PRINCIPLES OF COMMUNICATION

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PRINCIPLES OF COMMUNICATION

• GRADE XII
• OUR OWN HIGH SCHOOL, AL WARQAA
• DUBAI

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COMMUNICATION- Transmission of information
/message from one point to anotherCommunication
enters our lives in many ways- Telephone
makes us talk to any person anywhere.- Radio and
television entertain and educate.-
Communication signals as navigational aids
ships, aircrafts and satellites .- Weather
forecasting conditions measured by a multitude
of sensors are communicated to forecasters.-
Videophones, voicemail and satellite conferencing
enable seeing live images instantly and
communicate directly with people located far
away.- Digital data transmission and retrieval
has made realization of e-mail, FAX and
internet possible.We communicate through
speech. In modern communication systems, the
information is first converted into electrical
signals and then sent electronically.
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COMMUNICATION SYSTEMTwo persons talking to each
other constitute the simplest communication
system. The person who speaks is the source, the
person listening is the receiver and the
intervening air is the communication link
between them.A communication system consists of
three basic components- Transmitter (source)-
Communication channel (link medium)-
ReceiverNature/details of these components
depend on1. Nature of the signal/ message to
be communicated.2. Distance which separates the
source and the receiver.- Direct talking is
possible over short distances sound waves
attenuate fast.- Long distance communication
requires the signal/message to be converted
into an electrical signal/a set of signals
/electromagnetic waves.- Long distance
communication requires a link between the source
and the receiver
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Message signal
Communication channel
Out put signal
Transmitter
Receiver
NOISE
CUMMUNICATION SYSTEM
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Transmitter Transmits the message/signal
over the communication channel. Quite often the
original signal is not suitable for transmission
over the communication channel to the receiver.
It requires to be modified to a form suitable for
transmission.Communication Channel Provides
a link between the transmitter and the receiver.
It can be a transmission line (telephone and
telegraphy), an optical fibre (optical
communication) or free space in which the signal
is radiated in the form of electromagnetic
waves.Receiver Reconstructs the original
message/signal after propagation through the
communication channel.
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TRANSMITTER- We communicate through a message
or a signal.- A transmitter transmits the
message over the communication channel to the
receiver.- As a rule, the message produced by
the source is not suitable for transmission
over the communication channel. Accordingly, a
suitable transducer converts it into a time
varying electrical signal called the message
signal. A transmitter, in its simplest form,
is a setup which boosts the power of message
signal and feeds it into the communication
channel
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Antenna
Antenna
Loud speaker
Amplifier
Amplifier
Microphone
Receiver
Transmitter
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Basic constituents of a transmitter are 1.
Message signal 2. Modulation 3. Antenna
Message signal A single valued function of
time that conveys the information.
Analog Signals
Discrete or digitalAnalog Signal Is a
continuous function of time, with the amplitude
(instantaneous value of the signal) being
continuous.
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In its simplest form, the transmitter has
following problems1. Size of the antenna or
aerial For transmitting a signal we need an
antenna. It should have a size comparable to
the wavelength of the electromagnetic wave
representing the signal ( at least ?/4) so
that the time variation of the signal is properly
sensed by the antenna. For an
electromagnetic wave of frequency 20 kHz, the
wavelength ? is 15 km. Obviously such a long
antenna is not possible. Therefore, direct
transmission of such a signal is not
possible. If the frequency of the signal is 1MHz,
the corresponding wavelength is 300m and
transmission of such a signal is possible.
Therefore, there is a need of translating the
information contained in the original low
frequency signal into high or radio-frequencies
before transmission.2. Effective power
radiated by an antenna The power radiated
from a linear antenna ? For a good
transmission we need high power hence there is
need for high frequency transmission.
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3. Mixing up of signals from different
transmitters Direct transmission of baseband
signal leads to interference from multiple
transmitters. Thus multiple user friendly
communication is not possible. A possible
solution is provided by employing
communication at high frequencies and then
allotting a band of frequencies to each
user. The above arguments suggest that there
is a need for translating the original signal
( low frequency) into a high frequency wave
before transmission such that the translated
signal continues to possess the information
contained in the original signal. The high
frequency wave carrying the information is
called the carrier wave. The process of
transformation is called Modulation.Modulation
Transformation of the signal into a form
suitable for transmission through a given
communication channel
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Antenna
Antenna
To Speaker
Tunable Amplifier
Audio Amplifier
Demodu-lator
Modulator
Amplifier
Signal
Receiver
Transmitter
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Simplest form of an analog signal is a
sinusoidal signal having a single frequency
g(t) A sin ?t
Signals generated by different sources have their
own characteristics - amplitude, frequency or
nature. Nature Simple single frequency or a
complex superposition of several
frequency components The signals associated
with music or speech are complex can be
considered as superposition of several sinusoidal
signals of varying amplitudes and
frequencies. The range over which the
frequencies in a signal vary is called the
bandwidth (B) (the frequency range between the
lowest and highest frequency components).
Bandwidth for audio signals is 20 Hz to 20 kHz.
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Discrete Signals Discrete signals are
discontinuous in time they are defined only at
discrete times. In case of
discrete signals the independent variable (time)
takes only discrete values which are usually
uniformly spaced. Consequently, discrete-time
signals are described as sequences of samples
whose amplitudes may take a continuum of
values. When each sample of a discrete-time
signal is quantized I.e. its amplitude is
only allowed to take on a finite set of values
(e.g. in a binary representation low and high
signals are designated as 0 and 1) and then
coded, the resulting signal is referred to as a
digital signal.
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ModulationWe have seen- Direct transmission
of audio frequency signals is not possible
associated wavelengths are large. - Quite often,
the message signals require a carrier and a
communication channel for transmission from
source to the receiver. - An efficient
utilization of the communication channel requires
a shift of the range of base band frequencies
to another frequency range suitable for
transmission. A shift of the range of
frequencies in a signal is accomplished by
employing a process called modulation. In this
process the signal is superimposed over the
carrier wave. The frequency, fc, of the carrier
wave must be much higher than the highest
frequency component of the message signal.The
form of modulation depends on the specific
characteristics of the carrier wave, nature of
the message signal/data and the communication
channel. The carrier wave may be (I) continuous
or (ii) pulse. The signal may be analog or
digital.
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LESSON - 3

