9011041155 / 9011031155

• Live Webinars (online lectures) with

recordings.

• Online Query Solving

• Online MCQ tests with detailed

solutions

• Online Notes and Solved Exercises

• Career Counseling

Communication System

1

9011041155 / 9011031155

2

9011041155 / 9011031155

3

9011041155 / 9011031155

Communication System

There are two basic modes of communication

1) point – to – point communication and broadcast.

e.g.

2) Broadcast mode,

4

9011041155 / 9011031155

i. Single

analog or digital.

ii. Transmitter

iii. Transducer

5

9011041155 / 9011031155

Any device that converts one form of energy into

another

Electrical transducer :-a device which converts

some physical variable (Pressure, displacement,

temperature, force, etc.) into corresponding

variations in the electrical signal at its output.

iv. Attenuation

The loss of strength of a signal while propagating

through a medium is known as attenuation.

v. Amplification

Amplification is the process of increasing the

amplitude (and also strength)

6

9011041155 / 9011031155

vi. Noise

Noise is random, undesirable (unwanted) electric

energy that enters the communication system

vii. Receiver

A receiver extracts the desired message signals

from the received signals at the channel output. It

consists of a pickup antenna to pick up signal,

demodulator, an amplifier and the transducer.

7

9011041155 / 9011031155

viii. Range

The maximum (largest) distance between a source

and a destination upto which the signal is received

with sufficient strength is termed as range.

ix. Bandwidth

The frequency range over which equipment

operates or the portion of the spectrum occupied by

the signal is referred as bandwidth.

x. Modulation

The process of superimposing a low frequency

signal on a high frequency wave, which acts as a

carrier wave for long distance transmission is

known as modulation.

xi. Demodulation8

9011041155 / 9011031155

The process of regaining (retrieval) of information

from carrier wave at the receiver is termed as

demodulation. (This is the reverse process of

modulation).

xii. Repeater

A repeater is a combination of a receiver and

transmitter. Repeaters are used to extend the

range of a communication.

Bandwidth of Signals

Different frequency bands

9

9011041155 / 9011031155

Extremely Low Frequency

(ELF)

30 Hz to 300 Hz,

Voice Frequency (VF) - 300 Hz to 3000 Hz

Very Low Frequency (VLF) 3 kHz to 30 kHZ

Low Frequency (LF) 30 kHz to 300 kHz

Medium Frequency (MF) 300 kHz to 3000 kHz

Amplitude Modulation (AM) Band

High Frequency (HF) 3 MHz to 30 MHz

Very High Frequency (VHF) 30 MHz to 300 MHz

Ultra High Frequency (UHF) 300 MHz to 3000 MHz

Super High Frequency (SHF) 3 GHz to 30 GHz

(80 to 108 MHz)

Frequency Modulation (FM) Band

Extra High Frequency (EHF) 30 GHz to 300 GHz

Some important wireless communication

frequency bands

Service Frequency Comments

10

9011041155 / 9011031155

bandsStandard AM

broadcast

540 – 1600 kHz

FM broadcast 88 – 108 MHz

Television 54 – 72 MHz VHF (Very High

Frequency)

76 - 88 MHz TV

174 – 216 MHz UHF (Ultra High

Frequencies)

420 – 890 MHz TV

Cellular Mobile

Radio

896 – 901 MHz Mobile to base station

840 – 935 MHz Base station to mobile

Satellite

communication

5.925 – 6.425

GHz

Uplink

3.7 – 4.2 GHz Downlink

Need For Modulation

base band signals.

Size of the Antenna or aerial11

9011041155 / 9011031155

For efficient radiation and reception, the

transmitting antennas (or antennae) would have

lengths = of frequency used.

A vertical antenna of this size is impracticable and

hence direct transmission of such base band signals is

not practical.

Effective Power Radiated By An Antenna

According to theoretical study of radiation form a

linear antenna of length ‘ℓ’, the power radiated is

proportional to . This means, for the same

antenna length, the power radiated increases with

decreasing (i.e increasing frequency). Hence 12

9011041155 / 9011031155

effective power radiated by a long wavelength base

band signal would be small. For a good transmission,

we need high power and hence we will use high

frequency transmission.

Mixing up of signals different transmitters

Sound range:- from 20 Hz to 20 kHz.

