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TechGuru Technolic
Unit 4
Q1) Discuss the general communication system using block diagram.
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Block diagram of basic communication systems:-
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Communication systems convey information from one point to another physical
channels that propagate electromagnetic, acoustic, particles density or other waves.
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Communication system can not only link people or systems at great distances via
audio, visual, computer, or other messages but may link the various parts with in
systems and even within single semiconductor chips.
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The elements of a basic communication system are transmitter, communication
medium and the receiver.
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When the transmitted signal is travelling from the transmitter to the receiver, noise
gets added to it.
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The elements of basic communication system are as follows:
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Information or input signal:-
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The communication systems have been developed for communicating useful
information from one place to another.
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The information signal is also called as message signal.
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Input transducer:-
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The information in the form of sound, picture or data signals cannot be transmitted as
it is first to be converted into a suitable electrical signal.
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The input transducer block does this job. The input transducers commonly used in the
communication systems are television, camera etc.
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Transmitter:-
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The function of the transmitter block is to convert the electrical equivalent of the
information to be a suitable form.
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The transmitter consists of the electronic circuits such as amplifier, mixer, oscillator
and power amplifier.
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Communication channel:-
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The communication channel is the medium used for transmission of electrical signal
from one place to another.
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It can be conducting wires, cables, optical fibres or free space.
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Noise:-
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Noise is a random, undesirable electrical energy that enters the communication
system via the communication medium and interfers with transmitted message.
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Noise is an unwanted electrical signal which gets added to the transmitted signal
when it is travelling towards the receiver.
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The noise can be either natural or man-made sources.
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The sources of natural noise are lightning or radiation from the sun and stars etc.
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Man-made noise are welding machines, electric motors etc.
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Receiver:-
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The reception is exactly the opposite process of transmission. The received signal is
amplified demodulated and converted into a suitable form.
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It consists of electronic circuits like mixer, oscillator, detector, amplifier etc.
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Output transducers:-
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It converts the electrical signal at the output of the receiver back to the original from
i.e sound or tv pictures etc.
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Eg:- loudspeakers, picture tubes, computer monitor etc.
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Q2)Explain a typical radio transmitter system using block diagram.
Crystal
oscillator
Modulation in
Rf buffer
amplifier
Rf voltage
Rf output
And power
amplifiers
Power
amplifier
Modulation
processing
Modulation
Modulation
power
Voltage
amplifier
amplifier
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The message arriving from the information source needs to be processed before
transmitted.
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The modulated signal cannot travel long distance on its own.
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In order to enhance the modulated signal modulation voltage and power amplifiers are
used.
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The crystal oscillator generates a fixed frequency which is constant with respect to
time
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This carrier generated in the crystal oscillator is protected from other circuitries by
means of RF buffer amplifier.
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Finally, the modulated signal is applied to the RF (radio frequency) output power
amplifier, which in turn, is connected to the transmitting lower.
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Thus, to original message after being processed, is transmitted in the atmosphere or
through fibre optics or through cables.
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Q3) Explain typical radio receiver system using block diagram.
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The transmitting antenna radiated the radio waves
In the space in all direction.
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These radio waves travel with the velocity of light.
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Electrical energy can be radiated into space. On reaching the antenna, the radio waves
induced tiny e.g. in it. This small voltage is fed to the radio receiver.
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Here the radio waves first amplified and then signal is extracted from them by the
process of demodulation.
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The signal is amplified by audio amplifiers and then fed to the speaker for
reproduction into sound wave.
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Q4) what is the need for modulation in communication system?
MODULATION:•
Modulation is an operation performed at the transmitter to achieve efficient and
reliable information transmission.
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Modulation is a process of mixing a signal with a sinusoid of high frequency to
produce a new signal. This new signal will have certain benefits of a un- modulated
signal, especially during transmission. If we look at a general function for a sinusoid:
f(t) = A sin(ωt + φ)
NEED FOR MODULATION:•
Modulation for efficient Transmission:
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Signal transmission over appreciable distance always involves a traveling
electromagnetic wave, with or without a guiding medium.
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The efficiency of any particular transmission method depends upon the frequency of
the signal being transmitted.
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Typically, efficient line-of-sight radio propagation requires antennas whose physical
dimensions are at least 1/10 of the signal‘s wavelength.
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Unmodulated transmission of an audio signal containing frequency components
down to 100 Hz would thus require for antennas 300 km long. Modulated
transmission at 100 MHz, as in FM broadcasting, allows a practical antenna size of
about one meter.
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At frequencies below 100 MHz, other propagation modes have better efficiency with
reasonable antenna sizes.
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Modulation to Overcome Hardware limitations:
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The cost and availability of hardware is the constrained of communication system
which is frequency dependent.
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Modulation permits the designer to place a signal in some frequency range that avoids
hardware limitations.
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Hardware costs and complications are minimized by keeping the fractional bandwidth
within 1-10 %.
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Modulation for Frequency Assignment:
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When you tune a radio or television set to a particular station, you are selecting one of
the many signals being received at that time. Since each station has a different
assigned carrier frequency, the desired signal can be separated from the others by
filtering.
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Due to modulation it is possible to modulate different sound signals with different
frequency carriers thereby create the modulated signals that occupy different slots of
the frequency spectrum and avoid a jumble of signals.
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Modulation for Multiplexing:
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Multiplexing is a process in which two or more signals can be transmittted over the
same communication channel simultaneously.
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This is possiple only with the modulation. Th muliplexing allows the channel to be
used by many signals.
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Therefore many TV channels can use the same frequency range, without getting
mixed with each other.OR different signals can be transmitted at the same time.
