eyepattern

An eyepattern, typically referred to as an eye diagram in practice, is a graphical tool used in digital communications to assess signal integrity for high-speed serial links. It is produced by repeatedly sampling and overlaying portions of a received waveform aligned to the bit period, creating a composite view of many bit intervals.

Generation involves transmitting a sequence such as a pseudorandom binary sequence and recording the waveform with a high-bandwidth oscilloscope or signal integrity analyzer. The overlay of thousands or millions of bit periods yields the characteristic eye-shaped figure, whose openness reflects the channel performance.

Interpreting the eye: the vertical opening, or eye height, indicates the available voltage margin in the presence of noise and amplitude variations; the horizontal opening, or eye width, indicates timing margin and jitter tolerance. A wider, taller eye corresponds to easier and more robust data recovery. Reductions in eye opening reveal issues like intersymbol interference, attenuation, reflections, crosstalk, or insufficient equalization.

Applications and patterns: eyepatterns are widely used in the design, testing, and validation of high-speed serial interfaces such as PCIe, USB, HDMI, SATA, and Ethernet. They help set equalizers, decision thresholds, and baud rates, and they provide a quick visual assessment of how link performance changes with channel length, connectors, or data patterns.

Limitations: while informative, an eye diagram is a qualitative measure that depends on measurement bandwidth and instrumentation. It does not directly quantify bit error rate, and real-world BER may differ. Nevertheless, it remains a standard diagnostic tool in signal integrity.