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The Phase Locked Loop (PLL)
synchronizes a local
oscillator with a remote
one.
This ensures that the
local oscillator is at
the same frequency and
in phase with the remote
one.
The local oscillator is
voltage controlled (it
is a VCO).
This means that its
frequency is controlled
by varying a DC voltage
input.
The output signal of the
VCO is fed back to a
phase detector via a
buffer.
The buffer isolates the
VCO from the loading
caused by the detector
and external circuits.
It avoids "pulling" of
the oscillator
frequency.
If there is no reference
input signal then the
VCO will oscillate at
its natural "free
running" frequency.
The other input to the
phase detector is the
reference signal, which
we wish to lock the
frequency of the VCO
to.
If there is a difference
in frequency or phase
between the two inputs
then an error signal is
produced at the output
of the phase detector.
This error signal is fed
to the VCO via a filter
and a DC amplifier to
produce an error
correcting voltage.
The filter is a low pass
type which determines
the range of frequencies
over which the VCO can
vary.
The DC amplifier
amplifies the DC voltage
level to a value
suitable to control the
VCO.
The error correcting
voltage may be either
negative or positive
depending on whether the
VCO frequency is higher
or lower than the
reference frequency.
The effect of the error
correcting voltage is to
pull the VCO back to the
same frequency as the
reference frequency, and
in phase with it.
If the input signal is
an FM signal, then the
VCO follows the
deviations in frequency
caused by the
modulation, and the DC
output of the DC
amplifier is the
demodulated audio
signal.
The PLL can also be used
to keep an AC motor at a
constant speed.
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