An eye diagram displays the effects of intersymbol interference (ISI) in a data communication system. It is created by superimposing successive waveforms on an oscilloscope. The amount of ISI is seen by how open the eye pattern is - a wide open eye indicates less ISI, while a less open eye shows more ISI. An eye diagram provides information about the bit error rate, signal-to-noise ratio, optimal sampling time, and sensitivity to timing errors.

1. ISI and Eye Diagram

2. ISI

5. Eye diagram
• An eye pattern is a pattern displayed on the screen of a cathode ray oscilloscope (C.R.O.). The shape of this pattern resembles the
shape of the human eye. therefore, it is called an eye pattern.
• It is created simply by superimposing successive waveforms to form a composite image.
• The eye pattern is a practical way to study intersymbol interference (ISI) and its effects on a PCM or data communication system.
• The eye pattern is obtained on the C.R.O. by applying the received signal to vertical deflection plates (Y-Plates) of the C.R.O. and a
sawtooth wave at the transmission symbol rate i.e., (1/Tb) to the horizontal deflection plates (X-plates) as shown in figure(b) below.

8. Eeng 360 8
ISI on Eye Patterns
• The amount of ISI can be seen on an oscilloscope using an Eye
Diagram or Eye pattern.
Time (Tb)
Amplitude
Noise
Margin
Distortion
b
T Extension
Beyond Tb is
ISI

11. • An eye pattern provides the following information about a particular
system.
• Actual eye patterns are used to estimate the bit error rate and the
signal-to-noise ratio.
• The width of the eye opening defines the time interval over which
the received wave can be sampled without error from ISI.
• The instant of time when the eye opening is wide, will be the
preferred time for sampling.
• The rate of the closure of the eye, according to the sampling time,
determines how sensitive the system is to the timing error.
• The height of the eye opening, at a specified sampling time, defines
the margin over noise.

14. Eye diagram is wide open
Less ISI
Eye diagram is less open
More ISI