Getting more out of eye diagrams [circuit testing]

The eye diagram, so called for its resemblance to a human eye, is a well-established tool. The communications industry uses it to observe and analyze the performance of circuits that drive the transfer of digital data streams. In its simplest form, it is generated on the screen of a storage oscilloscope (analog or digital) by overlaying sweeps of different segments of a long data stream driven by a master clock. The author describes how the eye diagram is used to obtain information about the circuit under test. The advent of the digital oscilloscope has led to improvements in the use of the method. Digital oscilloscopes give engineers the ability to use timing jitter, rise time, pulse width, amplitude, and more than 40 other signal parameters as the basis for making decisions-or, even better, for having automatic test equipment make them. The most powerful digital scopes will even calculate and display such statistical quantities as the mean, minimum, maximum, and standard deviation of these parameters.