What is Eye Diagram

CreateDate:2016-08-08

Eye Measurements Basics

        Eye diagrams are avery successful way of quickly and intuitively assessing the quality of adigital signal. A properly constructed eye should contain every possible bitsequence from simple 101’s and 010’s, through to isolated ones after long runsof consecutive zeros and other problem sequences that often show up weaknessespresent in system design.

Figure 1. Overlayingof bit sequences to form an eye diagram


What does it show?

        Eye diagrams showparametric information about the signal – effects deriving from physics such assystem bandwidth health, etc. It will not show protocol or logical problems –if a logic 1 is healthy on the eye, this does not reveal the fact that the systemmeant to send a zero. However, if the physics of the system mean that a logicone becomes so distorted while passing through the system that the receiver atthe far end mistakes it for a zero, this should be shown in a good eye diagram.

Figure 2. EyeDiagram of ModuleTek’s XFP-10G-ZR

        Common ways of characterizingan eye are to measure the rise times, fall times, jitter at the middle of thecrossing point of the eye, the overshoot present and many other numericaldescriptions of eye behavior in order to compare devices being measured.Instruments usually offer automated measurements that simplify and speed up thetaking of such measurements.

 

Eye Diagrams and BER

        While eye diagramsprovide an accessible and intuitive view of parametric performance, systemsultimately are judged on their ability to pass bits faithfully, and withouterror. The BER, Bit Error Ratio, or Bit Error Rate as it is sometimes called,is a ratio of the number of bits received incorrectly (errors) divided by thetotal number of bits received. This provides an overall score for how well asystem is performing, but provides little help on why performance might bebelow expectations. It should be noted that BER tests logical problems as wellas parametric ones – whether the correct bit was sent in the first place.

        So why don’t eyediagrams and BER easily link together? A perfect eye diagram would show all parametricaspects of all possible bit sequences, irrespective of how infrequently someeffects show up. In other words, it would have a high information depth.Typically, eye diagrams are composed of voltage/time samples of the originaldata, acquired at some sample rate that is orders of magnitude below the datarate. For sampling oscilloscopes, this can be 105 samples per second at a 10Gb/s (1010 bits/second) rate. This means that most eye diagrams are composed ofshallow amounts of data.

        This becomes a problemwhen issues arise that are infrequently occurring. These can be patternrelated, noise related or deriving from other effects such as crosstalk andother forms of interference. These may not be visible in an oscilloscope eyediagram, but prevent link performance to desired levels. For example, links areoften required to work to better than one error in a million, million bits(1x10-12 BER), while eye diagrams struggle to show events with probabilitiesbelow an occurrence of 1x10-5.

        This leaves an informationgap that can be solved in a number of ways. The first is to construct the eyediagram from voltage/ time measurements taken with an architecture that has ahigher sampling efficiency. Systems such as the BERTScope are capable ofperhaps three orders of magnitude more depth than sampling scopes.

        The secondsolution is to use the fact that a BERT samples data directly at the line rate,and so is better equipped to see rare events that will limit systemperformance.


Conclusion

There can be animmense amount of information stored in an eye diagram if it is takencorrectly, and to enough data depth. This can tell a designer a lot about theparametric performance of his design, and a manufacturing engineer whetherparts will cause problems in the field later.


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