Answer1

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	WHAT'S YOUR ENGINEERING QUOTIENT?
Problem 2—What is an eye diagram? Answer: An eye diagram is used to evaluate serial communications links for signal quality, allowing one to visually estimate the likelihood of bit errors. It is generated on an oscilloscope by setting the horizontal timebase so that two or three bit times are displayed across the screen, and the recovered data clock is used as the horizontal trigger. Many samples of the incoming data stream will be overlaid on the screen, creating an image similar to that shown below. The center of the screen represents the point at which the one-or-zero decision is made by the receiver. However, there are tolerances in the design of the receiver that make this area a box instead of a point. Jitter in the receiver clock produces some uncertainty in the horizontal dimension, and errors in the refernce voltage of the comparator produce a similar uncertainty in the vertical dimension. In a wide-open eye diagram like this one, in which the waveform never crosses the box, the data reception should be error-free. However, if the waveform enters the box, a bit error may or may not occur, depending on the instantaneous circumstances within the receiver. Contributor: Dave Tweed 11-01 — NEXT Q&A For questions or comments about Test Your EQ, e-mail eq@circuitcellar.com.