Anatomy of an Eye Diagram - Application Note

Oct 25, 2026

Uploaded by Dipper

This application note explains what an eye diagram is, how to construct one, and the most common triggering approaches used to generate the display. It outlines how parameter information—such as effects of system bandwidth and signal distortion—can be extracted from the eye, while clarifying what logical or protocol issues may not be directly visible. It also describes how transmitters, channels, and receivers are tested using eye measurements and instrument automation.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages12

Reading Time30 min

AccessFree to download

Eye Measurements Basics

What does it show?

The Effects of Triggering on Eye Diagrams

Clock Trigger

Divided Clock Trigger

Pattern Trigger

Triggering on Data

Triggering from a Recovered Clock

Anatomy of an Eye Diagram Application Note Abstract This paper describes what an eye diagram is, how it is constructed, and common methods of triggering used to generate one. It then describes different ways that information from an eye diagram can be sliced to gain more insight. It also discusses some basic ways that transmitters, channels, and receivers are tested. It is designed to give an engineer new to this field a basic grasp of the concepts commonly used. Application Note Figure 1. Overlaying of bit sequences to form an eye diagram. Eye Measurements Basics Eye diagrams are a very successful way of quickly and intuitively assessing the quality of a digital signal. A properly constructed eye should contain every possible bit sequence from simple 101’s and 010’s, through to isolated ones after long runs of consecutive zeros and other problem sequences that often show up weaknesses present in system design. What does it show? Eye diagrams show parametric information about the signal – effects deriving from physics such as system 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 system meant to send a zero. However, if the physics of the system mean that a logic one becomes so distorted while passing through the system that the receiver at the far end mistakes it for a zero, this should be shown in a good eye diagram. Common ways of characterizing an eye are to measure the rise times, fall times, jitter at the middle of the crossing point of the eye, the overshoot present and many other numerical descriptions of eye behavior in order to compare devices being measured. Instruments usually offer automated measurements that simplify and speed up the taking of such measurements. [2](2 www.tektronix.com/bertscope)[ www.tektronix.com/bertscope](2 www.tektronix.com/bertscope) Anatomy of an Eye Diagram The Effects of Triggering on Eye Diagrams Many eye diagrams are constructed on test equipment that uses test patterns that repeat, such as PRBS patterns generated by a BERT (Bit Error Ratio Tester) . Such equipment can commonly generate a variety of trigger signals: 1. A clock trigger at the same rate as, and synchronous with the data signal 2. A divided clock trigger at some divide ratio of the data rate often related to a power of 2 such as ÷4, ÷16, etc. 3. A pattern trigger – a signal that provides a trigger once per pattern repetition. 4. The data itself can be used as a trigger. 5. The last option is to derive the trigger signal by using clock recovery on the data signal. See Figure 2. Each method provides different results when used to construct a waveform. Clock Trigger provides a classical eye diagram containing all possible bit transitions in one display. Divided Clock Trigger also produces an eye diagram, and this can be useful when the instrument being used to construct the eye has a trigger input bandwidth narrower than the data rate of the signal being viewed. This method can produce a good eye unless the pattern length divided by the divide ratio yields an integer – for example a 128-bit pattern viewed with a ÷4 clock. In this case, the trigger signal will coincide with the same bits in the pattern each time while consistently missing other parts of the pattern – leading to an incomplete eye. See Figure 3. Figure 2. An eye diagram formed with a full rate clock trigger. Figure 3. An eye diagram formed with a divided clock trigger. Figure 4. A bit sequence captured using a pattern trigger. Pattern Trigger: Used to display individual bits in the pattern. To view the whole pattern, the user must scroll through by altering either the time base of the scope or the pattern trigger position. Scrolling using the extent of a scope time base can lead to increased apparent jitter on the displayed signal due to weaknesses in time base circuitry; incrementinga pattern trigger, either with

FAQs

What is the purpose of an eye diagram?

An eye diagram provides a quick, intuitive assessment of digital signal quality by showing a wide range of possible bit sequences and transitions. Proper construction reveals weaknesses tied to system design and signal integrity.

What information does an eye diagram reveal, and what does it not show?

It shows parametric effects derived from physical behavior, such as bandwidth health and distortion. It does not directly reveal protocol or logical intent—for example, whether the transmitter meant to send a different bit value.

How do different triggering methods affect the resulting eye diagram?

Clock and divided-clock triggers typically produce a more complete view of transitions, while pattern triggers may require scrolling to view the full pattern. Triggering on data is less reliable for complete eyes, and using a recovered clock can introduce effects related to clock-recovery loop bandwidth and timing relationships.

Oct 25, 2026

Uploaded by Dipper