7.3 Principles of Eye Diagrams
289
uncertainties in clock timing. This timing uncertainty will cause a receiver to lose
synchronization with the incoming bit stream thereby incorrectly interpreting
logic 1 and 0 pulses. If the signal is sampled in the middle of the time interval
(i.e., midway between the times when the signal crosses the threshold level), then
the amount of distortion T at the threshold level indicates the amount of jitter.
Timing jitter is thus given by
Timing jitter (%) =
T
T b
× 100%
(7.25)
where T b is one bit interval.
• Traditionally, the rise time is defined as the time interval between the points where
the rising edge of the signal reaches 10% of its final amplitude to the time where
it reaches 90% of its final amplitude. However, when measuring optical signals,
these points are often obscured by noise and jitter effects. Thus, the more distinct
values at the 20 and 80% threshold points normally are measured. To convert from
the 20 to 80% rise time to a 10 to 90% rise time, one can use the approximate
relationship
T 10−90 = 1.25 × T 20−80
(7.26)
A similar approach is used to determine the fall time.
• Any nonlinear effects in the channel transfer characteristics will create an asymmetry in the eye pattern. If a purely random data stream is passed through a purely
linear system, all the eye openings will be identical and symmetrical.
Example 7.10 Consider an eye diagram in which the center opening is about 90%
due to intersymbol interference (ISI) degradation. What is the ISI degradation in
decibels?
Solution The ISI degradation is given by
ISI = 20 log
V 1
V 2
= 20 log 0.90 = 0.915 dB
Modern bit-error rate measurement instruments construct and display eye
diagrams such as the example shown in Fig. 7.14. Ideally, if the signal impairments
are small, the received pattern on the instrument display should exhibit sharp, clearly
defined lines. However, time-varying signal impairments in the transmission path can
lead to amplitude variations within the signal and to timing skews between the data
signal and the associated clock signal. Note that a clock signal, which typically is
encoded within a data signal, is used to help the receiver interpret the incoming data
correctly. Thus in an actual link the received pattern will become wider or distorted
on the sides and on the top and bottom, as shown in Fig. 7.14.
289
uncertainties in clock timing. This timing uncertainty will cause a receiver to lose
synchronization with the incoming bit stream thereby incorrectly interpreting
logic 1 and 0 pulses. If the signal is sampled in the middle of the time interval
(i.e., midway between the times when the signal crosses the threshold level), then
the amount of distortion T at the threshold level indicates the amount of jitter.
Timing jitter is thus given by
Timing jitter (%) =
T
T b
× 100%
(7.25)
where T b is one bit interval.
• Traditionally, the rise time is defined as the time interval between the points where
the rising edge of the signal reaches 10% of its final amplitude to the time where
it reaches 90% of its final amplitude. However, when measuring optical signals,
these points are often obscured by noise and jitter effects. Thus, the more distinct
values at the 20 and 80% threshold points normally are measured. To convert from
the 20 to 80% rise time to a 10 to 90% rise time, one can use the approximate
relationship
T 10−90 = 1.25 × T 20−80
(7.26)
A similar approach is used to determine the fall time.
• Any nonlinear effects in the channel transfer characteristics will create an asymmetry in the eye pattern. If a purely random data stream is passed through a purely
linear system, all the eye openings will be identical and symmetrical.
Example 7.10 Consider an eye diagram in which the center opening is about 90%
due to intersymbol interference (ISI) degradation. What is the ISI degradation in
decibels?
Solution The ISI degradation is given by
ISI = 20 log
V 1
V 2
= 20 log 0.90 = 0.915 dB
Modern bit-error rate measurement instruments construct and display eye
diagrams such as the example shown in Fig. 7.14. Ideally, if the signal impairments
are small, the received pattern on the instrument display should exhibit sharp, clearly
defined lines. However, time-varying signal impairments in the transmission path can
lead to amplitude variations within the signal and to timing skews between the data
signal and the associated clock signal. Note that a clock signal, which typically is
encoded within a data signal, is used to help the receiver interpret the incoming data
correctly. Thus in an actual link the received pattern will become wider or distorted
on the sides and on the top and bottom, as shown in Fig. 7.14.