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COMMUNICATION SYSTEM

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COMMUNICATION SYSTEM COMMUNICATION : It is a process of transmitting information or signal from one point known as source to another point known as destination. ... – PowerPoint PPT presentation

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Title: COMMUNICATION SYSTEM


1
COMMUNICATION SYSTEM
  • COMMUNICATION
  • It is a process of transmitting information
  • or signal from one point known as source
  • to another point known as destination.
  • Information can be continuous such as
  • music, picture etc. or discrete signals like
  • data from computer etc.

2
  • BLOCK DIAGRAM OF A COMMUNICATION
    SYSTEM
  • I I
  • N N
  • F F
  • O O
  • R R
  • M M
  • A A
  • T T
  • I I
  • O O
  • N N

3
  • As shown in the figure above, the first block
    at the source is an input transducer which is
    used to convert physical quantity (non
    electrical) to electrical quantity. For example
    voice is converted to electrical quantity using
    microphone. Similarly at the destination output
    transducer is used to convert electrical back to
    physical quantity. For example a loudspeaker is
    used to convert voice signal in the form of
    electrical back to physical quantity.

4
  • There are basically three essential blocks in
    a communication system.
  • Transmitter (TX)
  • Transmission Channel
  • Receiver (RX)

5
  • Transmitter
  • The output signal of the transducer is a
    complex signal. It is restricted to desired range
    of frequencies. On this signal modulation is
    performed. Modulation is a process of altering
    the characteristics of carrier signal in
    accordance with the information. There are
    basically three types of modulation technique
  • Amplitude modulation.
  • Frequency modulation.
  • Phase modulation.

6
  • Transmission channel
  • It is a medium over which the electronic
    signal is transmitted from one point to another.
    This medium can be either wired or wireless.
  • An example for wired communication is telephony
    where a pair of physical wires is running
    parallel between transmitter and receiver. Now a
    days optical fibers are used in between
    transmitter and receiver in which light carries
    the information. Similarly an example for
    wireless communication is radio communication
    where two antennas are employed, one at the
    transmitter and other at the receiver. The
    transmitter antenna transmits the modulated
    information into free space and the receiver
    antenna picks up the modulated information which
    is later demodulated to get the information back.
  • The modulated signal is then transmitted over
    a transmission channel.

7
  • Noise
  • It is a random, undesirable electrical energy
    that interferes with the transmitted signal. It
    can be either natural noise such as noise caused
    by lightning during rainy season or man made
    noise produced by ignition system of cars etc.
    Noise is a serious problem which cannot be
    eliminated but one can reduce the effect caused
    by it on the signal.

8
  • Receiver
  • It is a collection of electronic circuits
    designed to convert the modulated signal back to
    modulating signal. This process is known as
    demodulation. Finally an output transducer is
    employed to convert back the information in
    electrical form to physical form.

9
MODULATION
  • Modulation is a process of altering the
    characteristics of carrier signal in accordance
    with the information or modulating signal.
  • Carrier signal It is a high frequency signal
    that carries the information from transmitter to
    receiver.

10
Need for modulation
  • The height of the antenna required to transmit
    and receive radio waves is a function of
    wavelength of the frequency used. i.e.? c/f. At
    low frequency, ? is high and hence the height of
    the antenna should be more to transmit the signal
    (since ? is proportional to h). Therefore
    high frequencies are used to transmit the
    information which requires antenna of lesser
    height.
  • At low frequency radiation is poor and signal
    gets highly attenuated. Therefore signal
    cannot be transmitted over longer distance.
    Modulation effectively increases the frequency
    of the signal to be radiated and thus increases
    the distance over which signal can be transmitted
    faithfully.
  • The modulation permits multiplexing to be used.
    Multiplexing is method of transmitting two or
    more informations simultaneously over a single
    channel. In this method each message signal is
    modulated using different carrier signal
    frequencies and then transmitted over a single
    channel. At the receiver the message signals are
    extracted individually by tuning to their
    respective carrier frequencies.

11
AMPLITUDE MODULATION
  • It is defined as a process of varying the
  • amplitude of the carrier wave
  • proportional to the instantaneous
  • amplitude of modulating signal.

