The phase locked loop or PLL is a particularly useful circuit block that is
widely used in radio frequency or wireless applications.
In view of its usefulness, the phase locked loop or PLL is found in many
wireless, radio, and general electronic items from mobile phones to broadcast
radios, televisions to Wi-Fi routers, walkie talkie radios to professional
communications systems and vey much more.
1. Phase Locked Loop Application:
The phase locked loop take in a signal to which it locks and can then output
this signal from its own internal VCO. At first sight this may not appear
particularly useful, but with a little ingenuity, it is possible to develop a large
number of phase locked loop applications.
Some phase lock loop applications include:
FM demodulation: One major phase locked loop application is that
of a FM demodulator. With PLL chips now relatively cheap, this
PLL applications enables high quality audio to be demodulated from
an FM signal.
AM demodulation: Phase locked loops can be used in the
synchronous demodulation of amplitude modulated signals. Using
this approach, the PLL locks onto the carrier so that a reference
within the receiver can be generated. As this corresponds exactly to
the frequency of the carrier, it can be mixer with the incoming signal
to synchronous demodulate the AM.
Indirect frequency synthesizers: Use within a frequency
synthesizer is one of the most important phase locked loop
applications. Although direct digital synthesis is also used, indirect
frequency synthesis forms one of the major phase locked loop
applications.
Signal recovery: The fact that the phase locked loop is able to lock
to a signal enables it to provide a clean signal, and remember the
signal frequency if there is a short interruption. This phase locked
loop application is used in a number of areas where signals may be
interrupted for short periods of time, for example when using pulsed
transmissions.
Timing distribution: Another phase locked loop application is in
the distribution precisely timed clock pulses in digital logic circuits
and system, for example within a microprocessor system.
2. Basic Concept – Phase:
The key to the operation of a phase locked loop, PLL, is the phase difference
between two signals, and the ability to detect it. The information about the
error in phase or the phase difference between the two signals is then used to
control the frequency of the loop.
To understand more about the concept of phase and phase difference, it is
possible to visualise two waveforms, normally seen as sine waves, as they
might appear on an oscilloscope. If the trigger is fired at the same time for
both signals they will appear at different points on the screen.
The linear plot can also be represented in the form of a circle. The beginning
of the cycle can be represented as a particular point on the circle and as a time
progresses the point on the waveform moves around the circle. Thus a
complete cycle is equivalent to 360° or 2π radians. The instantaneous position
on the circle represents the phase at that given moment relative to the
beginning of the cycle.
The concept of phase difference takes this concept a little further. Although
the two signals we looked at before have the same frequency, the peaks and
troughs do not occur in the same place.
There is said to be a phase difference between the two signals. This phase
difference is measured as the angle between them. It can be seen that it is the
angle between the same point on the two waveforms. In this case a zero
crossing point has been taken, but any point will suffice provided that it is the
same on both.
This phase difference can also be represented on a circle because the two
waveforms will be at different points on the cycle as a result of their phase
difference. The phase difference measured as an angle: it is the angle between
the two lines from the centre of the circle to the point where the waveform is
represented.
When there two signals have different frequencies it is found that the phase
difference between the two signals is always varying. The reason for this is
that the time for each cycle is different and accordingly they are moving
around the circle at different rates.
It can be inferred from this that the definition of two signals having exactly
the same frequency is that the phase difference between them is constant.
There may be a phase difference between the two signals. This only means
that they do not reach the same point on the waveform at the same time. If the
phase difference is fixed it means that one is lagging behind or leading the
other signal by the same amount, i.e. they are on the same frequency.
3. PLL Basic:
A phase locked loop, PLL, is basically of form of servo loop. Although a PLL
performs its actions on a radio frequency signal, all the basic criteria for loop
stability and other parameters are the same. In this way the same theory can
be applied to a phase locked loop as is applied to servo loops.
A basic phase locked loop, PLL, consists of three basic elements:
Phase comparator / detector: As the name implies, this circuit
block within the PLL compares the phase of two signals and
generates a voltage according to the phase difference between the
two signals. This circuit can take a variety of forms.
Voltage controlled oscillator, VCO: The voltage controlled
oscillator is the circuit block that generates the radio frequency
signal that is normally considered as the output of the loop. Its
frequency can be controlled over the operational frequency band
required for the loop.
Loop filter: This filter is used to filter the output from the phase
comparator in the phase locked loop, PLL. It is used to remove any
components of the signals of which the phase is being compared
from the VCO line, i.e. the reference and VCO input. It also governs
many of the characteristics of the loop including the loop stability,
speed of lock, etc.
4. PLL Operation:
The basic concept of the operation of the PLL is relatively simple, although
the mathematical analysis and many elements of its operation are quite
complicated
The diagram for a basic phase locked loop shows the three main element of
the PLL: phase detector, voltage controlled oscillator and the loop filter.
In the basic PLL, reference signal and the signal from the voltage controlled
oscillator are connected to the two input ports of the phase detector. The
output from the phase detector is passed to the loop filter and then filtered
signal is applied to the voltage controlled oscillator.
The Voltage Controlled Oscillator, VCO, within the PLL produces a signal
which enters the phase detector. Here the phase of the signals from the VCO
and the incoming reference signal are compared and a resulting difference or
error voltage is produced. This corresponds to the phase difference between
the two signals.
The error signal from the phase detector passes through a low pass filter
which governs many of the properties of the loop and removes any high
frequency elements on the signal. Once through the filter the error signal is
applied to the control terminal of the VCO as its tuning voltage. The sense of
any change in this voltage is such that it tries to reduce the phase difference
and hence the frequency between the two signals. Initially the loop will be out
of lock, and the error voltage will pull the frequency of the VCO towards that
of the reference, until it cannot reduce the error any further and the loop is
locked.
When the PLL, phase locked loop, is in lock a steady state error voltage is
produced. By using an amplifier between the phase detector and the VCO, the
actual error between the signals can be reduced to very small levels. However
some voltage must always be present at the control terminal of the VCO as
this is what puts onto the correct frequency.
The fact that a steady error voltage is present means that the phase difference
between the reference signal and the VCO is not changing. As the phase
between these two signals is not changing means that the two signals are on
exactly the same frequency.
The phase locked loop, PLL is a very useful building block, particularly for
radio frequency applications. The PLL forms the basis of a number of RF
systems including the indirect frequency synthesizer, a form of FM
demodulator and it enables the recovery of a stable continuous carrier from a
pulse waveform. In this way, the phase locked loop, PLL is an essential RF
building tool.
