High speed Designs - Part 2

Eye Diagram in communications field is one of the indicator of channel performance. The quality of the signal and in turn channel can be explained by the eye diagram. the eye diagram is a source to analyze the high speed designs. An eye diagram is formed by overlaying of number of bits that are transmitted across the channel. An eye diagram is a pattern that is formed in an oscilloscope which resembles the shape of an eye. If you have validated or performed any certification of high speed interfaces like PCIe, USB, etc., then you know well that eye diagram is very crucial to such analysis. In such certifications/validation a fixed data pattern is transmitted from the scope and received back. Example of such patterns is PRBS. Eye diagram basically is a voltage/time samples of the actual data. the eye diagram is representation of some million samples of data patterns captured. Oscilloscope is to be used in persistence mode to capture the sequence of samples.

What can be measured using an eye diagram?

1. Rise Time

2. Fall Time

3. Jitter

4. Over shoot

5. undershoot

6. Bit error rate

7. Cross talk

8. Inter symbol interference

9. Signal-to-Noise Ratio

What do we do/How do we analyze using eye diagram?

1. When we are measuring a interface performance, like for example, USB, we have to determine the acceptable levels of signal that help for exact reproduction of the signal at the receiver. For this we draw a small diagram called eye mask. If the generated eye diagram is well beyond this eye mask, then channel is performing well and doesn't have any signal integrity issues. If any of the eye diagram overlaps eye mask then there is a serious issue in the channel.

2. An eye opening (so called height of the eye or peak-to-peak value) is an indication of the channel performance. If an eye diagram generated is observed to be closed, then there is signal integrity issue in the channel.

3. The overshoot and undershoot indicates the impedance mismatch that is present in the channel because of which reflections occurred.

4. The eye width indicates the unit interval, and is an indication of any jitter present in the channel.

The basic test setup for generating eye diagram is:

The following image is a capture of eye diagram. Snapshot is taken from edn.com/keysight.com to give a view of the eye diagram analysis and capture.

Definitions used in eye diagram:

One level: The highest level (logic high voltage) in a peak-to-peak representation of the signal

Zero level: The lowest level (logic low voltage) in a peak-to-peak representation of the signal

Amplitude: The difference between one level and zero level.

Eye Height: Indication of eye opening. Under ideal conditions, Amplitude is equal to eye height. 

Eye crossing: Indication of jitter that is present in the channel.

Disadvantages of eye diagram:

1. Eye diagram can only judge but can't guarantee the channel performance.

2. Eye diagram is generally captured with a standard data pattern but in real time the pattern can vary.

3. Eye diagram can't determine the actual cause of the signal degradation in the channel.