7.2 Performance Characteristics of Digital Receivers
287
powers. However, this can be misleading because the limitation on real components
is based on peak and not average power.
7.3 Principles of Eye Diagrams
The eye diagram is a powerful measurement tool for assessing the data-handling
ability of a digital transmission system. This method has been used extensively for
evaluating the performance of wire line systems and also applies to optical fiber
data links. Chapter 14 gives more details on BER test equipment and measurement
methods.
7.3.1 Features of Eye Patterns
The eye pattern measurements are made in the time domain and allow the effects
of waveform distortion to be shown immediately on the display screen of standard
BER test equipment. Figure 7.12 shows a typical display pattern, which is known as
an eye pattern or an eye diagram. The logic 1 and 0 levels, shown by b on and b off ,
respectively, determine the basic upper and lower bounds.
A great deal of system-performance information can be deduced from the eye
pattern display. To interpret the eye pattern, consider Fig. 7.12 and the simplified drawing shown in Fig. 7.13. The following information regarding the signal
amplitude distortion, timing jitter, and system rise time can be derived:
• The width of the eye opening defines the time interval over which the received
signal can be sampled without error due to interference from adjacent pulses
(known as intersymbol interference).
• The best time to sample the received waveform is when the height of the eye
opening is largest. This height is reduced as a result of amplitude distortion in
the data signal. The vertical distance between the top of the eye opening and the
maximum signal level gives the degree of distortion. The more the eye closes, the
more difficult it is to distinguish between ones and zeros in the signal.
• The height of the eye opening at the specified sampling time shows the noise
margin or immunity to noise. Noise margin is the percentage ratio of the peak
signal voltage V 1 for an alternating bit sequence (defined by the height of the eye
opening) to the maximum signal voltage V 2 as measured from the threshold level,
as shown in Fig. 7.13. That is
Noise margin (%) =
V 1
V 2
× 100 %
(7.24)
287
powers. However, this can be misleading because the limitation on real components
is based on peak and not average power.
7.3 Principles of Eye Diagrams
The eye diagram is a powerful measurement tool for assessing the data-handling
ability of a digital transmission system. This method has been used extensively for
evaluating the performance of wire line systems and also applies to optical fiber
data links. Chapter 14 gives more details on BER test equipment and measurement
methods.
7.3.1 Features of Eye Patterns
The eye pattern measurements are made in the time domain and allow the effects
of waveform distortion to be shown immediately on the display screen of standard
BER test equipment. Figure 7.12 shows a typical display pattern, which is known as
an eye pattern or an eye diagram. The logic 1 and 0 levels, shown by b on and b off ,
respectively, determine the basic upper and lower bounds.
A great deal of system-performance information can be deduced from the eye
pattern display. To interpret the eye pattern, consider Fig. 7.12 and the simplified drawing shown in Fig. 7.13. The following information regarding the signal
amplitude distortion, timing jitter, and system rise time can be derived:
• The width of the eye opening defines the time interval over which the received
signal can be sampled without error due to interference from adjacent pulses
(known as intersymbol interference).
• The best time to sample the received waveform is when the height of the eye
opening is largest. This height is reduced as a result of amplitude distortion in
the data signal. The vertical distance between the top of the eye opening and the
maximum signal level gives the degree of distortion. The more the eye closes, the
more difficult it is to distinguish between ones and zeros in the signal.
• The height of the eye opening at the specified sampling time shows the noise
margin or immunity to noise. Noise margin is the percentage ratio of the peak
signal voltage V 1 for an alternating bit sequence (defined by the height of the eye
opening) to the maximum signal voltage V 2 as measured from the threshold level,
as shown in Fig. 7.13. That is
Noise margin (%) =
V 1
V 2
× 100 %
(7.24)