A PLL or phase-locked loop is a control system that generates an output signal whose phase is related to the phase of an input signal. It consists of three basic elements: a phase detector that compares the phase of two signals and generates an error signal, a loop filter that filters the error signal, and a voltage-controlled oscillator whose frequency is controlled by the filtered error signal. PLLs are commonly used in applications such as frequency synthesis, signal demodulation, and motor speed control.

1. Phase locked loop basics
• PLL stands for 'Phase-Locked Loop' and is basically a closed
loop frequency control system,
• which functioning is based on the phase sensitive detection of
phase difference between the input and output signals of the
controlled oscillator (CO)
• The Phase Locked Loop method of frequency synthesis is now
the most commonly used method of producing high frequency
oscillations in modern communications equipment
• PLL circuits are now frequently being used to demodulate FM
signals, making obsolete the Foster-Seerly and radio detectors of
the early years. Other applications for PLL circuits include AM
demodulators, FSK decoders, two-tone decoders and motor
speed controls.
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3. 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.
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5.  A PD is a circuit that senses two periodic inputs and produces an output
whose average value is proportional to the difference between the phases of
the inputs
 The input/output characteristic of the PD is ideally a straight line, with a
slope called the “gain” and denoted by KPD

6.  We seek a circuit whose average output is proportional to the input phase
difference.
 An Exclusive-OR (XOR) gate can serve this purpose. It generates pulses
whose width is equal to Δϕ

7. Loop filter:
This filter is used to filter the output from the phase comparator in
the PLL. It is used to remove any components of the signals of
which the phase is being compared from the VCO line. It also
governs many of the characteristics of the loop and its stability.
Voltage controlled oscillator (VCO): The voltage controlled
oscillator is the circuit block that generates the output radio
frequency signal. Its frequency can be controlled and swung over
the operational frequency band for the loop.
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9.  Negative feedback loop: if the “loop gain” is sufficiently high, the
circuit minimizes the input error.
 The PD produces repetitive pulses at its output, modulating the VCO
frequency and generating large sidebands.
 Interpose a low-pass filter between the PD and the VCO to suppress
these pulses.
A student reasons that the negative feedback loop must force the phase error to zero, in
which case the PD generates no pulses and the VCO is not disturbed. Thus, a low-pass
filter is not necessary.
As explained later, this feedback system suffers from a finite loop gain, exhibiting a finite phase
error in the steady state. Even PLLs having an infinite loop gain contain nonidealities that disturb
Vcont

10.  We say the loop is “locked” if ϕout(t)-ϕin(t) is constant with time.
 An important and unique consequence of phase locking is that the input
and output frequencies of the PLL are exactly equal.

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13. A voltage-controlled oscillator or VCO is
an electronic oscillator whose oscillation frequency is
controlled by a voltage input. The applied input
voltage determines the instantaneous oscillation
frequency. Consequently, modulating signals applied
to control input may cause frequency
modulation (FM) or phase modulation (PM).
A VCO may also be part of a phase-locked loop.
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15.  What is a PLL?
 Control System Representation
 Parts of a PLL
 PLL in Simulink
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16.  Digital frequency control system
 Generate high speed oscillations
 Acquire and track signals
 Radio Frequency Demodulation
 DX-ing
 RF communications
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20.  Phase Detector
 Filter
 Voltage Controlled Oscillator
 Programmable Counter
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21.  Phase Detector
 Acts as comparator
 Produces a voltage proportional to the phase difference
between input and output signal
 Voltage becomes a control signal
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22.  Filter
 Determines dynamic characteristics of PLL
 Specify Capture Range (bandwidth)
 Specify Tracking Range
 Receives signal from Phase Detector and filters
accordingly
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23.  Voltage Controlled Oscillator
 Set tuning range
 Set noise margin
 Creates low noise clock oscillation
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24.  Divider
 Divides the VCO output by the degree of the open loop
gain
 Feedback loop allows phase comparison
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25. (mō´dem) :- Short for modulator-demodulator. A
modem is adevice or program that enables
a computer to transmit data over, for example,
telephone or cable lines. Computer information is
stored digitally, whereas information transmitted
over telephone lines is transmitted in the form
of analog waves. A modem converts between these
two forms.
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26. Standard Modem Interface
Fortunately, there is one standard interface for
connecting external modems to computers
called RS-232.
Consequently, any external modem can be
attached to any computer that has an RS-
232 port, which almost allpersonal
computers have.
There are also modems that come as
an expansion board that you can insert into a
vacant expansion slot. These are sometimes
called onboard or internal modems.
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27. •bps : How fast the modem can transmit and receive
data. At slow rates, modems are measured in terms
of baud rates. The slowest rate is 300 baud (about
25 cps). At higher speeds, modems are measured in
terms of bits per second (bps). The fastest
modems run at 57,600 bps, although they can achieve
even higher data transfer ratesby compressing the data.
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28. Obviously, the faster the transmission rate,
the faster you can send and receive data.
Note, however, that you cannot receive data
any faster than it is being sent. If, for
example, the device sending data to your
computer is sending it at 2,400 bps, you must
receive it at 2,400 bps. It does not always pay,
therefore, to have a very fast modem. In
addition, some telephone lines are unable to
transmit data reliably at very high rates.
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29. •voice/data: Many modems support
a switch to change between voice and
data modes. In data mode, the modem acts
like a regular modem. In voice mode, the
modem acts like a regular telephone.
Modems that support a voice/data switch
have a built-in loudspeaker and
microphone for voice communication.
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30. •auto-answer :An auto-answer modem enables your
computer to receive calls in your absence. This is only
necessary if you are offering some type of computer service
that people can call in to use.
•data compression :Some modems perform data
compression, which enables them to send data at faster rates.
However, the modem at the receiving end must be able to
decompress the data using the same compression technique.
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31. •flash memory : Some modems come
with flash memory rather than
conventional ROM, which means that
the communications protocolscan be easily
updated if necessary.
•Fax capability: Most modern modems
are fax modems, which means that they can
send and receive faxes.
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