12
CARRIER AND MODULATING SIGNALS
13
AMPLITUDE MODULATED SIGNAL
14
Time domain analysis
  • Let the equation of carrier signal be c(t)
    Accos(2?fct) where Ac is the peak amplitude of
    carrier signal and fc is the frequency of the
    carrier signal.
  • Let the equation of modulating signal be
    m(t) Amcos(2?fmt) where Am is the peak
    amplitude of modulating signal and fm is the
    frequency of the modulating signal.
  • Then by the definition of AM
  • VAM (t) AcAmcos(2?fmt)cos(2?fct)
  • Accos(2?fct) (Am) cos(2?fmt)
    cos(2?fct)
  • Accos(2?fct) (mAC/2)
    cos2?(fc fm)t cos2?(fc - fm)t
  • Where m is the modulation index of AM
    signal which is defined as
  • ratio of amplitude of modulating signal to
    that of carrier signal.

15
  • SIGNIFICANCE OF MODULATION INDEX
  • The significance of modulation index is, it
    decides the depth of modulation. If it is less
    than one, then AM signal is known as under
    modulated signal. If it is more than one, then AM
    signal is known as over modulated signal. If it
    is equal to one , then AM signal is known as
    perfect modulated signal. To obtain the original
    information, modulation index should always be
    less than or equal to one.

16
  • Spectrum of AM signal
  • Ac
  • mAc/2 mAc/2
  • fc -fm fc fc
    fm f (Hz)

17
  • As shown in the figure above, the spectrum
    consisted of three frequency components, one at
    fc and other two at fc fm , fc- fm. The
    frequencies fc fm and fc- fm are known as
    sideband frequencies i.e. fc fm is known as
    upper sideband frequency and fc- fmis known as
    lower sideband frequency . The separation between
    these two frequencies is defined as bandwidth of
    AM signal. Therefore the bandwidth of AM signal
    is 2fm.

18
Total power required to transmit AM signal
  • The total power required to transmit AM
    signal ( PT ) is sum of power
  • required to transmit carrier signal ( PC )
    and power required to
  • transmit side band signals ( PTSB ).
  • Therefore PT PC PTSB


  • PC PLSB
    PUSB
  • (AC2/2R)
    (m2AC2/8R) (m2AC2/8R)
  • PC 1
    ( m2/2)

19
  • The above equation gives the total power
    required to transmit AM signal in terms of
    carrier power and modulation index.
  • For 100 modulation m 1,
  • Therefore PT PC 1 ( ½)
  • PC 0.6666 PT

    Or PC 66.66PT
  • i.e. 66.66 of total power is wasted in
    transmitting carrier signal.

20
  • Current calculation
  • PT PC 1 (m2/2)
  • IT2R IC2R 1 (m2/2)
  • IT IC 1 (m2/2) 0.5
  • where IT is the current with modulation , IC
    is the current without modulation and R is the
    resistance of the antenna.

21
Modulation by several sine waves
  • In modulation by several sine waves,
    modulating signal consists of several sine waves
    i.e.
  • m(t) Am1cos(2?fm1t) Am2cos(2?fm2 t) .
    . .
  • For modulation by several sine waves overall
    modulation index will be
  • mt (Am12 Am22 ..)/Ac20.5
  • (m12 m22 .)0.5

22
  • Therefore Total power will be
  • Similarly current with modulation will
  • be

PT PC 1 (mt2/2)
IT IC 1 (mt2/2) 0.5
23
Problems
  • 1. An audio signal 10sin (2?1000t) amplitude
    modulates a carrier of 40sin (2?2000t). Find
  • Modulation index
  • Sideband frequencies.
  • Bandwidth.
  • Total power delivered if RL 1K?.
  • Amplitude of each side band components

24
  • Solution
  • i) Modulation index m 0.25.
  • ii) Sideband frequencies
  • Upper side band fC fm
    300Hz.
  • Lower side band fC - fm
    1000Hz.
  • iii) Bandwidth 2fm 2KHz.
  • iv) Total power delivered
  • PT 1 (0.252)/2
  • 1.03125
  • v) Amplitude of each sideband
    mAc/25V

25
  • 2. The antenna current of an AM transmitter is
    8A when only carrier is transmitted, but
    increases to 8.93A when carrier is modulated by a
    single sine wave. Find the percentage modulation.
    Determine the antenna current when the depth of
    modulation changes to 0.8A.
  • Solution
  • i) Given IT 8.93A.
  • IC 8A.
  • IT IC 1 m2/2
  • m 0.701 70.1.

