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Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
Unit 4: Communication systems
Introduction to Analog Communications, Modulation Schemes :- Amplitude modulation (Derivation, frequency
spectrum only), Frequency Modulation (qualitative analysis only), AM and FM: A comparison, Introduction to Digital
Communications :- Digital System Block design , analog to digital and digital to analog conversions, error detection
and digital encoding system, Introduction to optical fibre: Advantages and disadvantages of optical fibre, optical
communication systems, Applications of fibre optic communication, Numerical problems as applicable.
10hrs
Chapter 14.1, 14.2.1, 14.2.2, 14.2.4, 14.4, 15.1.1, 15.3.1, 15.3.5
Introduction to Analog Communications
Communication is the basic process of exchanging information. The term communication refers to transmission,
reception and processing of information by electrical means.
The subject has its origin in the beginning of wire telegraphy in the middle of 19th century and radio
communication at the beginning of 20th century. Radio communication is the process of sending information from
one place and receiving at another place without using any connecting wires.it is also called wireless
communication.
Basic block diagram communication system
Information
The communication systems communicate messages. The message comes from the information sources.
Example: human voice, picture, code, data, music etc...
Transmitter
The transmitter is a collection of electronic circuits designed to convert the information into a signal
suitable for transmission over a given communication medium.
Communication channel
The communication channel is the medium by which the electronic signal is transmitted from one place to
Another.it can be a pair of conducting wire, coaxial cable, optical fiber or free space.
Noise
Noise is a random, undesirable electrical energy that enters the communication system via medium and
interferes with the transmitted message.
Receiver
A receiver is a collection of electronic circuits designed to convert the signal back to the original
information.
Communication systems can be fundamentally divided into two categories: Analog and Digital.
Analog refers to a continuous, smooth change of signal. Digital system consists of discrete, distinguishable steps.
Digital is further defined in terms of 2 possible steps known as 1 and 0, on and off, or true and false.
 In a historical context, Communications began as a strictly analog process. Original radio, telegraph, and
even telephone systems were analog simply because a digital technique does not exist.
 Analog communication had an advantage since all information transmitted at that time was analog in
nature.
 As solid state electronics evolved into integrated circuits and other devices, they showed an inherent
advantage in speed, size and power consumption when compared to analog devices.
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
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Digital electronics led to the development of desktop and laptop computer, calculators and miniature
radios.
Digital environment is better than that of analog in many ways.
Modulation
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Modulation is an important step of communication system.
Modulation is a process by which a high frequency sine wave (carrier) is made to carry a modulating signal
(like audio signal).
Here modulating signal is a message signal/information signal or base band signal.
Modulation is also defined as the process whereby some characteristic like amplitude, frequency, phase of
a carrier wave is varied in accordance with the intensity of modulating wave.
Need for modulation: (i) To separate signal from different transmitters:
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Audio frequencies are within the range of 20 Hz to 20 kHz. Without modulation all signals at same
frequencies from different transmitters would be mixed up. There by giving impossible situation to tune to
any one of them.
In order to separate the various signals, radio stations must broadcast at different frequencies.
Each radio station must be given its own frequency band.
This is achieved by frequency translation as a result of modulation process.
(ii) To reduce the height of the antenna: -
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
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For efficient transmission the transmitting antennas should have length at least equal to a quarter of
the wavelength of the signal to be transmitted.
For an electromagnetic wave of frequency 15 kHz, the wavelength λ is 20 km and one-quarter of this will
be equal to 5 km.
Obviously, a vertical antenna of this size is impractible.
On the other hand, for a frequency of 1 MHz, this height is reduced to 75m.
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wavelength 
Velocity
3  10 8
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metres
frequency frequency ( Hz)
(iii)To Increase Operating Range
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The power radiated by an antenna of length l is proportional to (l/λ)2.
This shows that for the same antenna length, power radiated is large for shorter wavelength.
Thus, our signal which is of low frequency must be translated to the high frequency spectrum of the
electromagnetic wave.
This is achieved by the process of modulation.
(iv) Modulation also increases distance of communication and reduces the effect of noise.
Amplitude modulation
AM is a process in which the amplitude of the carrier wave is varied in accordance with the instantaneous
values of the modulating signal , i.e. amplitude of the carrier is made proportional to the instantaneous
amplitude of the modulating voltage, Keeping frequency and phase of the carrier constant.
Let the Unmodulated Carrier and Message signal voltages be respectively represented as
ec = Ec cos(ωc t )--------(1) and em = Em cos(ωm t )------------(2)
Where Ec --------amplitude of carrier signal
ωc --------angular frequency of carrier signal
Em --------amplitude of message signal
ωm --------angular frequency of message signal
The carrier frequency ωc is much greater than frequency of modulating signal, ωm. the resulting
modulated wave has form
ec = [Ec + KaEm cos(ωm t)] cos(ωc t)---------------(3)
Where Ka is a proportionality factor which determines maximum variation in amplitude for a
given signal voltage Em and the amplitude factor [Ec + KaEm cos(ωm t )] represents the variation of
the wave.
ec = Ec [1+ {KaEm/Ec } cos(ωm t)] cos(ωc t)
ec = Ec [1+ ma cos(ωm t )] cos(ωc t)
where ma= KaEm/Ec is called modulation index,
ec = Ec cos(ωc t)+ ma Ec cos(ωm t)cos(ωc t)
it is used calculate percentage of modulation.
ec = Ec cos(ωc t)+ ma Ec/2 cos(ωc - ωm)t + ma Ec/2 cos(ωc+ ωm)t
Carrier
lower sideband
upper sideband
The waveforms and frequency spectrum is as shown below.
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
Frequency spectrum of AM Wave
The following points are to be noted in amplitude modulation.
