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 width of the eye is affected by timing variations such as jitter and by variations in setup and hold times. The maximum allowable shrinkage in eye width is called the jitter budget. The height of the eye is affected by voltage variations or noise. The maximum allowable closure in eye height is called the noise margin. The jitter budget and noise margin depend on the maximum allowable bit error rate (BER), the ratio of correctly received bits to the total number of bits sent. In high-speed channels, the required BER becomes very small, on the order of one error in 1012 bits. Generating a useful eye diagram requires transient simulation over multiple terabits.
In practice, however, designers are interested only in the probability that a waveform violates the eye window. For this purpose, faster statistical techniques can be used instead of the full transient simulation.
- Quick Eye analysis uses simplifying assumptions to calculate an eye diagram from a transient analysis of single transitions.
- VerifEye analysis uses a fully statistical approach to calculate the BER directly, similar to the public domain Stat Eye.
Quick Eye Analysis
Quick Eye analysis employs pattern-dependent convolution. QuickEye uses a brief transient simulation to calculate the channel’s step responses to a single rising edge and a single falling edge. QuickEye combines the step responses with the data stream to approximate the output of the channel. The eye diagram is formed by overlaying UIs in the usual manner. With QuickEye, the effect of inter-symbol interference (ISI) on the bit error rate is easy to calculate.
VerifEye Analysis
VerifEye analysis is a statistical eye-analysis algorithm. VerifEye also calculates the rising and falling edge responses. VerifEye calculates the probability density function (PDF) for the receiver voltage, taking into account the conditional probabilities for the various kinds of transitions. The BER at a given sample time is the proportion of the voltage PDF that results in an incorrect bit being read. A plot of the BERs over all sample times generates the familiar bathtub curve.VerifEye simulates transmit jitter by adding the jitter distribution to the PDFs for the edge responses. With edge responses, random jitter is truly random (timings of edges are independent) and duty cycle distortion (DCD) is easy to calculate. QuickEye simulates transmit jitter using Monte Carlo techniques.
Channel Equalization
Both QuickEye and VerifEye have the ability to simulate the effects of equalization on the channel. Both analyses can add feed-forward equalization (FFE) and decision-feedback equalization (DFE).
Peak Distortion Analysis
Peak distortion analysis (PDA) to find the worst case bit pattern is automatically performed for both VerifEye and QuickEye analyses. For Quick Eye analyses, the worst case bit pattern can be pushed to all eye sources, overriding any bit lists set in the eye sources. For both QuickEye and VerifEye analyses, the worst case bit pattern can be displayed in reports.