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3-11
3.5.2.
Phased Locked Loop
terminology (PLL)
Phase
Detector (PD)
A
circuit
which compares
the input
and
VCO
signals
and produces an
error voltage
which
is
dependent
on
their
relative
phase
difference.
This
error voltage corrects
the
VCO
frequency
during
tracking. Also
called Phase Comparator.
Low-Pass
Filter
(LPF)
A
low-pass
filter in
the loop
which
permits only dc
and low frequency
voltages to
travel
around
the
loop.
It
controls the capture range
and
the
noise
and outband
signal
rejection characteristics.
Voltage
Controlled
Oscillator
(VCO)
An
oscillator
whose
frequency
is
determined by an
applied control voltage.
Lock
Range
(2a; L)
The
range of frequencies over
which
the loop
will
remain
in
lock.
Normally
the lock range
is
centered
at
the
free-running
frequency
unless there
is
some
non-linearity
in
the
system which
limits
the
frequen-
cy
deviation
on one
side fo.
The
deviations
from
fo are referred to
as
the
Tracking
Range
or Hold-in
Range.
Free-Running Frequency
(fo,coo)
Also
called
tne Center
Frequency;
this
Is
the
frequency
at
which
the loop
VCO
operates
when
not
locked
to
an
input
signal.
VCO
Conversion Gain (Ko)
The
conversion
factor
between
VCO
frequency and
control voltage
in
radians/second/voit.
Phase
Detector Gain Factor (Kd)
The
conversion
factor
between
the
phase
detector
output
voltage
and
the phase difference
between
input and
VCO
signals
in
volts/radian.
At
low
input
signal
amplitudes, the
gain
is
also
a
function
of
input
level.
BLOCK DIAGRAM OF
PHASE
LOCKED LOOP
«•
INPUT
SIGNAL
ViW
OUTPUT
SIGNAL
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