øjendiagram

An eye diagram is a visualization used in digital communications to assess signal integrity. It is created by overlaying many successive samples of a digital signal taken at regular intervals, typically aligned to the bit boundaries of a transmitted data stream. The result resembles an eye-shaped opening, hence the name.

The diagram is usually captured with an oscilloscope or a dedicated measurement tool. In practice, the instrument triggers on a reference edge and repeatedly samples the signal over many bit periods, displaying a composite image that highlights how the signal would be interpreted at the receiver under real operating conditions. For differential lines, the diagram can be generated from the differential pair or from a single-ended representation depending on the setup.

What the eye diagram reveals includes the vertical eye opening, which indicates the noise margin in amplitude, and the horizontal eye opening, which reflects timing jitter and how consistently bits are placed in time. A wide, clear eye suggests low noise, minimal jitter, and good signal integrity, while a closed or distorted eye indicates problems such as noise, intersymbol interference, bandwidth limitations, or nonlinearity.

Applications of eye diagrams span high-speed digital design and testing, including serial interfaces and data links (for example, USB, PCIe, Ethernet, and optical transceivers). They help engineers evaluate whether a channel meets performance specifications and guide adjustments to bandwidth, equalization, or shielding to improve reliability and reduce bit error rates.

Limitations include sensitivity to measurement setup, sampling rate, and averaging; the diagram is a diagnostic tool rather than a complete performance metric on its own.