Eye Diagrams 101 “The Basics” Jonathan Nerger Applications Engineer High Speed Interface

What is an Eye Diagram? An Eye Diagram is a representation of a digital signal for use in both qualitative analysis and at-a-glance evaluation of the signal’s quality. A Sampling Oscilloscope is used to create an Eye Diagram by displaying and overlaying the results of a continuous sampling of the signal. This is done at a resolution of a single Unit Interval (UI) of a one data bit to allow for all combinations of low-to-high and high-low transitions to occur. There are 8 basic 3-bit transition combinations that will create a traditional eye.

Formation of an Eye Diagram

Formation of an Eye Diagram

Formation of an Eye Diagram The Resultant Eye Diagram: where all 8 bit transitions are superimposed to create diagram that resembles an “eye.”

Extracting Data from the “EYE”

What Data Can I Obtain From The Eye? Jitter: Jitter is a measure of signal quality that is derived from the amount of variance in the signal’s characteristics. The more precise a signal’s transitions are in time relative to the other transitions, the less jitter there will be. For example, in the theoretical “zero” jitter case, all of the bit transitions will lay perfectly on top of each other in the eye diagram for very precise transition times. If however, the signal transitions are not precise, the eye diagram edges will appear thicker because the signal is transitioning earlier or later in time relative to the other bit transitions. Jitter is comprised of Random Jitter (RJ) and Deterministic Jitter (DJ) components. The majority of the DJ is caused by the signal loss due to the PCB trace. The specific data pattern also causes the DJ to change because of it’s unique and different combinations of low-high and high-low transitions.

What Data Can I Obtain From The Eye? JITTER This signal has a Total of 48.2pS of Jitter after 4 inches of FR-4 PCB trace. This signal has a Total of 84.4pS of Jitter after 14 inches of FR-4 PCB trace.

What Data Can I Obtain From The Eye? Voltage Swing: The VOH (Voltage High Level) and the VOL (Voltage Low Level) can easily be observed for the various bit transitions to ensure the signal meets the required “high” and “low” levels. The VOD (Differential Output Voltage) can also be seen in the case of a differential signal eye diagram. The “Eye Height” is the height of the vertical opening typically found in the center of the eye and hopefully in the middle of the UI. The “Eye Amplitude” is the distance between the two eye levels (the level “1” and the level “0”)

What Data Can I Obtain From The Eye? VOLTAGE SWING Eye Height This signal is being measured after 4 inches of FR-4 PCB trace and has an Eye Height of 435.7mV. This signal has an Eye Height of only 200mV after being run through 14 inches of FR-4 PCB trace.

What Data Can I Obtain From The Eye? VOLTAGE SWING Eye Amplitude This signal is being measured after 4 inches of FR-4 PCB trace and has an Eye Amplitude of 493.2mV. This signal has an Eye Height of only 335.4mV after being run through 14 inches of FR-4 PCB trace.

What Data Can I Obtain From The Eye? Transition Time: The transition time is the rise and fall time that is associated with the signals and measured using the rising and falling slope of the transitioning signals in the eye diagram. The faster the rise time, the more steep the slope. A “zero” rise time will have a 90 degree slope, however, this is not possible because all transitions have some time associated with them. Faster rise times result in shorter transition times and more “open” eye diagrams. This is because the digital signal is valid for longer time during that bit period. The “Eye Width” measurement is typically used to analyze the quality of the transition times. The faster the transition time, the larger the eye width will be. In contrast, the slower the transition, the smaller the eye width will be.

What Data Can I Obtain From The Eye? TRANSITION TIME This signal has relatively fast transitions times resulting in an Eye Width of 298pS after 4 inches of FR-4 PCB trace. This signal has relatively slow transition times resulting in an Eye Width of 211pS after 14 inches of FR-4 PCB trace.

Channel Loss

What causes the Eye to close? There are many different reasons an Eye “closes” and it is generically referred to as “Channel Loss” Parasitic effects associated with the transmission media such as capacitance and inductance are responsible for the majority of the signal loss. Generically, the longer the transmission length is, the more smaller the eye will become. Other more complicated causes of eye closure exist that are beyond the scope of this basic discussion. Techniques such as Pre-emphasis, De-emphasis, and Receiver Equalization can overcome some of the parasitic effects of the transmission channel that will allow for reliable data sampling from an otherwise overly-closed eye.

Effects of Channel Loss on the Eye 4 inches of FR-4 PCB Trace 8 inches of FR-4 PCB Trace 12 inches of FR-4 PCB Trace 16 inches of FR-4 PCB Trace

Effects of Channel Loss on the Eye 20 inches of FR-4 PCB Trace 24 inches of FR-4 PCB Trace Notice how the Eye Closes up both vertically due to the signal swing diminishing, and horizontally as the rise and fall times increase and the transition times become longer.