Electronic Design

Eye Diagrams, Bathtub Curves, And Bit-Error Rates

On an oscilloscope's eye-pattern diagram, the distance between peak deterministic and random jitter amplitudes (the unit interval, or UI) is often called a "bathtub," which functions as a bit-error-rate (BER) indicator (see the figure, a). An eye-pattern diagram is a good method for quickly visualizing and accurately measuring timing jitter (see the figure, b).

Typical communications applications usually seek a BER of about 1 × 10-12. The eye pattern is formed by applying the repetitively sampled digital data signal to the oscilloscope's vertical input and the data clock rate that triggers the oscilloscope's sweep speed to the horizontal input.

Timing jitter is a critical measurement. A communication-system receiver's ability to track jitter in the incoming data stream is related directly to the error performance of that system. Jitter is typically defined in terms of UIs. A UI is the timing of the digital data when it transitions from its ideal (jitter-free) locations. The data transitions occur nominally at multiples of one UI, which is equal to the inverse of the bit rate.

Bit-error-rate testers (BERTs) can generate the distinctive bathtub curve characterizing the relationship between the BER and the fraction of the UI that's available for strobing data. As a result, designers can provide evidence that a design meets its intended applications.

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