AprTraining Basic Concept.pptx

STP 163/121/COM MTC VHF CATC INDIA
Apprentice Training April-May 2021
Basic Communication Concepts

Contents
1.AM Modulation
2.Introduction to Tx Line
3.Antenna Concepts
4.Wave propagation theory
5.Transmitter related Concepts
6.Receiver related concepts

Concept of AM
Amplitude Modulation
STP 163/121/COM MTC VHF CATC INDIA PPT 1.5

Use of Radio waves or frequency to communicate or
exchange or pass information.
Components needed are:
1. Transmitter-Amplitude Modulated.
2. Receiver
3. Audio or voice or information
Radio Communication

1. Modulation for ease of radiation.
2. Reduces noise and interference.
3. Frequency Assignment.
4. Multiplexing.
5. To overcome equipment limitations.
Need of Modulation and its definition

Types of Modulation:
1. CW and Pulse Modulation.
2. Analog and Digital Modulation.
3. Linear Modulation and Non Linear Modulation
Radio Communication- Modulation

Types of CW Modulation.
• A Continuous Wave can be characterized by the
following three parameters.
• The Amplitude
• The Frequency
• The Phase

Accordingly there are three types of modulation
–Amplitude modulation (AM)
–Frequency Modulation (FM)
– Phase Modulation (PM).
FM and PM are together called Angle Modulation.

AM WAVE FORM

• Amplitude modulation(AM):
Has two components
1.Carrier
2.Modulating signal
• Carrier- A sine wave carrier is of the form
ec = Ec Sin ( c t + )
or ec = Ec Sin ( c t)
• Modulating signal: em = Em Sin ( m t )
Radio Communication

Since the amplitude of the carrier wave varies at the
signal frequency, therefore the amplitude of AM wave is
given by
E = Ec + em = Ec + Em Sin ( m t )
Instantaneous value (value at any instant in time) is
e = E Sin ( c t )
e = {Ec + Em Sin ( m t )} * Sin ( c t )
e = Ec Sin ( c t ) + Em Sin ( m t ) * Sin ( c t )
e = Ec Sin ( c t ) + ½ Em Cos ( (c - m) t ) - ½
Em Cos ((c + m)t)
Radio Communication- Amplitude
Modulation

Frequency Spectrum of AM

Example:A 1 MHz carrier is amplitude modulated by an audio
signal which contains all frequencies in the range 300 Hz to 5
kHz. What are the frequency bands which are output? What
is the output bandwidth? Draw the spectral diagram of these
signals.
A.M. Waveform

Answer: The carrier is 1 MHz
The Upper Side Band is all frequencies in the range
1,000,300 to 1,005,000 Hz
The Lower Side Band is all frequencies in the range 995,000
to 999, 700 Hz
The Bandwidth is 1,005,000 - 995,000 = 10,000 Hz = 10 kHz.
A.M. Waveform

A.M. Waveform

• Modulation Index
m = Em / Ec
or
Emax – Emin
m = ----------------
Emax + Emin
Modulation Index (or Modulation Factor or
Depth of Modulation)

Average Power: Total average power contained in a sine wave and
each side band
PT = PC + 2 x PSF
= PC (1 + m2/2)
Effective Voltage and Current for Sinusoidal AM
E = EC  (1 + m2/2)
I = IC  (1 + m2/2)
Non sinusoidal Modulation.
– total average power.
– effective modulation index.
Power for Sinusoidal AM

• em(t) = E1 cos 2f1t + E2 cos 2f2t + E3 cos 2f3t + ….
• PT = PC ( 1 + m1
2/2 + m2
2/2 + m3
2/2 +…… )
• Hence an effective modulation index or modulation depth
can be defined in this case as
• m effect. = √ m1
2 + m2
2 + m3
2 + ……
• It follows that the effective voltage and current in this case
is
• E = Ec √ 1 + m2
eff / 2
• I = Ic √ 1 + m2
eff / 2
Multi Tone Modulation

• There are three basic types of modulation methods for
transmission of digital signals.
• These methods are based on the three attributes of a
sinusoidal signal, amplitude, frequency and phase:
–Amplitude Shift Keying (ASK)
–Frequency Shift Keying (FSK)
–Phase Shift Keying (PSK)
• A combination of ASK and PSK is employed at high bit rates.
This method is called Quadrature Amplitude Modulation
(QAM).
Digital modulation technique

• In ASK, the carrier amplitude is multiplied by the binary
"1" or "0" .
• The digital input is a unipolar NRZ signal.
• The amplitude modulated carrier signal can be written as
v(t) = d sin (2fct)
where fc is the carrier frequency and d is the data bit
variable which can take values "1" or "0", m the state of
the digital signal
Amplitude Shift Keying (ASK)

Amplitude Shift Keying (ASK)

Frequency spectrum of an ASK signal

• In FSK the frequency of the carrier is shifted between two
discrete values, one representing binary "1" and the other
representing binary "0"
• The carrier amplitude does not change.
• The instantaneous value of the FSK signal is given by
v(t) = d sin (2πf1t) + d sin (2 π f0t)
where f1 and f0 are the frequencies corresponding to
binary "1" and "0" respectively and d is the data signal
variable as before.
Frequency Shift Keying (FSK)

Frequency Shift Keying (FSK)

Frequency spectrum of a FSK signal.

