Previous Page
Home
Contact Us

Eye Diagram

An eye diagram is an oscilloscope display in which a digital data signal from a receiver is repetitively sampled and applied to the vertical input, while the data rate is used to trigger the horizontal sweep. It is so called because, for several types of coding, the pattern looks like a series of eyes between a pair of rails. Several system performance measures can be derived by analyzing the display. If the signals are too long, too short, poorly synchronized with the system clock, too high, too low, too noisy too slow to change, or have too much undershoot or overshoot, this can be observed from the eye diagram. An open eye pattern corresponds to minimal signal distortion. Distortion of the signal waveform due to intersymbol interference and noise appears as closure of the eye pattern.

This photo shows the Jitterlyzer eye diagram and is performed on real-life traffic. It provides much more information than just a vertical eye opening or a horizontal eye opening. Instead, it provides a first indication of parameters such as dispersion in the signal path, rise-time issues, etc. As can be seen, an indication of noise throughout the whole eye is made, as opposed to, for example, only at the center. In this example the diamond in the center is the PCI Express mask, showing what the minimum eye is at 2.5 Gb/s.