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Baseband Pulse and Digital Signaling Chap. 3
deleted. Alternatively, as described in Sec. 4–3, a phase-locked loop (PLL) can be used to
extract the sync signal from the unipolar RZ line code by locking the PLL to the discrete spectral
line at f = R. For a polar NRZ line code, the bit synchronizer is slightly more complicated,
as shown in Fig. 3–20. Here the filtered polar NRZ waveform (Fig. 3–20b) is converted to a
unipolar RZ waveform (Fig. 3–20c) by using a square-law (or, alternatively, a full-wave rectifier)
circuit. The clock signal is easily recovered by using a filter or a PLL, since the unipolar
RZ code has a delta function in its spectrum at f = R. All of the bit synchronizers discussed
thus far use some technique for detecting the spectral line at f = R.
Another technique utilizes the symmetry property of the line code itself [Carlson,
1986]. Referring to Fig. 3–18, which illustrates the eye pattern for a polar NRZ code, we realize
that a properly filtered line code has a pulse shape that is symmetrical about the optimum
clocking (sampling) time, provided that the data are alternating between 1’s and 0’s. From
Fig. 3–21, let w 1 (t) denote the filtered polar NRZ line code, and let w 1 ( t 0 + nT b ) denote a
sample value of the line code at the maximum (positive or negative) of the eye opening, where
n is an integer, R = 1T b is the bit rate, t is the relative clocking time (i.e., clock phase), and
t0 is the optimum value corresponding to samples at the maximum of the eye opening.
Because the pulse shape of the line code is approximately symmetrical about the optimum
clocking time for alternating data,
|w 1 (t 0 + nT b -¢)| L |w 1 (t 0 + nT b +¢)|
where t0 is the optimum clocking phase and 0 6 ∆ 6 2 T b . The quantity w 1 ( t + nT b - ∆) is
called the early sample, and w 1 ( t + nT b + ∆) is called the late sample. These samples can be
used to derive the optimum clocking signal, as illustrated in the early–late bit synchronizer
1
Instantaneous
sampler
and hold
Late sample
|w 1 (tc+ nT b +)|
Full-wave
rectifier
|w 1 (tc+ nT b +)|
Late clock
Filtered
polar NRZ
waveform
w 1 (t) w 4 (t) w 3 (t) filter
clock (VCC)
w 2 (t)
Eye pattern
-
for w 1 (t)
Delay, –
Delay, +
Early clock
Clock
Voltage
controlled
Control
voltage
Low-pass
+
Output
clock
w 1 (t)
0 + nT b
Instantaneous
sampler
and hold
Early sample
|w 1 (t c + nT b -)|
Full-wave
rectifier
|w 1 (t c + nT b -)|
T b
Figure 3–21
Early–late bit synchronizer for polar NRZ signaling.