• In PSK, phase of the carrier is modulated to represent
the binary values.
• In simplest form of PSK called Binary PSK (BPSK), the
carrier phase is changed between 0 and  by the bipolar
digital signal. Binary states "1" and "0" are represented
by the negative and positive polarities of the digital
signal.
Phase Shift Keying (PSK)

• The instantaneous value of the BPSK signal can be
written as
–V(t) = Sin (2fct) when d = 1 for binary state "0"
–V(t) = - Sin (2fct) = Sin (2fct + ) when d = -1 for
binary state "1“
• In other words
V(t) = d Sin (2fct) where d = ± 1
Binary Phase Shift Keying (BPSK)

Binary phase shift keying.

Frequency spectrum of a BPSK signal

• The problem of generating the carrier with a fixed absolute
phase can be circumvented by encoding the digital
information as the phase change rather than as the absolute
phase.
• This modulation scheme is called differential PSK.
• If Φt-1 is the previous phase state and Φt is the new phase
state of the carrier when data bits modulate the carrier, the
phase change is defined as
ΔΦ = Φt - Φt-1
ΔΦ is coded to represent the data bits
Differential PSK

• The concept of differential phase shift keying can be
generalized to M equally spaced phase states.
• The bit rate will become n x (baud rate), where n is such
that 2n = M.
• This is called M-ary PSK or simply MPSK.
• The phase states of the MPSK signal are separated by
2/M radians .
• As M is increased, the phase states come closer and
result in degraded error rate performance because of
reduced phase detection margin.
Quadrature Amplitude Modulation (QAM)

• Quadrature Amplitude Modulation (QAM) is one
approach in which separation of the phase states is
increased by utilizing combination of amplitude and
phase modulations.
• Each state can be represented as the sum of two carriers
in quadrature.
Quadrature Amplitude Modulation
(QAM)

Introduction to Transmission Line
Antenna concept

• A transmission line is a device designed to guide electrical
energy from one point to another.
• It is used to transfer the output RF energy of a transmitter to
an antenna.
• The transmission line has a single purpose to transfer the
energy of the source to the load with the least possible
power loss.
Transmission line

• Input Impedance (Zin).
• Output Impedance (Zout).
• Characteristic Impedance. (Zo) : defined as a steady-state vector
ratio of voltage to the current at the input of an infinite line.
Transmission line

• Short section of two-wire transmission line and
equivalent circuit
Transmission line

Transmission line

 Four line parameters R, L, C, and G are termed as “Primary
constants”.
 Two complex constants  and Z0 are termed as “Secondary
Constants” of the line.
1.  is called the Propagation Constant .
2. Z0 is called the Characteristic impedance.
Primary & Secondary Constants of a
Transmission Line

• A transmission line is considered to be electrically
short when its physical length is short compared to a
quarter-wavelength ( / 4) of the energy it is to carry
• A transmission line is electrically long when its
physical length is long compared to a quarter-
wavelength of the energy it is to carry.
Length of a Transmission Line

• Characteristic impedance of a uniform transmission line
may be defined as a steady-state vector ratio of voltage
to the current at the input of an infinite line.
• Alternatively, it can simply be defined as the impedance
looking into an infinite length of the line. Its unit is ohms.
It is also known as Surge impedance.
Characteristic Impedance

1) Transmission line terminated in its characteristic
impedance (ZL= ZO)
2) Transmission line terminated in an open circuit
(Z L = )
3) Transmission line terminated in a short circuit
(Z L = 0)
Characteristics of Line of different lengths
terminated with different loads

• When a transmission line is not correctly terminated,
the traveling electromagnetic wave from generator
at the sending end is reflected completely or
partially at the termination.
• The combination of incident and reflected waves
gives rise to standing waves of current and voltage
along the line, with definite maxima and minima of
current and voltage along the line,
Nodes and Antinodes of Voltage and
Current

Nodes and Antinodes

• The ratio of the adjacent maximum voltage to minimum
voltage on a line is called the VOLTAGE STANDING-WAVE
RATIO (VSWR).
 E+  +  E- 
S = ------------
 E+  -  E- 
Voltage Standing-Wave Ratio (VSWR)

