Understanding eye diagram

With the recent trends of increase in complexity of designs, accelerated data rates going up to Gbps and reduced time to market, the designers must have a tool in hand which can predict the problems and report before production. Eye diagram is one such tool which analyzes the quality of high speed signal over a channel. It may be any interface standard, any data rate, it has to comply with the certification standard before releasing to market. Eye diagram comes handy in these cases. 

Eye diagram can be thought of as a amplitude vs time graph over a oscilloscope formed by continuously sampling a high speed signal. Using this user can analyze,

• Transmission line effects
• Termination requirements and effects
• Any impedance mismatch over the line
• Routing problems on a PCB
• Grounding requirements
• Jitter of the signal
• Any crosstalk, inter symbol interference effects
• Signal-to-Noise ratio of a channel
• Overshoot and Undershoot 

The below diagram shows a noisy eye diagram visualized over a oscilloscope:

Few points to remember while using a scope for measuring eye diagram:

• Scope used must have sufficient bandwidth
• Probes with low capacitance
• Scope calibration before measurement
• Keep a check on the probes, oscilloscope vendor may suggest a specific probes for measurement
• To do the eye diagram, user has to transmit data pattern specific to the interface being tested
• Oscilloscope vendors like Agilent, Tektronix provide different applications for interface wise eye diagram measurements
• For eye diagram analysis of a specific interface, user has to transmit data at a rate specific to that interface for unbiased analysis

Eye diagram measurements:

One level:Mean value of high level measured over a channel

Zero Level:Mean value of zero level measured over a channel

Eye Amplitude: Difference between high and low level in the eye diagram. It should not exceed interface specific standards.

Eye height: Measure of eye vertical opening. Lower the height, higher the noise over the channel. So, an eye must never get closed, which indicates noisy channel.

Eye width:Mean of crossing points of sampled digital signals.

Bit Period: This indicates horizontal opening and indicates the data rate in the channel.

Rise Time:Mean transition time in the upward direction

Fall time:Mean transition time in the downward direction

Jitter: Measured in picoseconds, this indicates deviation from the actual bit event.

Compliance mask overlay:

For a specific interface, a standard will be defined and the signal timing and other parameters have to comply to that standards. Compliance mask is like a ideal situation as per the interface standard.The time and amplitude limits as per the standards need to be maintained. By seeing the above figure, you can see a keep out area for eye and then maximum and minimum amplitude requirements. If signal either comes into keepout area or crosses the maximum and minimum limits, then that interface is not compliant as per interface standard.

So, regarding eye diagram we have to keep in mind that it is indicator of system performance.