• DIFFERENT TYPES OF MODULATION TECHNIQUES

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Forms of Modulation
Amplitude Modulation Analog Signals
Angle/Frequency Modulation
Pulse Modulation
Amplitude shift
keying (ASK)Digital Signals
Frequency shift keying (FSK)
Phase shift keying (PSK)
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ANALOG SIGNALSAmplitude Frequency
ModulationA sinusoidal wave conveys no
information. To transmit information by the usual
sinusoidal waveform, the characteristics of the
wave must be varied in some manner. A sinusoidal
carrier wave C(t) is defined by
C(t) Ac cos (??c t ?o )The
modulation of the carrier wave can be
accomplished in two ways (1) The amplitude of
the carrier wave is varied about a mean value,
linearly with the baseband signal m(t), the
angular frequency ??c remaining constant.
This mode of modulation is termed as amplitude
modulation. (2) The phase angle ? of the
carrier wave is varied according to the
baseband signal, the amplitude of the carrier
wave being kept constant. This mode of
modulation is termed as angle modulation.
There are two variations of angle modulation -
phase modulation (PM) and frequency
modulation (FM).
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Amplitude Modulation- Employed for commercial
broadcasting of voice signals. Carrier
frequencies 0.5 to 20 MHz.- Broadcast noisy
noise signals created by atmospheric static or
man made electric discharges also get
amplitude modulated.Frequency Modulation- TV
broadcast , VHF, UHF, SHF and EHF broadcasts.-
Requires higher carrier wave frequencies.- Noise
generated by atmospheric or man made electric
discharges does no harm to intelligence. -
Higher Signal/Noise ratio, quality of broadcast
very good. FM Radio 88 to 108 MHz VHF TV
47 to 230 MHz UHF TV 470 to 960 MHz
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HOMEWORK

• A carrier wave of peak voltage 12V is used to
  transmit a message signal. What should be the
  peak voltage of the modulating signal in order to
  have a modulation index of 75?
• For an amplitude modulated wave , the maximum
  amplitude is found to be 10V while the minimum
  amplitude is found to be 2V. Determine the
  modulation index,µ.