• Return loss is a measure of impedance mismatch in an RF
interface. Return loss represents the attenuation of the
reflected RF signal in an RF interface, compared to the
incident RF signal in the same interface.
• Return loss in a radio system's cable-antenna interface
reduces effective radiated power. The lower the return loss,
the greater the ERP reduction.
Return Loss

Quarter wave line

• A balun, or
balance-to-
unbalance
transformer, is a
circuit element
used to connect a
balanced line to an
unbalanced line or
antenna.
Balanced to unbalanced transformation
(Baluns)

Baluns

• It is often necessary to
measure the power being
delivered to a load or an
antenna through a
transmission line. This is often
done by a sampling technique,
in which a known fraction of
the power is measured, so that
the total may be calculated.
Directional Couplers

• Antenna or Aerial is defined as the structure associated with
the region of transition between a guided wave and a free
space wave or between a free space wave and a guided
wave.
• To radiate or receive electromagnetic waves, an antenna is
required.
Introduction of antenna

An antenna differs from a transmission line in the following two
respects.
1. Power is radiated by an antenna where as a negligible power
is assumed to be radiated by a transmission line.
2. Inductance (L), capacitance (C) and characteristic impedance
(Z0) are varying along the line in an antenna because it
represents a non-uniform transmission line whereas L, C,
and Z0 are constant in a uniform transmission line
Difference between Antenna & Transmission
line

1. Power Density
2. Radiation Intensity
3. Gain
4. Directive Gain
5. Power Gain
6. Antenna Efficiency
7. Directivity(D)
8. Bandwidth
9. Beam width
10.Polarization
Typical Antenna Terms

• “The orientation of the electric
field with respect to earth is
known as the polarization of the
wave”.
• If the plane of the electric field is
horizontal with respect to earth,
the wave is said to be
horizontally polarized.
• If the plane of the electric field is
oriented vertically with respect
to earth, the wave is said to be
vertically polarized
Polarization

• Radiation pattern of an antenna is nothing but a graph
which shows the variation in actual field strength of
electromagnetic field at all points which are at equal
distance from the antenna.
• The graph of the received power at a constant radius
from the transmitting antenna is called the power
pattern.
• A cross-section of this field pattern in any particular plane
is called the radiation pattern in that plane.
Radiation Pattern

Radiation Pattern

1. Omni directional Pattern
2. Pencil Beam Pattern
3. Fan-Beam Pattern
4. Shaped Beam Pattern
5. Limacon or Cardioid Pattern
6. Figure of eight pattern
Types of radiation patterns

1. Resonant and Non–resonant
Antenna
2. Grounded and Ungrounded
Antenna
3. Half-Wave Dipole Antenna
4. Folded Dipole Antenna
Different types of Antenna

• The gain of a
folded dipole
antenna can be
increased by
stacking two
dipoles.
• While stacking,
the optimum gain
occurs at one
wavelength
center to center
spacing.
Stacking

Introduction to
Radio wave propagation
Wave Propagation

1. Ground waves 2. Sky Waves 3. Space waves
Principles of Electromagnetic
Propagation

The radio horizon for space waves is about four-thirds as far
as the optical horizon.
This beneficial effect is caused by the varying density of the
atmosphere, and because of diffraction around the curvature
of the earth.
The radio horizon of an antenna is given, with good
approximation, by the empirical formula
dt = 4 (ht)1/2
Radio Horizon

• where dt = distance from transmitting antenna, km
• ht = height of transmitting antenna above ground,
• The same formula naturally applies to the receiving
antenna. Thus the total distance will be given by addition,
and by the empirical formula
• d = dt + dr = 4 √ht + 4 √hr
Radio Horizon

• Also known as troposcatter, or forward scatter propagation,
tropospheric scatter propagation is a means of beyond-the-
horizon propagation for UHF signals.
Tropospheric Scatter

• Synthesizer
• Methods
1]. Direct synthesizers.
2]. Indirect (phase locked loop) synthesizers
a) Basics.
b) Block Diagram.
Frequency Generation

1. Presence of spurious frequencies generated by the many
mixers used.
2. Extensive filtering and extremely careful selection of
operating frequencies are required.
3. Spurious frequency problems increase as the output
frequency range increases and channel spacing is
reduced.
Drawbacks-Direct synthesizers.

• Basics. A frequency synthesizer is based around a phase
locked loop.
Indirect (phase locked loop) synthesizers

• A programmable divider added into a phase locked loop
enables the frequency to be changed.
Indirect synthesizers-a programmable
divider

• Comparison frequency reduced by adding a fixed divider after
the reference oscillator
Indirect synthesizers-a programmable
divider

• Conceptual Block Diagram of Amplitude modulated
Transmitter.
• Modulator.
• Antenna-Introduction
Transmitter system

Conceptual Block Diagram

Receiver System
 The Components of a Receiver System
 Super heterodyne Receiver
 RF Section and Characteristics
 Reasons for use and functions of RF amplifier
 Mixer.
 Intermediate Frequencies and IF Amplifiers
 Choice of frequency(IF).

