Digital Communication
Eye Diagram
Ashutosh Rastogi
Quality Education for all…
Outline
• Inter-symbol Interference
• Strategies for Combating ISI
• Eye Diagram
• Formation of Eye Diagram
• What is Jitter?
• Application of Eye Diagram
22-11-2016 Good Education,Strong Nation 2
Inter-symbol Interference
• Inter-symbol interference (ISI) occurs when a pulse spreads
out in such a way that it interferes with adjacent pulses at the
sample instant.
• Example: assume polar NRZ line code. The channel outputs
are shown as spreaded (width Tb becomes 2Tb) pulses shown
(Spreading due to band-limited channel characteristics).
Channel Input Channel Output
Pulse width Tb Pulse width Tb
Data 1
Tb 0 Tb Tb 0 Tb
Data 0
Tb 0 Tb Tb 0 Tb
22-11-2016 Good Education,Strong Nation 3
Inter-symbol Interference
• For the input data stream:
1 0 1 1 0 1
A
0 Tb 2Tb 3Tb 4Tb 5Tb
• The channel output is the superposition of each bit’s output:
0 Tb 2Tb 3Tb 4Tb 5Tb Resultant Channel
Output Waveform
1 0 1 1 0 1
0 Tb 2Tb 3Tb 4Tb 5Tb
22-11-2016 Good Education,Strong Nation 4
Strategies for Combating ISI
• Three strategies for eliminating ISI:
• Use a line code that is absolutely bandlimited.
• Would require Sinc pulse shape.
• Can’t actually do this (but can approximate).
• Use a line code that is zero during adjacent sample
instants.
• It’s okay for pulses to overlap somewhat, as long as there is no
overlap at the sample instants.
• Can come up with pulse shapes that don’t overlap during adjacent
sample instants.
• Raised-Cosine Rolloff pulse shaping
• Use a filter at the receiver to “undo” the distortion
introduced by the channel.
• Equalizer.
22-11-2016 Good Education,Strong Nation 5
Introduction to Eye Diagram
• Today's serial data links operate at gigahertz transmission
frequencies.
• A serial digital signal can suffer impairments as it travels
from a transmitter to a receiver. The transmitter,
connectors, and cables will introduce interference that
will degrade a signal both in its amplitude and timing.
• By using an oscilloscope we can create an eye diagram
through which engineers can quickly evaluate system
performance and gain insight into the nature of channel
imperfections.
22-11-2016 Good Education,Strong Nation 6
Eye Diagram
• The ISI and other signal degradation can be easily studied
with the help of Eye Diagram.
• A random binary pulse sequence is sent over the
channel.
• An oscilloscope generates an eye diagram by overlaying
sweeps of different segments of a long data stream.
• The triggering edge may be positive or negative, but the
displayed pulse that appears after a delay period may go
either way.
• By either way we mean that there is no way of knowing
beforehand the value of an arbitrary bit.
22-11-2016 Good Education,Strong Nation 7
Eye Diagram
• Many such transitions have been overlaid, positive and
negative pulses are superimposed on each other.
• Overlaying many bits produces an eye diagram, so
called because the resulting image looks like the
opening of an eye.
• The shape of an eye diagram will depend upon various
types of triggering signals, such as clock triggers,
divided clock triggers, and pattern triggers. Differences
in timing and amplitude from bit to bit cause the eye
opening to shrink.
22-11-2016 Good Education,Strong Nation 8
Formation of Eye Diagram
• Eye diagrams usually include voltage and time
samples of the data acquired at some sample rate
below the data rate.
• The figure on the next slide shows the bit sequences
011, 001, 100, and 110 are superimposed over one
another to obtain the final eye diagram.
22-11-2016 Good Education,Strong Nation 9
Formation of Eye Diagram
Illustration on how an eye diagram is formed
22-11-2016 Good Education,Strong Nation 10
Formation of Eye Diagram
Illustration on how an eye diagram is formed
22-11-2016 Good Education,Strong Nation 11
What is Jitter?
• When high-speed digital signals are transmitted, the
impairments introduced at various stages lead to timing
errors.
• Jitter is timing error which results from the misalignment
of rise and fall times.
• Jitter occurs when a riding or falling edges occur at times
that differ from the ideal time.
• The deviation of the digital signals as a result of
reflections, inter-symbol interference, crosstalk, PVT
(process-voltage-temperature) variations, and other
factors amounts to jitter.
22-11-2016 Good Education,Strong Nation 12
Visualization of Jitter
a) Finite rise and fall times cause eye diagrams to look like this image rather than like
a rectangle.
b) Jitter results from the misalignment of rise and fall times.
c) Although the absolute timing error or jitter margin is less than that in image b,
this eye opening is smaller because of a higher bit rate.
22-11-2016 Good Education,Strong Nation 13
How to interpret a signal using Eye
Diagram
• An eye diagram can help you interpret a signal and determine the
best time for making a measurement.
22-11-2016 Good Education,Strong Nation 14
Applications of Eye-Diagram
• An eye diagram can reveal the important
information such as:
• It can indicate the best point for sampling, divulge
the SNR (signal-to-noise ratio) at the sampling
point
• indicate the amount of jitter and distortion.
• it can show the time variation at zero crossing,
which is a measure of jitter.
22-11-2016 Good Education,Strong Nation 15
References
• Lathi B.P. “Modern Digital and Analog
Communication Systems” Oxford Publications,Fourth
Edition
• Taub H.,Schilling D.L.,Saha G. “Taub’s Principle of
Communication Systems”, McGraw-Hill, Third edition
• http://www.tmworld.com/design/manufacturing/43
89368/Eye-Diagram-Basics-Reading-and-applying-
eye-diagrams
22-11-2016 Good Education,Strong Nation 16
22-11-2016 Good Education,Strong Nation 17