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LESSON - 4

• BANDWIDTH PROPAGATION OF E.M WAVES

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BANDWIDTH OF SIGNALS

• Bandwidth refers the frequency range over which
  the portion of the spectrum occupied by the
  signal.
• In a communication system, the message signal can
  be voice, music, picture or computer data.
• Each of these has different ranges of
  frequencies. The type of communication system
  needed for a given signal depends on the band of
  frequencies which is considered essential for the
  communication process.
• For speech signals, frequency range 300Hz to
  3100Hz is considered adequate. Therefore speech
  signal requires a bandwidth of 2800Hz(3100Hz-300Hz
  ) for commercial telephonic communication.

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• For speech signals, frequency range 300Hz to
  3100Hz is considered adequate. Therefore speech
  signal requires a bandwidth of 2800Hz(3100Hz-300Hz
  ) for commercial telephonic communication.
• To transmit music an approximate bandwidth of
  20khz is required.
• Video signals for transmission of pictures
  require about 4.2MHz of bandwidth.
• A T V signal contains both voice and picture and
  is allocated 6 MHz of bandwidth for transmission.

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Digital signals are in the form of rectangular
waves as shown in fig. This rectangular
wave can be assumed to be the superposition
of sinusoidal waves of frequencies n?o. This
implies an infinite band width. For practical
purposes, the contribution from higher and
lower frequencies can be neglected thus
limiting the bandwidth.
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Bandwidth of Transmission medium

• Different types of media offer different
  bandwidths.
• Commonly used transmission media are wire, free
  space and optic cable.
• Coaxial cable is widely used wire medium which
  offers a bandwidth of 750MHz.
• An optical fibre can offer a transmission
  bandwidth in excess of 100GHz.
• Spectrum allocations are arrived at by an
  international agreement. The international
  Telecommunication Union(ITU) administers the
  present system of frequency allocations.

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Communication through free space
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Propagation of EM waves

• Ground wave propagation
• A radio wave that can travel directly from one
  point to another following the surface of the
  earth is called a ground wave.
• Ground wave propagation is possible only when the
  transmitting and receiving antenna are close to
  the surface of the earth.
• When the em wave glides over the earth surface,
  its electric field component induces charges
  hence current on the earths surface.
• Ground wave gets attenuated due to diffraction
  effect.

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• Ground wave propagation is possible at low
  frequencies, 500 kHz 1500 kHz. This region of
  the AM band is called medium wave band.
• Maximum range of ground wave propagation depends
  on two factors a) the freq. of the transmitted
  wave and b) the power of the transmitter.
• ADVANTAGES i) Given enough transmitting power,
  ground waves can be used to communicate between
  any two locations on the earth. (ii) Ground waves
  are relatively unaffected by changing atmospheric
  conditions.
• Disadvantages
• i) Ground waves require a relatively high
  transmitting power.
• ii) Ground losses vary with surface material
  and composition.
• iii) Large antennas are required for
  transmission.

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Space wave propagation

• If a radio wave transmitted from antenna,
  travelling in a straight line, directly reaches
  the receiving antenna, it is called a space wave.
  
• Its also called tropospheric or line of sight
  propagation.
• The range of space wave propagation is limited by
  the line of sight distance and the curvature of
  the earth.

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Range of a transmitting antenna

• The Range of coverage of transmission from an
  antenna is
• d v2hR, where R radius of the earth and h
  height of the antenna.
• The range of space wave propagation between two
  towers on earths surface is dM dT dR.

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SKY WAVE PROPAGTION

• A radio wave directed towards the sky and
  reflected by the atmosphere towards the desired
  location of the earth is called a sky wave.
• Radio waves freq. 2MHz 20MHz can be reflected
  by the ionosphere.
• This region of the AM wave band is called short
  wave band.
• The radio waves of different frequencies sent
  from earth will be reflected back the
  appropriate layers of the ionosphere. The
  critical frequency for reflection is, therefore,
  given by

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HOME WORK

• The TV signals have a bandwidth of 4.7MHz. What
  is the number of channels that can be
  accommodated in a bandwidth of 4700GHz.
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  (106 channels)
• A transmitting antenna at the top of a tower has
  a height 32m and that of the receiving antenna is
  100m. What is the maximum distance between them
  for satisfactory communication in LOS mode?
  Radius of earth 6400km.
• The TV tower at a particular station has a
  height of 160m.
• a) What is its coverage range?
• b) How much population is covered by the
  transmission. If the average
• density around the tower is 1200
  /km2.
• c) By how much should the height be
  increased to double its coverage
• range? Given radius of the earth
  6400km.
• ( 45km, 76.37lakhs, 480m)