The eye opening looks at the region in the interior of the eye diagram. However, the typical metric used to read an eye diagram is its mask, or eye opening. This gives you a measure of noise that exists at the receiver due to intersymbol interence, crosstalk, and any phenomenological noise added to the channel (level jitter on the driver's I/O power rail). The eye diagram you generate for a high-speed channel illustrates the statistics of signal transitions between different level and the statistics for voltages at each logic level. The ability to see this is what makes eye diagrams such a fundamental part of signal integrity! How to Read an Eye Diagram In other words, the output signals exactly match the input signals. This raises a converse question: what would constitute an objectively desirable eye diagram? Ideally, you would have zero signal distortion, zero jitter, zero pulse spreading, and zero amplitude noise. The ISI you find in a channel is a summative metric, Jason Ellison provides a good overview and comparison with insertion loss deviation in this article. By examining intersymbol interference resulting from successive bits, it’s possible to identify specific problems in a digital channel. The condition where successive signals interfere with each other due to signal integrity problems is intersymbol interference. For example, dynamic feedback equalization (DFE) is used for 400G with PAM-4. Techniques like equalization and pre-emphasis are two ways to decrease BER values. This value will depend on several factors, but a desirable value can be as small as 10-12 or lower. Bit error rate (BER): By comparing logic thresholds with the received bits in the eye diagram, it’s possible to determine a bit error rate.Average symbol duration: This is the time between the midpoints of consecutive signal crossings.If there is strong reflection, noise, or ISI, the rise/fall times may not be smooth and could exhibit plateaus or strong variance. It is related to both the channel response and noise in the system. Average rise/fall time: This is equal to the time between the average 90% signal level time and the average 10% signal level time.Signal levels can also vary depending on impedance mismatches. This is, in general, some function of the timing jitter plus other random noise. Signal level variance: You’ll be able to easily see how the signal level varies. This would account for both random noise and timing skew in a differential pair. Timing jitter: The variation in rise/fall initiation can be seen directly from an eye diagram when you look at the signal crossings during switching.You can extract the following information directly from an eye diagram measurement: This diagram helps you quantify a wealth of information from a single measurement. This image was adapted from the eye diagram in Jason Ellison’s article on COM. The eye diagram (left) shown here was used to capture statistics for the 0 level (right). The resulting normal distribution is overlaid on the data below. From this histogram you could fit the data to a normal distribution using calculations of the sample standard deviation and the average signal level. The image below shows an example eye diagram and a histogram of measurements taken from the LOW signal level in the trace. With overlaid signal traces, it’s possible to take statistics at various points along the time domain measurements. Variances that can lead to bit error rates are the main quantities to be determined from this measurement. By overlaying the rising and falling edges, it’s easy to visualize the level of variance in signal behavior. A signal integrity simulator can perform the same type of superposition of signal levels. This involves superimposing rising and falling edges of a bitstream in a time domain sampling trace, such as with an oscilloscope. One of the fundamental measurements used to qualify channel designs in digital systems is an eye diagram. Eye Diagrams in Signal Integrity Analysis What is an Eye Diagram? In this article, I’ll run over some of the fundamental measurements that you could manually extract from an eye diagram and how they reveal some strategies for improving channel designs. The measurement shows many different factors that can affect signal behavior simultaneously, ultimately allowing for qualification of errors and losses in a channel. The eye diagram is a useful measurement or simulation as part of channel compliance. Things like S-parameters and impedance are useful, but there is one important measurement to be evaluated with a digital bitstream: an eye diagram. There are many ways to characterize high speed digital channels, with the goal being to verify specific signal integrity metrics that illustrate channel compliance.
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