How to Study Communication Systems for XII-Physics?

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Communication System
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Communication System
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There are two basic modes of communication
1) point – to – point communication and broadcast.
e.g.
2) Broadcast mode,
i.
Single
analog or digital.
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ii. Transmitter
iii. Transducer
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.
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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)
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
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consists of a pickup antenna to pick up signal,
demodulator, an amplifier and the transducer.
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.
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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. Demodulation
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.
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Bandwidth of Signals
Different frequency bands
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
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(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
bands
Standard
AM 540 – 1600 kHz
broadcast
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
Cellular Mobile
TV
896 – 901 MHz
Mobile to base station
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Radio
840 – 935 MHz
Base station to mobile
Satellite
5.925 – 6.425 Uplink
communication
GHz
3.7 – 4.2 GHz
Downlink
Need For Modulation
base band signals.
Size of the Antenna or aerial
For
efficient
radiation
and
reception,
the
transmitting antennas (or antennae) would have
lengths =
4
of frequency used.
A vertical antenna of this size is impracticable and
hence direct transmission of such base band signals is
not practical.
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3 108
15 103
20 km
4
5 km
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

2
. This means, for the same
antenna length, the power radiated increases with
decreasing
(i.e
increasing
frequency).
Hence
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.
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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
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
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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
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
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modified carrier waves called modulated waves. The
carrier wave may be continuous (sinusoidal) or in the
form of pulses as shown in fig
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.
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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.
Amplitude Modulation
Let c(t) = Ac sin ωct represents a carrier wave and
m (t) = Am sin ωm t represents the message of the
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modulating signal where ωm = 2πfm is the angular
frequency of the message signal.
The modulated signal cm (t) can be written as
=
c (t) + m (t) sin ωc t
=
cm (t)
(Ac + Am sin ωm t) sin ωc t
=
cm (t) = Ac sin ωc t + uAc sin ωc t ……. (1)
Where
Am
is the modulation index. In practice, μ
Ac
is kept ≤ 1 to avoid distortion.
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By using sin A . sin B =
1
cos A
2
B
cos A B
equation (1) becomes
Cm (t)
A c sin
c
t
uA c
cos (
2
uA c
cos (
2
c
m)
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t
c
m)
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Production and Detection Amplitude modulated
wave
Production of A.M. Wave
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 nonlinear device and produces an output.
Y (t) = B x (t) + C x2 (t)
…….. (2)
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Where B and C are constants.
Thus,
y t
B A m sin
C
m
t
B A c sin
2
A m sin2
BAm sin
m
m
c
t
t sin
c
t
m
B A c sin
CAm A c cos (
CAm A c cos (
c
t
C 2
(Am
2
...... 3
A2 )
c
m )t
c
c
t
2
A c sin2
t
2 A m A c sin
t
c
m)
t
......... 4
1 cos2A
[ sin A
and sin A sin B
2
1
cos (A B) cos (A B)
2
2
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In equation (4), there is a d,c term
C 2
(Am
2
A2 ) and
c
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
sinusoids of frequencies ωm, 2ωm and 2ωc and retains
the frequencies ωc, (ωc ∓ωm). The output of the band
pass filter is same as equation and is therefore an AM
wave.
Detection AM wave
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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
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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.
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.
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Space Communication
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
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Ne2
n' = n 1 e0m w
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, fC = g Nmax where Nmax is the
maximum
electron
density
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of
the
ionosphere.

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Depending on the properties of ionosphere, the critical
frequency changes from 5MHz to 10MHz.
Space wave propagation
The electromagnetic waves
which
travel
transmitting
receiving
directly from
antenna
antenna
to
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
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the line of sight. But, due to the curvature of the Earth,
these waves cannot be received beyond horizon.
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.
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∴ OA =
2
2
OR1
+ AR1
2
2
2
But, OA = R + h and AR1 = h + d
2
2
2
∴ (R + h) = R + h + d
2
2
2
2
2
∴ R + 2hR + h = R + h + d
∴ 2hR = d
d =
2
2
2
2hR
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.
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Satellite Communication
The electromagnetic waves having frequencies beyond
30MHz are very useful in communication because of
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
communication by these waves.
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are
used
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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.
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Global communication.
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
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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.
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7. In
oceanography
to
study
ocean
temperature of the ocean surface.
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