563489578934

176
Baseband Pulse and Digital Signaling Chap. 3
Waveform
0 1 1 0 0 1
Eye pattern
t
t
(a) Ideal Filtering
T b
T b
t
t
(b) Filtering with ISI
Best sampling time
t
Noise
margin
(c) Noise Plus ISI
Figure 3–18
Maximum distortion
Distorted polar NRZ waveform and corresponding eye pattern.
detected bit errors), the eye will be open. If there is a great deal of noise or ISI, the eye will
close; this indicates that bit errors will be produced at the receiver output. The eye pattern
provides an excellent way of assessing the quality of the received line code and the ability of
the receiver to combat bit errors. As shown in the figure, the eye pattern provides the following
information:
• The timing error allowed on the sampler at the receiver is given by the width inside the
eye, called the eye opening. Of course, the preferred time for sampling is at the point
where the vertical opening of the eye is the largest.
• The sensitivity to timing error is given by the slope of the open eye (evaluated at, or
near, the zero-crossing point).
• The noise margin of the system is given by the height of the eye opening.
Regenerative Repeaters
When a line code digital signal (such as PCM) is transmitted over a hardwire channel (such as
a twisted-pair telephone line), it is attenuated, filtered, and corrupted by noise. Consequently,
for long lines, the data cannot be recovered at the receiving end unless repeaters are placed in