TRF (Tuned Radio frequency) Receiver
STP 163/121/COM MTC VHF CATC INDIA PPT 126
Tuned radio
frequency
amplifier
detector
A. F.
amplifier
Modulati
ng signal

• TRF receiver includes an
• RF stage
• a detector stage
• and an audio stage .
• Two or three RF amplifiers are required to filter and
amplify the received signal to a level sufficient to drive
the detector stage.
Receivers

Tuned Radio Frequency (TRF) Receiver:
• Composed of RF amplifiers and detectors.
• No frequency conversion
• It is not often used.
• Difficult to design tunable RF stages.
• Difficult to obtain high gain RF amplifiers
Receivers

• TRF receivers are simple to design and allow the
broadcast frequency 535 KHz to 1640 KHz.
• High sensitivity..
Advantages of TRF
STP 163/121/COM MTC VHF CATC INDIA PPT 2।10

Disadvantages of TRF
• At the higher frequency, it produces difficulty in design.
• It has poor audio quality.
• Drawbacks
Instability
Variation in BW
Poor Selectivity

POOR SELECTIVITY
• The gains are not uniform over a very wide frequency
range.
• Due to higher frequencies ability to select desired signal is
affected.
Due to these drawbacks TRF are rarely used.
Disadvantages of TRF

Super Hetrodyne Receiver
The shortcomings of the TRF receiver are overcome by the
super heterodyne receiver.
RF
amplifier
Local
oscillator
mixer
IF
amplifier
detector
AF
amplifier
Modulatin
g signal
Ganged
tuning
fs
fo
IF=fo- fs

Heterodyne – to mix two frequencies together in a
nonlinear device or to translate one frequency to another
frequency using nonlinear mixing.
It also known as frequency conversion , high frequency
down converted to low frequency.(IF)
A super heterodyne receiver converts all incoming radio
frequency (RF) signals to a lower frequency known as an
intermediate frequency (IF).

Comparison
• TRF Receiver
• No frequency conversion
• No IF frequency
• Instability , variation in BW
and poor selectivity due to
high frequencies
• Difficult to design tunable
RF stages.
• Rarely used
• Super Hetrodyne Receiver
• Frequency conversion
• Downconvert RF signal to
lower IF frequency
• No instability, variation in
BW and poor selectivity as
IF introduced.
• Main amplifixcation takes
place at IF
• Mostly used

Receiver System
Characteristics of a receiver.
 Sensitivity
 Selectivity
 Fidelity
 AGC
 Noise Limiter
 Squelch(Muting)

Sensitivity
Ability to amplify weak signals.
Minimum RF signal level that can be detected at the input
to the receiver and still produce a usable demodulated
information signal.
Broadcast receivers/ radio receivers should have
reasonably high sensitivity so that it may have good
response to the desired signal
But should not have excessively high sensitivity otherwise
it will pick up all undesired noise signals.
 It is function of receiver gain and measures in decibels.

Sensitivity of a receiver is expressed in micro volts of the
received signal.
Typical sensitivity for commercial broadcast-band AM
receiver is 50 μV.
Sensitivity of the receiver depends on :
•Noise power present at the input to the receiver
•Receiver noise figure
•Bandwidth improvement factor of the receiver
•The best way to improve the sensitivity is to
•reduce the noise level.

Selectivity of radio receiver is its ability to differentiate
desired signal from unwanted signals.
Selectivity

Fidelity
 the ability of radio receiver to give faithful reproduction of
the output over a large frequency input range
 The signal to noise ratio should be infinite.
 Flat frequency response for complete audio range.
 Non-Linear distortion (or amplitude distortion) should be
nil.

Image Frequency
 is defined as the signal frequency plus twice the
intermediate frequency. Reiterating, we have
 fsi = fs + 2 fi,
 IF Amplifiers
 Double conversion

STP 163/121/COM MTC VHF CATC INDIA PPT 2..23
 Adjust the IF amplifier gain according to signal level(to
the average amplitude signal almost constant).
 AGC is a system by means of which the overall gain of
radio receiver is varied automatically with the variations
in the strength of received signals, to maintain the output
constant.
AGC ( Automatic Gain Control )

 AGC circuit is used to adjust and stabilize the
frequency of local oscillator.
 Types of AGC –
 No AGC
 Simple AGC

Delayed AGC

Detection
Simple Diode Detection

Noise Limiter: A circuit for reducing the interfering noise
pulses created by ignition systems, electrical storms or
electrical machinery of various types
Squelch: enables the receiver's output to remain cut off
unless the carrier is present.

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