So all signals from the different sources would be

hopelessly and inseparably mixed up. In any city, the

broadcasting stations alone would completely blanket

the “air” and yet they represent a very small proportion

of the total number of transmitters in use

In order to separate the various signals, it is

necessary to covert them all too different portions of

electromagnetic spectrum. Each must be given its own

frequency location. This also overcomes the difficulties 13

9011041155 / 9011031155

of power radiation at low frequencies and reduces

interference.

An un-modulated carrier has constant amplitude, a

constant frequency and a constant phase relationship

with respect to some reference. A message consists of

ever-varying quantities. Speech, for instance is made

up of rapid and unpredictable variations in amplitude

(volume) and frequency (pitch). Since it is impossible to

represent these two variables by a set if three constant

parameters, an un-modulated carrier cannot be used to

convey information,

The above discussion suggests that there is a need

for translating the original low frequency base band

signal or information message into high frequency

wave before transmission such that the translated

signal continues to possess the information contained

in the original signal 14

9011041155 / 9011031155

To achieve this, signals to be transmitted are

superimposed on high frequency (small wavelength)

waves called carrier waves. This process is termed as

modulation, which attaches information to it. The

information is then transmitted by radiating these

modified carrier waves called modulated waves. The

modified carrier waves called modulated waves. The

carrier wave may be continuous (sinusoidal) or in the

form of pulses as shown in fig

15

9011041155 / 9011031155

A sinusoidal carrier wave can be represented as

c(t) = Ac sin (ωct + Ф)

Where c (t) is the signal strength (voltage or

current), Ac is the amplitude, ωc(=2πfc)is the angular

frequency and Ф is the initial phase of carrier wave.

During the process of modulation, any of three

parameters viz. Ac, ωc and Ф of the carrier wave can be

controlled by the message or information signal. This

result in three types of modulation as shown in fig.16

9011041155 / 9011031155

Amplitude Modulation

Let c(t) = Ac sin ωct represents a carrier wave and

m (t) = Am sin ωm t represents the message of the

modulating signal where ωm = 2πfm is the angular

frequency of the message signal.

The modulated signal cm (t) can be written as

cm (t) = c (t) + m (t) sin ωc t

17

9011041155 / 9011031155

= (Ac + Am sin ωm t) sin ωc t

=

cm (t) = Ac sin ωc t + uAc sin ωc t ……. (1)

Where is the modulation index. In practice, μ

is kept ≤ 1 to avoid distortion.

By using sin A . sin B =

equation (1) becomes

18

9011041155 / 9011031155

Production and Detection Amplitude modulated

wave

Production of A.M. Wave

19

9011041155 / 9011031155

Here the modulating signal Am sin ωm t is added to the

carrier signal Ac sin ωm t to produce the signal x (t). This

signal x (t) = Am sin ωt sin (t) + Ac sin ωc t is passed

through a square law device which is a non-linear

device and produces an output.

Y (t) = B x (t) + C x2 (t) …….. (2)

Where B and C are constants.

20

9011041155 / 9011031155

Thus,

In equation (4), there is a d,c term and

sinusoids of frequencies ωm, 2ωm, ωc, 2ωc, (ωc – ωm)

and (ωc + ωm). As shown in, this signal is passed

through a band pass filter which reject d.c. and the

21

9011041155 / 9011031155

sinusoids of frequencies ωm, 2ωm and 2ωc and retains

the frequencies ωc, (ωc ω∓ m). The output of the band

pass filter is same as equation and is therefore an AM

wave.

Detection AM wave

22

9011041155 / 9011031155

Common Am Applications

i. AM-radio Broadcasting

ii. TV picture (video)

iii. Two way radio

a. air-craft b. Amateur radio(SSB)

c. Citizen’s band radio d. Military

Drawback in AM

1. Low efficiency – only 20 to 30% is useful.

2. Noisy reception – AM signal is easily affected by

external atmosphere and electrical disturbances.

23

9011041155 / 9011031155

3. Operating range is small.

4. Quality: The allowed AM bandwidth is only 10kHz

and for transmission of all audio frequencies about

30 kHz bandwidth is required which affects fidelity.

Due to limited bandwidth stereotype transmission is

not possible.

Space Communication

24

9011041155 / 9011031155

There are three main types of space communication.

Ground wave propagation

ground wave communication is used for low

frequencies (500kHz to 1500kHz). This type of

communication is used for medium wave radio

transmission, ship communication or radio navigation.