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Increases the range of communication:
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The frequency of base band signal is low. Therefore it cannot travel over a long
distance.
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When such signals are transmitted, they get heavily attenuated. The attenuation of the
signal reduces with increase in frequency of transmitted signal and they travel larger
distance.
The modulation process up shifts the frequency of the signal to be transmitted. Therefore it
increases the range of the communication.
Q5)Define amplitude modulation and modulation index for amplitude modulation.
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Amplitude modulation is the process of changing the amplitude of a high
frequency carrier signal in proportion with the instantaneous value of the modulating
signal
Modulation index for amplitude modulation is given by:In AM wave the modulation Index (m) is defined as the ratio of amplitudes
of the modulating and carrier waves as follows:
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Where
is the peak amplitude
is the peak carrier amplitude
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% Modulation =
*100
Q6)Derive the power relation for amplitude modulated waves?
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power in A.M wave
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The power dissipated in any circuit is a fuction of the square of voltage across the circuit
and the effective resistance of the circuit.
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equation of A.M wave reveals that it has the three componenrs of amplitude EC ; mEC/2 and
m Ec/2.
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clearly power output must be distributed among this components.
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Carrier power ,PCC/2/R =E2C/2R ……………..(1)
Total power of sidebands,Ps=(mEc/22)/R +(mEc/2/2/R
= m2E2c/8R +m2E2c/4R…………………………….(2)
Total power of AM wave ,Pt= Pc+ Ps
=E2c/2R +m2E2c/4R = E2/2R [1+m2/2]
Or Pt = E2c/2R [2+m2]/2……………………………………….(3)
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Fraction of total power carried by sidebands is PS/PT =m2/2+m2...................................(4)
As the signal is contained in the sideband frequencies ,therefore useful power is in the
sidebands .
Inspection of (4) reveals that sideband power depends upon the modulation factor m.
The greater is the useful power carried by the sidebands.
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This emphasies th e importance of modulation factor.
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When m=0,power carried by sidebands= 022+02=0.
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When m= 0.5 ,power carried by sidebands =(0.5)2/2+(0.5)2=11.1% of total power of
wave.
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When m=1,power carried by sidebands =(1)2/2+(1)2 =33% of total power of AM wave.
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As an example ,suppose the total power of an AM wave is 600 watts and modulation is
100%.
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Then sideband power is 600/3=200 watts and carrier power will be 600-200=400 watts.
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which is required as the means of transmission.
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Note : pc=E2c/2R and ps = m2E2c/4R
Therfore Ps/Pc =1/2 m2
Or Ps=1/2 m2Pc…....................................................(5)
Expression (5) gives th e relation between the total sideband power(Ps) and carrier power(Pc).
Q7)explain SSB in detail.
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SSB is a single sideband or single sideband signal.
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The transmission bandwidth of standard AM as well as DSB-SC modulated wave is
2WHZ or 2fm HZ.
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In both these system; one half of the transmission bandwidth is occupied by the upper
sideband (USB) and the other half is occupied by the lower sideband (LSB).
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When only one side band is transmitted, the modulation is referred to as single side
band modulation . it is also called as SSB or SSB-SC
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The DSB-FC signal is power efficient but bandwidth inefficient so the next step is to
suppress the redundant sideband.
Observation:•
The SSB waves is a single frequency wave . it’s frequency is equal to the sideband
frequency
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The amplitude of the SSB waves is proportional to the amplitude of the modulating
signal (fm ).
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The frequency of the SSB waves varies with the frequency of the modulating signal
(fm ). The upper side band is increase the frequency with increase fm .
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Modulated signal:-
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SSB signal:-
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Frequency domain representation:-
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The frequency spectrum of the SSB signal. This shows that the bandwidth of the
SSB signal will be ‘fm’ instead of ‘2fm’.
Advantages:•
Less bandwidth requirement as SSB requires a BW of fm. this will allow more number
of signals to be transmitted in the same frequency range.
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Lots of power saving. This is due to the transmission of only one sideband
component.
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Disadvantages:•
This generation and reception of SSB signal is complicated.
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The SSB transmitter and receiver need to have an excellent frequency stability. A
slight change in frequency will hamper the quality of transmitted and received signal.
Transmission bandwidth of SSB-SC:Since we are transmitting the frequency only in the range (fc+fm) or (fc-fm), the transmission
bandwidth for the SSB-BC will be,
Bandwidth
B= (fc+ fm) -fc = fm Hz
Or
B=fc -(fc - fm) = fm Hz
Applications of SSB:SSB transmission is used in the applications where the power saving and low bandwidth
requirements are important. The application are land and air mobile communication,
telemetry, military communication, navigation and amateur radio.
Q8)Derive the relation for percentage power savings in ssb
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single sideband
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Amplitude modlate signal had a carrier and two side bands this is called double side band
full carrier .
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the carrier signals does not contain any information in its self.so we can suppressed the
carrier.
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the resultant signal is called double sideband suppressed carrier.
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the two sided band carry the same information.
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hence it is sufficient to send one of the sideband.
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This is called as single sideband transmission.
The power relation in AM is given by Pt=Pc(1+m2/2)
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The maximum power exists in the carrier signal .the two sidebands have1/4 carrier power
each.thus if carrier is supressed & one sideband is eliminated then the 2/3 power saving is
archieved.
Percentage power saving in ssb.
=power saved/total power.
=Pc+m2/4 Pc/Pc(1+m2/2)
=Pc(1+m2/4)/Pc(1+m2/2)
=(1+m2/4)/(1+m2/2)