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Sec. 3–5 Line Codes and Spectra 177
Low-level
distorted
input
Amplified filter
Filter
Sample and hold
Decision-making circuit
+
Comparator
–
High-level
regenerated
output
Clock
t
Figure 3–19
Bit synchronizer
(timing circuit)
V T
Regenerative repeater for unipolar NRZ signaling.
t
cascade along the line and at the receiver, as illustrated in Fig. 3–7. These repeaters amplify
and “clean up” the signal periodically. If the signal were analog instead of digital, only linear
amplifiers with appropriate filters could be used, since relative amplitude values would need
to be preserved. In this case, in-band distortion would accumulate from linear repeater to
linear repeater. This is one of the disadvantages of analog signaling. However, with digital
signaling, nonlinear processing can be used to regenerate a “noise-free” digital signal. This
type of nonlinear processing is called a regenerative repeater. A simplified block diagram of
a regenerative repeater for unipolar NRZ signaling is shown in Fig. 3–19. The amplifying
filter increases the amplitude of the low-level input signal to a level that is compatible with the
remaining circuitry and filters the signal in such a way as to minimize the effects of channel
noise and ISI. (The filter that reduces ISI is called an equalizing filter and is discussed in
Sec. 3–6.) The bit synchronizer generates a clocking signal at the bit rate that is synchronized,
so that the amplified distorted signal is sampled at a point where the eye opening is a maximum.
(Bit synchronizers are discussed in detail in the next section.) For each clock pulse, the
sample-and-hold circuit produces the sample value that is held (for T b , a 1-bit interval) until
the next clock pulse occurs. The comparator produces a high-level output only when the
sample value is larger than the threshold level V T . The latter is usually selected to be one-half
the expected peak-to-peak variation of the sample values. † If the input noise is small and there
is negligible ISI, the comparator output will be high only when there is a binary 1 (i.e., a high
level) on the corrupted unipolar NRZ line code at the input to the repeater. The comparator—
a threshold apparatus—acts as a decision-making device. Thus, the unipolar NRZ line code is
regenerated “noise free,” except for bit errors that are caused when the input noise and ISI
alter the sample values sufficiently so that the sample values occur on the wrong side of V T .
Chapter 7 shows how the probability of bit error is influenced by the SNR at the input to the
repeater, by the filter that is used, and by the value of V T that is selected. ‡
In long-distance digital communication systems, many repeaters may be used in cascade,
as shown in Fig. 3–7. Of course, the spacing between the repeaters is governed by the path loss
of the transmission medium and the amount of noise that is added. A repeater is required when
the SNR at a point along the channel becomes lower than the value that is needed to maintain the
† This is the optimum V T when the binary 1’s and 0’s are equally likely.
‡ For minimum probability of bit error, the sample-and-hold circuit of Fig. 3–19 is replaced by an optimum
sample-detection circuit called a matched filter (MF), described in Sec. 6–8.