PHASE LOCKED LOOP (PLL - IC 565)

A PLL is a feedback system that includes a VCO (voltage controlled oscillator), phase
detector, and low pass filter within its loop. Its purpose is to force the VCO to replicate and
track the frequency and phase at the input when in lock. The PLL is a control system
allowing one oscillator to track with another. It is possible to have a phase offset between
input and output, but when locked, the frequencies must exactly track.

The above equations indicate that that when locked, the input and output frequencies and
phases are same. The PLL output can be taken from either Vcont, the filtered (almost DC)
VCO control voltage, or from the output of the VCO depending on the application. The
former provides a baseband output that tracks the phase variation at the input. The VCO
output can be used as a local oscillator or to generate a clock signal for a digital system.
Either phase or frequency can be used as the input or output variables.

i.e the frequency of a signal can be obtained by differentiating the phase of signal over the
time period of the signal.

Applications: There are many applications for the PLL, but we will study:
a. Clock generation
b. Frequency synthesizer
c. Clock recovery in a serial data link

BASIC BUILDING BLOCKS OF PLL

1. Phase Detector
Phase Detector compares the phase at each input and generates an error signal, vc(t)
proportional to the phase difference between the two inputs. Assume KD is the gain of
phase detector in volts/radians.

Ekambir Sidhu (AP, ECE)

The function of VCO is to generate an output signal with frequency which will be directly
proportional to the VCO input signal voltage i.e. higher the input voltage, more will be the
output frequency of VCO.
The below figure shows the transfer characteristics of phase detector which is plotted
between Φ (output phase difference between the output of phase detector and the input signal
frequency vs the error output voltage Ve of a phase detector).

It can be observed from the above transfer characteristics that when the phase difference
between the two input signals of phase detector is pi/2, the average output of phase detector is
zero. The average output error voltage increases linearly with increase in the phase difference
for phase differences above pi/2 and for the phase difference below pi/2, the output voltage of
phase detector decreases linearly. The maximum error voltage arises when the phase
difference between the two input signals is pi and equals KD pi/2 and minimum phase
difference arises when the difference between two input signals is zero and equals - KD pi/2.
The slope of the characteristic curve in either case is KD.

2. VCO (Voltage Controlled Oscillator)

In PLL, the VCO is treated as linear, time invariant system where the output signal
frequency is a linear function of input signal voltage.

Ekambir Sidhu (AP, ECE)

NOTE – A simple XOR Gate is nothing but a phase detector.

Ekambir Sidhu (AP, ECE)

Ekambir Sidhu (AP, ECE)
Ekambir Sidhu (AP, ECE)
 PIN DIAGRAM OF PLL 565

NOTE:
The range over which the loop system will follow changes in the input frequency is called
the lock range. On the other hand, the frequency range in which the loop acquires phase-lock
is the capture range, and is never greater than the lock range.

SPECIFICATIONS OF PLL LM-565

1. 200ppm/°C frequency stability (drifting) of the VCO

2. Power supply range of 5 to 12 volts
3. Frequency range 0.001 Hz to 500 KHz
4. Highly linear triangle wave output

Ekambir Sidhu (AP, ECE)

IMPORTANT PRACTICAL APPLICATION AREAS OF PLL 565:
1. Data and Tape Synchronization
2. Modems
3. FSK Modulation
4. FM Demodulation
5. Frequency Synthesizer
6. Tone Decoding
7. Frequency Multiplication and Division
8. Telemetry Receivers
9.
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TOPIC – 2
FREQUENCY SYNTHESIZER (FS)

A frequency synthesizer is a device (an electronic system) that generates a large number of
precise frequencies from a single reference frequency. A frequency synthesizer can replace
the expensive array of crystal resonators in a multichannel radio receiver. A single-crystal
oscillator provides a reference frequency, and the frequency synthesizer generates the other
frequencies.
Because they are relatively inexpensive and because they can be easily controlled by digital
circuitry, the frequency synthesizers are being included in many new communication system
designs.
Frequency synthesizers are found in many devices, including radio receivers, mobile
telephones, radiotelephones, walkie-talkies, satellite receivers, GPS systems, etc.
A frequency synthesizer can combine frequency multiplication, frequency division, and
frequency mixing (the frequency mixing process generates sum and difference frequencies)
operations to produce the desired output signal frequency.

PRINCIPLE OF OPERATION OF FREQUENCY SYNTHSIZER

A phase locked loop is a feedback control system. It compares the phases of two input signals
and produces an error signal that is proportional to the difference between their phases. The
error signal is then low pass filtered and used to drive a voltage-controlled oscillator (VCO)
which creates an output frequency. The output frequency is fed through a frequency
divider back to the input of the system, producing a negative feedback loop. If the output
frequency drifts, the phase error signal will increase, driving the frequency in the opposite
direction so as to reduce the error. Thus, the output is locked to the frequency at the other
input. This other input is called the reference and is usually derived from a crystal oscillator,
which is very stable in frequency. The block diagram below shows the basic elements and
arrangement of a PLL based frequency synthesizer.

Ekambir Sidhu (AP, ECE)

The key to the ability of a frequency synthesizer to generate multiple frequencies is the
divider placed between the output and the feedback input. This is usually in the form of
a digital counter, with the output signal acting as a clock signal. The counter is pre-set to
some initial count value, and counts down at each cycle of the clock signal. When it reaches
zero, the counter output changes state and the count value is reloaded. This circuit is
straightforward to implement using flip-flops, and because it is digital in nature, is very easy
to interface to other digital components or a microprocessor. This allows the frequency output
by the synthesizer to be easily controlled by a digital system.

TYPES OF FREQUENY SYNTHESIZERS

There are several different types of categories of synthesizer. Each of them obviously has its
own advantages and disadvantages. There are often choices that can be made about which
type to choose.

1. Direct frequency synthesizers:

The direct forms of frequency synthesizer, are as the name suggests implemented by
creating a waveform directly without any form of frequency transforming element.
The direct techniques employ an oscillator and mixer to generate frequency.

2. Indirect: Indirect frequency synthesis is based around phase locked loop technology.
Here, the output signal is generated indirectly. In other words, the final signal is
generated by an oscillator that is controlled by other signals. In this way, the signals
used in creating the output are indirectly replicated by the output oscillator, thereby
giving the name to this technique.

TOPIC 3
NORTON AMPLIFIER
OR
CURRENT DIFFERENCING TRANSCONDUCTANCE AMPLIFIER (CDTA)

The CDTA element with its schematic symbol in Fig below has a pair of low-impedance current
inputs and p, n and an auxiliary terminal z, whose outgoing current is the difference of input
currents.

Here, output terminal currents are equal in magnitude, but flow in opposite directions, and the
product of transconductance ( g m {\displaystyle gm\,}gm) and the voltage at the z terminal gives
their magnitudes. Therefore, this active element can be characterized with the following
equations:

Ekambir Sidhu (AP, ECE)

CDTA can be thought as a combination of a current differencing unit followed by a dual-output
operational transconductance amplifier, DO-OTA. Ideally, the OTA is assumed as an ideal voltage-
controlled current source and can be described by:

where Ix is output current, V + {\displaystyle V+\,} where Ix is output current, V + and V − denote
non-inverting and inverting input voltage of the OTA, respectively. Note that gm is a function of the
bias current. When this element is used in CDTA, one of its input terminals is grounded (e.g., V − =
0). With dual output availability, I x + = − I x − , condition is assumed.

Ekambir Sidhu (AP, ECE)