Sky wave propagation

very long distance

25

9011041155 / 9011031155

For very low and high values of

frequencies of em waves, the sky

waves are either absorbed or

escape from the ionosphere.

Hence, following two terms are

important for sky wave communication.

Critical Frequency (fc)

It is the maximum value of frequency of the radio

waves, reflected back to the Earth from the ionosphere,

when directed normally to the ionosphere. It is

approximately given as, where Nmax is

the maximum electron density of the ionosphere.

Depending on the properties of ionosphere, the critical

frequency changes from 5MHz to 10MHz.

26

9011041155 / 9011031155

Space wave propagation

The electromagnetic waves

which travel directly from

transmitting antenna to

receiving antenna without

being influenced by the Earth

are called space waves. In

this propagation, the em waves move in Earth’s

troposphere, within about 15km over the surface of the

Earth. Hence, they are also known as tropospheric

waves. Their frequency range is in between 30MHz to

300MHz. These waves travel in straight line. Hence,

the transmitting and the receiving antenna must be in

the line of sight. But, due to the curvature of the Earth,

these waves cannot be received beyond horizon.

27

9011041155 / 9011031155

Hence, the effective reception or

the range of these waves is up to

line of sight only. Hence, the

communication is also called line

of sight communication.

The figure shows curved surface of the Earth R1 B R2.

At B there is a transmitting antenna of height h (BA). R

is the radius of the Earth, i.e. distance of R1, B and R2

from centre of the Earth O. C is the midpoint of the line

joining R1 and R2. Hence, R1 and R2 are at distance d

from C. Triangles OR1A and OCR1 are right angled

triangles.

∴ OA2 = OR12 + AR12

But, OA = R + h and AR12 = h2 + d2

28

9011041155 / 9011031155

∴ (R + h)2 = R2 + h2 + d2

∴ R2 + 2hR + h2 = R2 + h2 + d2

∴ 2hR = d2

Thus, transmitting antenna is installed at the top of the

mountains to increase height h to increase the range.

For a 100m high antenna, the range is approximately

35km.

When these waves are reflected from ground, there

is a phase reversal of 180o. If the direct waves and

ground reflected waves reach the receiving antenna in

anti phase, they cancel each other.

Satellite Communication

The electromagnetic waves having frequencies beyond

30MHz are very useful in communication because of 29

9011041155 / 9011031155

their higher band widths. But, these waves cannot

transmitted as ground waves or space waves and also

as sky waves because they escape from the

ionosphere. Hence, satellites are used for

communication by these waves.

30

9011041155 / 9011031155

The satellite receives them, amplifies them and sends

them back towards the Earth, which are received by

the receiving antenna.

The process of sending the signal from the Earth to

the satellite is called uplinking and receiving the signals

from the satellite is called downlinking. The uplink and

down link frequencies have atleast 2MHz frequency

difference between them to avoid confusion. The

transmitting and receiving antennae are tuned to the

corresponding frequencies. As the geostationary

satellite doesn’t change its position relative to the

Earth, there is no Doppler’s shift in the downlink

frequencies.

Global communication. 31

9011041155 / 9011031155

Advantages

1. long distance wide spread communication.

2. A 24 hour communication is possible in remote

and hilly areas with excellent quality

3. As the band width is high a large amount of

information can be send at a faster rate.

4. It is cheaper and maintenance free as compared to

cable communication.

5. It can be used in G.P.S. (Global Positioning

System) to decide position of any object accurately.

Application of remote sensing

32

9011041155 / 9011031155

Remote sensing A remote sensing satellite is orbiting

in polar orbit at nearly 1000km from the surface of the

Earth.

Uses

1. In meteorology for weather forecasting, prediction

of storm, snow fall etc.

2. In collection of scientific data such as changes in

Earth’s magnetic field, gravity, ionosphere etc.

3. In geological survey of underground water, oil,

radioactive substances etc.

4. In military operations such as movement of troops,

deployment of tanks etc. and for spying.

5. Aerial survey of flood, storm, draught affected

areas. Survey of crop yields, crop diseases.

6. For finding fishing zones in sea, to observe

development of forest.

33

9011041155 / 9011031155

7. In oceanography to study ocean currents,

temperature of the ocean surface.

• Ask Your Doubts

• For inquiry and registration, call 9011041155 /

9011031155.

34