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18 DESIGN007 MAGAZINE I JANUARY 2023 An eye diagram is a useful tool for the analy- sis of signals used in digital transmission. It provides a quick scan of system performance and can offer insight into the nature of chan- nel imperfections. An eye diagram is simply a graphical display of a serial data signal with respect to time that shows a pattern that resem- bles an eye. Careful scrutiny of this visual dis- play can give one a first-order approximation of signal-to-noise, clock timing jitter, reflec- tions and skew. In this month's column, I will take an eye-ball look at the eye diagram. An eye diagram overlays the signal wave- form over many cycles. e stimulus is nor- mally a pseudo-random bit stream (PRBS). Each cycle waveform is aligned to a common timing reference, typically a clock. An eye dia- gram provides a visual indication of the volt- The Eye Diagram age and timing uncertainty associated with the signal. In an ideal world, eye diagrams would look like rectangular boxes. However, in real- ity, communications are not perfect, so the transitions do not align perfectly, resulting in an eye-shaped pattern. A pseudo-random bit stream is a program that applies mathematical algorithms to simu- late randomness. It generates a sequence of binary numbers, synchronized by a clock, approximating the properties of random num- bers. e triggering edge may be positive or negative, but the resultant pulse that appears aer a delay period may go either way. ere- fore, when many such transitions are overlaid, positive and negative pulses are superimposed on each other. Overlaying many such bits pro- duces an eye diagram. is sequence (up to Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Eye diagram of a serial data stream with an eye mask.