Digital Signal Processing Reference
In-Depth Information
Fig. 3.8 Spectra associated
with SSBSC AM generator
X
(
f
)
1
f
0
P
(
f
)
0.5
f
0
−
f
c
f
c
Q
(
f
)
0.5
−0.5
f
0
−
f
c
f
c
Y
L
(
f
)
1
f
0
−
f
c
f
c
r
(
t
)
s
(
t
) =
y
L
(
t
)
LPF
0.5
x
(
t
)
cos (
ω
c
t
)
Fig. 3.9
SSBSC AM signal demodulation
The LPF in Fig.
3.9
will then filter out all the signal terms given in (
3.14
) except
the 0.5x(t) term. This remaining term is simply a scaled version of the original
signal.
3.4 Phase-Locked Loops
The phase-locked loop (PLL) represents a very important application in signal
processing and communications. The initial idea for the PLL came as early as 1919
in the context of synchronization of oscillators. The theory of PLL was based
initially on the theory of feedback amplifiers, and found early application in
communications and motor servo systems. Due to the rapid development of
integrated circuits (ICs) subsequent to the 1970s, PLLs gained a strong foothold in
many areas of applications. Some of these applications include filtering, frequency
synthesis,
motor-speed
control,
frequency
modulation,
demodulation,
signal
detection, frequency tracking, etc.
The PLL is basically a device that tracks the phase and frequency of an
incoming signal. It is a feedback system that adjusts the phase and frequency
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