26
  • ii) IT ?, when m 0.8
  • IT 8 1(.7)2/2 9.19A .
  • EXERCISE
  • 1. A certain transmitter radiates 9KW with
    carrier unmodulated and 10.125KW when carrier is
    sinusoidally modulated. Calculate modulation
    index. If another sine wave corresponding to 40
    modulation is transmitted simultaneously,
    determine the total power radiated.

27
FREQUENCY MODULATION
  • It is defined as a process of altering the
  • frequency of the carrier signal w.r.t.
  • instantaneous amplitude of modulating
  • signal.

28
CARRIER AND MODULATING SIGNALS
29
FREQUENCY MODULATED SIGNAL
30
  • Time domain analysis
  • From the definition
  • fFM fC
    Kfm(t)
  • Where Kf is known as frequency sensitivity.
  • fFM fC Kf
    Amcos(2?fmt)
  • fC ?f
    cos(2?fmt)
  • where ?f is known as frequency deviation. Its
    signifies , by how much amount carrier frequency
    gets deviated.

31
  • Multiplying by 2? on both sides
  • 2?fFM 2?fC 2??f cos(2?fmt)
  • ?FM ? C ? ? cos(2?fmt)
  • ? FM d?(t)/dt , integrating both the sides
  • ? FMt ? Ct (? ? / ? m)sin(2?fmt)
  • Therefore equation of FM Signal
  • VFM(t) ACcos?(t)
  • ACcos? Ct ß
    sin(2?fmt)

32
  • where ß ?f/fm is defined as modulation index of
    FM. Unlike AM modulation index is not restricted
    to one. It can be more than unity.
  • Frequency spectrum
  • The frequency spectrum of FM signal consisted of
    infinite number of sideband components ( using
    Fourier Transform ). Hence theoretically the
    bandwidth of FM signal is infinity. But
    practically, the bandwidth of FM signal is
    restricted using Carsons rule. According to
    Carsons rule the bandwidth of FM signal is given
    by 2(?f fm).

33
Problems
  • Given a FM equation
  • VFM(t) 10 cos 2?108t 5 sin(2?15000t)
    , Calculate
  • Carrier frequency.
  • Modulating frequency.
  • Frequency deviation.
  • Bandwidth using Carsons rule.

34
  • Solution
  • Carrier frequency fC 108Hz.
  • Modulating frequency fm 15KHz.
  • Frequency deviation ?f ß fm 5 15
    75KHz.
  • Bandwidth 2(?f fm ) 2( 75 15 )
    180KHz.
  • In an FM system when the audio frequency
    is 50Hz , modulating voltage is 2.5V , the
    deviation produced is 5KHz. If the modulating
    voltage is now increased to 7.5V , calculate the
    new value of frequency deviation. If the AF
    voltage is raised to 10V while the modulating
    frequency is dropped to 250Hz , what is the
    frequency deviation produced. Also calculate
    modulation index in each case.

35
  • Solution
  • Given fm 50Hz , Am 2.5V , ?f 5KHz.
  • Modulation index ß ?f/fm 5103/50 100
  • ii) If Am 7.5V , ?f ?
  • Kf ?f/Am2KHz/V.
  • ?f Kf Am 27.5 15KHz.
  • Modulation index ß ?f/fm 300.
  • iii) ?f Kf Am 210KHz 20KHz.
  • Modulation index ß ?f/fm 800.

36
EXERCISE
  • A carrier of amplitude 5V and frequency 90MHz is
    frequency modulated by asinusoidal voltage of
    amplitude 5V and frequency 15KHz. The frequency
    sensitivity is 1Hz/V. Calculate the frequency
    deviation and modulation index.
  • Compare and contrast AM and FM.

37
  • END
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