Eye‐Diagram - espotek-org/Labrador GitHub Wiki

An eye-diagram is a plot of a digital communication signal, formed by superimposing integer number of symbols (usually 2) on top of each other. Since the eye-diagram plot is triggered by a synchronous clock, some serial communication protocols (e.g. SPI, I2C) transmit the clock signal next to the data line and it makes eye-diagram calculation easier. Where as other communication protocols (e.g. UART, SERDES) embed the clock signal within the data line, and thus clock recovery is the first operation before calculating the eye-diagram.

An eye-diagram is used to asses the quality and integrity of a digital communication. An open eye indicates a high-quality signal with good integrity. A "closed" or distorted eye, on the other hand, signifies problems.

A noisy signal, having fluctuations in the signal amplitude, appears to have a vertical blurring in the eye-diagram.

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A signal full of jitter on the clock line or on the embedded clock-data line, appears to have a horizontal blurring in the eye-diagram.

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A signal traveling long distance is low pass filtered because of the high capacitive wire load, thereby flattening the sharp transitions, and thus appears to have a closed eye-diagram.

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We have seen so far the use of eye-diagrams for measuring signal integrity. On top of that, an eye-diagram is also used for compliance testing. Here companies specify eye-masks (e.g., HDMI, PCI Express, Ethernet, DDR), there by ensuring a communication system meets industry standards.

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Engineers also use eye-diagrams to troubleshoot and debug the root cause of signal degradation in a communication system, thereby allowing them to apply equalization or add pre-emphasis blocks.

Last but not least, eye-diagram is also used to estimate the bit error rate (BER) of a communication channel. Although it is not used for direct BER measurement, an open eye has a lower BER compared to a closed eye, thus allowing subjective BER estimations.