1. The amplitude of the carrier wave changes according to the intensity of the signal.
2. The amplitude variations of the carrier wave are at the signal frequency .
3. The frequency of the amplitude modulated wave remains the same ie.carrier frequency .
Modulation index or degree of modulation or percentage of modulation or depth of modulation
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It is an indication of the strength of the message signal.
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
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The greater the modulation index ma the stronger and clearer will be the message signal.
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Thus for ma=1 or 100% the message signal being transmitted will be strongest.
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For amplitude modulation ma= Emax - Emin
Emax + Emin
Frequency modulation
“When the frequency of carrier wave is changed in accordance with the intensity of the signal, it is called
frequency modulation”.
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Here the amplitude of the modulated wave remains the same ie carrier wave amplitude.
The frequency variations of carrier wave depend upon the instantaneous amplitude of the signal.
When the signal approaches positive peaks as the B and F, the carrier frequency is increased to maximum
and during negative peak, the carrier frequency is reduced to minimum as shown by widely spaced cycles.
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
Disadvantages of FM over AM
Comparison of Amplitude Modulation and Frequency Modulation
The Advantages of AM
1.
The modulators and demodulators are very simple and straightforward to build. This makes the cost of
receivers very low and therefore available to the majority of people.
2. AM requires only a very small bandwidth to transmit the same information compared to FM where the
bandwidth requirement is significantly higher.
The Disadvantages of AM
1.
All communication signals pick up interference and noise, and these usually affect the amplitude of the
carrier. In an AM system (where the information is contained in the amplitude) the noise adds to the
information and thus pollutes it. It is virtually impossible to remove this additional component unless the
original signal is available for comparison and this is hardly ever the case.
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
2. In AM transmissions a significant amount of the transmitted power is contained within the carrier, which is
eventually discarded at the receiver. Only a limited amount (¼) of the original power is located in the
sideband where the information signal is located. This makes AM an inefficient system.
The Advantages of FM
1.
In an FM system, the amplitude of the carrier remains constant; the information is contained in the
frequency deviation of the carrier. During transmission it will pick up noise just like any other signal,
however, as there is no information contained in the amplitude of the carrier, it is relatively
straightforward to remove any noise picked up during transmission at the receiver. FM transmissions
therefore have greater immunity to the effect of noise.
2. In a wideband FM signal, the amplitude of the carrier component is relatively small: most of the
transmitted power goes into the sidebands, where the information is carried. Therefore FM is a much more
efficient process than AM.
3. Using FM, it is possible to transmit the same quality of information using a lower power transmitter.
The Disadvantages of FM
1.
The circuitry involved in modulating and demodulating FM signals is much more complex that of AM
receivers. This makes FM transmitting and receiving equipment more expensive, and less attractive to
purchase.
2. The transmission range of FM signals is lower than that of AM signals, meaning that more repeater stations
will be required to cover a large area.
3. At high frequency, the carrier signal travels in straight lines, sometimes referred to as ‘line-of-sight’
communication, meaning that in hilly areas more repeaters will be required.
Introduction to Digital Communications
Digital system block design
A typical communication system involves a transmitting station (sender), a receiving station (user), and a
connecting medium called a channel.
A block diagram of the principal functions that may be present in a digital communication system is illustrated in
the fig as the overall system is digital, the transmitting subsystem can accept such signals directly.
Analog-to-Digital (A/D) converter
 It can also work with analog signals if they are converted into digital form in the analog-to-digital (A/D)
converter. A/D conversion involves periodically sampling the analog waveform and quantizing the samples.
 Quantization amounts to rounding to the nearest of a number amplitudes.
 However in the rounding process some information may be lost that limits the accuracy with which the
analog signal can be reconstructed in the receiver.
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
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The actual output of the A/D converter at point A in the fig is a discrete voltage level. These levels can be
represented by a sequence of binary representation, often binary digits 0 and 1.
Source encoder
 The purpose of source encoder is to effectively convert each discrete level into a suitable digital
representation, often binary.
 Digital messages are said to process redundancy if their levels are not equally probable or are not
statistically independent.
 The practical purpose of source encoder is to remove redundancy.
Channel encoder
 This block helps to reduce the effects of channel caused errors.
 This is achieved by adding controlled redundancy to the source encoder’s digital representation in a known
manner such that errors may be reduced.
Channel
The transmitted signal moves along with some errors/noise/interferences etc.. through a medium called channel
Channel decoder
The purpose is to reconstruct the signal to the best extent possible, the output that was generated by the source
encoder and channel encoder.
Source decoder
It performs exact inverse of the source encoding function. For digital messages its output becomes the final
receiver output.
Digital-to-Analog converter
If the original message was analog, the source decoder output is passed through a Digital-to-Analog converter which
reconstructs the original message using sampling theory.
Analog-to-Digital (A/D) converter
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Since modern communications are hybrid in nature, some begin as analog and some as digital, a method is
required to convert analog signals to digital signals and vice versa.
A/D conversion involves 2 stages
1. Sampling the message.
2. Quantizing the samples.
To obtain in digital form, an analog continuous time signal is converted into a list of numbers by sampling
the time function.
Then the lists of numbers are coded into discrete code words. Each sample is represented by a digital code.
Smaller samples and more bits in the code increase the accuracy of the conversion.
In majority of digital communication system, the actual form used for code words is binary number
composed of 1,s and 0’s.
For example, if the samples ranged from 0 to 10v, each sample could be rounded to the nearest integer.
This would result in code words drawn from the 11 integers between 0 and 10 with the binary
restriction,the converter would operate on the 0 to 10v samples by first rounding each sample value to the
nearest volt.it would then convert the resulting integer into 4-bit binary number.
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE
Elements Of Electronics Engg. Notes by Sowmya Sunkara, Asst. Prof. Dept. Of E&C, BMSCE