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VerifEye and QuickEye Analyses

The eye diagram is a convenient way to analyze the performance of a serial communications channel. In a traditional eye diagram, copies of the waveform generated by transient analysis are overlaid at a spacing of one unit interval (UI). The required width of the eye depends on the bit error rate (BER) due to timing variations such as jitter and variations in setup and hold times. The required height of the eye depends on the noise margin. Slicing through the eye at the midpoint of the amplitude displays the BER as a “bathtub” curve. In high-speed channels, the required BER becomes very small, and generating a useful eye diagram or bathtub curve requires long simulation runs.

In practice, however, designers are interested only in the probability that a waveform will violate the eye window. For this purpose, faster statistical techniques can be used instead of the full transient simulation. Nexxim’s QuickEye analysis uses simplifying assumptions to calculate the BER from a transient analysis of single transitions. Nexxim’s VerifEye analysis uses a fully statistical approach to calculate the BER.

QuickEye analysis employs pattern-dependent convolution. A step response to a single transition is taken for a single rising edge and a single falling edge, and the step responses are then combined to create an intermediate waveform that approximates the behavior of the fully-simulated channel. With QuickEye, inter-symbol interference (ISI) is easy to calculate.

VerifEye analysis is a statistical eye-analysis algorithm, similar to the public domain StatEye (see QuickEye and VerifEye References for a citation).VerifEye calculates a cumulative distribution function (CDF) for deviations from the step responses in the intermediate waveform described above, taking into account the conditional probabilities for the various kinds of transitions. A slice through the CDF generates the bathtub curve. Moreover, random transmit jitter can be simulated by using a random distribution for the deviations in response. VerifEye analysis uses the edges for calculations (rather than the pulses as in standard StatEye). Using the edges allows random jitter to be truly random (timings of edges are independent) and also enables an easy calculation of duty cycle distortion (DCD).

Peak distortion analysis (PDA) to find the worst case bit pattern is automatically performed for both VerifEye and QuickEye analyses. For QuickEye analyses, the worst case bit pattern can be pushed to all Eye Sources, overriding any bitlists set in the Eye Sources. For both QuickEye and VerifEye analyses, the worst case bit pattern can be displayed in reports.

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