Unit – III

The need of Modulation in Communication System:


1. The purpose of a communication system is to transmit information or message signals. Message signals are also called baseband signals, which essentially designate the band of frequencies representing the original signal, as delivered by the source of information.
2. No signal, in general, is a single frequency sinusoid, but it spreads over a range of frequencies called the signal bandwidth.
3. Suppose we wish to transmit an electronic signal in the audio frequency (AF) range (baseband signal frequency less than 20 kHz) over a long distance directly. Let us find what
factors prevent us from doing so and how we overcome these factors.

4. Size of the antenna or aerial:

a) For transmitting signals antenna is required whose length should be at least (λ/4), to sense the time variation of the signal.
b) The antenna size for considered baseband signal frequency is of impracticable length. Hence direct transmission at baseband signal frequency is not practical.

5. Effective Power Radiated by Antenna:

a) The power radiated is proportional to (l /λ)2. Where ‘l’ is the size of the antenna. Thus for the same antenna, the power radiated is inversely proportional to the wavelength of the wave i.e. directly proportional to the frequency of the wave.
b) Thus the effective power radiated by long wavelength is small. Hence direct transmission at baseband signal frequency is not practical.
c) To get more power radiated the high-frequency transmission should be used.

6. Mixing Up of Signals From Different Transmitters:

a) For considered baseband frequency there are full chances of mixing up of the signals from the transmitters operating in the same band. It is just like many people talking simultaneously.
b) Thus, all these signals will get mixed up and there is no simple way to distinguish between them.
Due to these three reasons modulation of the wave is required.

The concept of Modulation:

1. There is a need for translating the original low-frequency baseband message or information signal into the high-frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal.
2. In doing so, we take the help of a high-frequency signal, known as the carrier wave, and a process known as modulation which attaches information to it. The carrier wave
may be continuous (sinusoidal) or in the form of pulses as shown in Fig.


3. Continuous (sinusoidal) carrier wave:

A sinusoidal carrier wave can be represented as c(t ) = Asin (ωt +Φ) where c(t) is the signal strength (voltage or current), AC is the amplitude, ω( = 2πfC) is the angular frequency and Æ is the initial phase of the carrier wave. During the process of modulation, any of the three parameters, viz AC, w, and f, of the carrier wave can be controlled by the message or information signal. This results in three types of modulation:
(i) Amplitude modulation (AM),
(ii) Frequency modulation (FM) and
(iii) Phase modulation (PM)

4. Pulse carrier wave:

The significant characteristics of a pulse are:
pulse amplitude, pulse duration or pulse Width, and pulse position (denoting the time of rise or fall of the pulse amplitude as shown.
Hence, different types of pulse modulation are:
(i) pulse amplitude modulation (PAM),
(ii) pulse duration modulation (PDM) or pulse width modulation (PWM),and
(iii) pulse position modulation (PPM)

Amplitude Modulation:
1. In amplitude modulation, the amplitude of the carrier is varied in accordance with the information signal. Here we explain amplitude modulation process using a sinusoidal signal as the modulating signal.
2. A carrier is described by

Thus the modulated signal now contains message signal.

3. The quantity Am/Ais called modulation index and is denoted by symbol μ. Thus Am= μASubstituting in equation (1)

The quantities  (ωc –ωm) and (ωc + ωm) are called the lower side and upper side frequencies.

4.  The modulated signal now consists of the carrier wave of frequency ωc  plus two sinusoidal waves each with the frequency slightly different from ωc  known as sidebands. The frequency spectrum of modulated wave is as shown below.

5. As long as the broadcast frequencies are sufficiently spaced out, the sideband does not overlap so that different stations can operate without interfering with each other.

Production of Amplitude Modulated Wave:

1. Following block diagram shows a simple method of production of amplitude modulate wave.

Production of Amplitude Modulated Wave

2. Here the modulating signal Asin ωmt added to carrier signal Ac  sin ωc t to produce signal x(t).
Thus x(t) = A sin ωc t + A sin ωmt

This signal is passed through a square law device which is nonlinear device produces an output given by  y(t) = Bx(t)+Cx2(t)

Where B and C are constant.

Production of Amplitude Modulated Wave

Block Diagram for Transmitter:

Block Diagram for Transmitter

Block Diagram for Receiver:

Block Diagram for Receiver

Block Digram for Detecting AM signal:

Detection of Amplitude Modulated waves:

1. The transmitted message gets attenuated in propagating through the channel. The receiving antenna is therefore to be followed by an amplifier and a detector.
2. In addition, to facilitate further processing, the carrier frequency is usually changed to a lower frequency by what is called an intermediate frequency (IF) stage preceding the detection.
The detected signal may not be strong enough to be made use of and hence is required to be amplified. A block diagram of a typical receiver is shown in Fig.
3. Detection is the process of recovering the modulating signal from the modulated carrier wave. In order to obtain the original message signal m(t ) of angular frequency Ém, a simple method is shown in the form of a block diagram.
4. The modulated signal of the form given in is passed through a rectifier to produce the output shown in fig.. This envelope of signal is the message signal. In order to retrieve m(t), the signal is passed through an envelope detector (which may consist of a simple RC circuit).

The Internet

1. It is a system with billions of users worldwide. It permits communication and sharing of all types of information between any two or more computers connected through a large and complex network. It was started in 1960’s and opened for public use in 1990’s. With the passage of time it has witnessed tremendous growth and it is still expanding its reach. Its applications include:

2. E-mail:
It permits the exchange of text/graphic material using email software. We can write a letter and send it to the recipient through ISP’s (Internet Service Providers) who work like the dispatching and receiving post offices.
3.File transfer:
An FTP (File Transfer Programmes) allows transfer of files/software from one computer to another connected to the Internet.
4.World Wide Web (WWW):
Computers that store specific information for sharing with others provide websites either directly or through web service providers. Government departments, companies, NGO’s (Non-Government Organisations) and individuals can post information about their activities for restricted or free use on their websites. This information becomes accessible to the users. Several search engines like Google, Yahoo! etc. help us in finding information by listing the related websites. Hypertext is a powerful feature of the web that automatically links relevant information from one page on the web to another using HTML (hypertext markup language).
5. E-commerce:
Use of the Internet to promote a business using electronic means such as using credit cards is called E-commerce. Customers view images and receive all the information about various
products or services of companies through their websites. They can do online shopping from home/office. Goods are dispatched or services are provided by the company through mail/courier.
6.Chat :
Real time conversation among people with common interests through typed messages is called chat. Everyone belonging to the chat group gets the message instantaneously and can respond rapidly.

Facsimile (FAX):

1. It scans the contents of a document (as an image, not text) to create electronic signals. These signals are then sent to the destination (another FAX machine) in an orderly manner using telephone lines. At the destination, the signals are reconverted into a replica of the original document.
2. Note that FAX provides an image of a static document unlike the image provided by television of objects that might be dynamic.

Mobile telephony:

1. The concept of mobile telephony was developed first in 1970’s and it was fully
implemented in the following decade.
2. The central concept of this system is to divide the service area into a suitable number of cells centered on an office called MTSO (Mobile Telephone Switching Office).
3. Each cell contains a low-power transmitter called a base station and caters to a large number of mobile receivers (popularly called cell phones). Each cell could have a service area of a few square kilometers or even less depending on the number of customers.
4. When a mobile receiver crosses the coverage area of one base station, it is necessary for the mobile user to be transferred to another base station. This procedure is called handover or handoff. This process is carried out very rapidly, to the extent that the consumer does not even notice it.
5. Mobile telephones operate typically in the UHF range of frequencies (about 800-950 MHz).

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