B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

4.8 Phase-locked Loop and Some Applications 173
Fi g ure 4.25
L-carrier
hierarchical
long-haul analog
telephone
frequency
division
multiplexing
system.
60 64 68 72
Voice channels
76 80 84 88 92 96
100 104 108
Frequency, kHz
Frequency, kHz
312
552
Frequency, kHz
564
3084
We will first describe the analog telephone hierarchy that utilizes FDM and SSB modulation
here and defer the digital hierarchy discussion until later (Chapter 6).
In the analog L-carrier hierarchy, 4 each voice channel is modulated using SSB+C. Twelve
voice channels form a basic channel group occupying the bandwidth of 60 to 108 kHz. As
shown in Fig. 4.25, each user channel uses LSB, and frequency division multiplexing (FDM)
is achieved by maintaining the channel carrier separation of 4 kHz.
Further up the hierarchy, 5 five groups form a supergroup, via FDM. Multiplexing 10
supergroups generates a mastergroup, and multiplexing six supergroups forms a jumbo
group, which consists of 3600 voice channels over a frequency band of 16.984 MHz in the
L4 system. At each level of the hierarchy from the supergroup, additional frequency gaps are
provided for interference reduction and for inserting pilot frequencies. The multiplexed signal
can be fed into the baseband input of a microwave radio channel or directly into a coaxial
transmission system.
4.8 PHASE-LOCKED LOOP AND SOME
APPLICATIONS
Phase-Locked Loop (PLL)
The phase-locked loop (PLL) is a very important device typically used to track the phase and
the frequency of the carrier component of an incoming signal. It is, therefore, a useful device
for synchronous demodulation of AM signals with a suppressed carrier or with a little carrier
(the pilot). It can also be used for the demodulation of angle-modulated signals, especially
under conditions of low signal-to-noise ratio (SNR). It also has important applications in a
number of clock recovery systems including timing recovery in digital receivers. For these
reasons, the PLL plays a key role in nearly every modem digital and analog communication
system.
A PLL has three basic components:
1. A voltage-controlled oscillator (VCO).
2. A multiplier, serving as a phase detector (PD) or a phase comparator.
3. A loop filter H (s).