersymbol interference (ISI), noise, and jitter, can be judged from the appearance of the eye. In the following, we discuss how to measure and simulate eye diagrams and how to determine the eye openings and eye margins. In Appendix C, we discuss the related subject of jitter and its measurement.
Cutting and Overlaying Waveforms
The waveform of a communication signal, such as a non-return-to-zero (NRZ), a return-to-zero (RZ), or a 4-level pulse amplitude modulation (4-PAM) signal, can be turned into an eye diagram (a.k.a. eye pattern) by cutting it up into segments that are two bit (or symbol) intervals long and overlaying them. For example, in Fig. B.1, the waveform of an NRZ signal with mild ISI is cut up into segments with half a bit interval on the left, a full bit interval at the center, and half a bit interval on the right. Then, all segments are overlaid to form the eye diagram, as shown at the center of Fig. B.1. Because the ISI in our example is limited to just one bit to the right and left, there are only eight distinct segments corresponding to the three-bit binary words: 000, 001, 010, 011, 100, 101, 110, and 111 (see the left- and right-hand side of Fig. B.1). In the case of a signal with stronger ISI, there are more distinct segments.
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