Phase locked
loop (PLL) is used for synchronous demodulation of AM
signals with suppressed carrier or little carrier. It
tracks the phase and the frequency of the carrier components
of an incoming signal. PLL is also used for the demodulation
of the angle modulated signal. It is used where loss
along the transmission path is very large. It is suited
best under low signal to noise ratio. PLL is especially
used for space-vehicle to earth data link.
A PLL has
three basic blocks:
A loop filter
A voltage controlled oscillator (VCO)
A phase detector or phase comparator
Block diagram
of basic PLL operation is given below. It is a feedback
system; the output is compared with the input. If the
signal feedback is not equal to the input signal, the
difference i.e. error will change the signal
feedback until it is close to the input signal. The
feedback energy is phase; it is not the amplitude as
in normal system. A voltage controlled oscillator (VCO)
is an oscillator whose frequency can be controlled by
an external voltage. In PLL the VCO adjust its own frequency
until it is equal to the input sinusoidal frequency,
with the result, the frequency and phase of the two
signals are in synchronism. A mathematical expression
will explain how a PLL tracks the incoming signal angle
and instantaneous frequency.
Block
diagram of basic PLL operation
Let the input to the PLL
be
and let the VCO output be a sinusoid of,
then two are multiplied in a multiplier, which is further
low-pass
filtered by the loop filter
and generate out .
The multiplier output m (t) is given by:
The last term
in the upper expression is the high frequency signal
term, which is suppressed by narrow-band low-pass filter.
Hence the output,
Where
is the phase error.
A plot of graph
shows
how PLL track the incoming signal.
Suppose the
loop is locked it means frequencies of both the input
and output are
identical
i.e.
are in phase. Let O be the operating point in graph.
Suppose
further
that the input sinusoidal frequency suddenly increases
from,
the
incoming
signal, changes to
Thus the increase in the incoming
frequency
causes
to increase to ,
thereby increasing.
Now the operating
point
O in the graph shifts upward, this increases.
Now the output of the VCO
is
a sinusoid of frequency
given by
Where, C is
a constant of the VCO and
is the free running frequency of the VCO.
Thus, the output
of the VCO matches the increase in the input frequency.
Thus, the PLL tracks the input sinusoid. The two signals
are said to be mutually phase coherent or in phase lock.
A PLL can track the incoming frequency only over a finite
range of frequency shift. This range is called the lock
range or hold in range. Lock range is different
for various types of PLL. Also if the input frequency
changes too, lock is called the pull-inor capture range.
The PLL is very useful in communication circuits. It
can also be used as an FM demodulator and frequency
synthesizer. Frequency multipliers and dividers can
also be built using the PLL IC's.
and let the VCO output be a sinusoid of, 
then two are multiplied in a multiplier, which is further
low-pass 
.
The multiplier output m (t) is given by:
is the phase error. 
shows
how PLL track the incoming signal.
are in phase. Let O be the operating point in graph.
Suppose
,
the
Thus the increase in the incoming 
to increase to 
,
thereby increasing
.
Now the operating
.
Now the output of the VCO
given by
is the free running frequency of the VCO.