Communication is the act of transmission of information. Every living creature in
the world experiences the need to impart or receive information almost continuously
with others in the surrounding world. For communication to be successful
Communication is the act of transmission of information. Every living creature in
the world experiences the need to impart or receive information almost continuously
with others in the surrounding world. For communication to be successful
Communication pervades all stages of life of all living creatures. Irrespective of its nature, every communication
system has three essential elements transmitter, medium/channel and receiver. The block diagram shown in
figure-1 depicts the general form of a communication system.
BASIC TERMINOLOGY USED IN ELECTRONIC COMMUNICATION SYSTEMS
In a communication system, the message signal can be voice, music picture or computer data. Each of these
signals 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.
Similar to message signals, different type of transmission media offer different bandwidths. The commonly
used transmission media are wire, free space and fiber optic cable.Coaxial cable is a widely used wire medium,
which offers a bandwidth of approximately 750 MHz. Such cables are normally operated below
18 GHz. Communication through free space using radio waves takes place over a very wide range of frequencies:
In communication using radio waves, an anetnna at the transmitter radiates the electromagnetic wave (em
waves), which travel through the space and reach the receiving antenna at the other end. As the em wave travels
away from the transmitter, the strength of the wave keeps on decreasing. Several factors influenece the propagation
of em waves and the path they follow. At this point, it is also important to understand the composition of the
earth's atmosphere as it plays a vital role in the propagation of em waves.
In the frequency range from a few MHz up to 30 to 40 MHz, long distance communication can be achieved by
ionospheric reflection of radio waves back towards the earth. This mode of propagation is called sky wave
propagation and is used by short wave broadcast services. The ionosphere is so called because of the presence
of a large number of ions or charged particles. It extends from a height of ~65 Km to about 400 Km above the
earth's surface. Ionisation occurs due to the absorption of the ultraviolet and other high-energy radiation coming
from the sun by air molecules. The ionosphere is further subdivided into several layers.
Another mode of radio wave propagation is by space waves. A space wave travels in a straight line from transmitting
antenna to the receiving antenna. Space waves are used for line-of-sight (LOS) communication as well as
satellite communication. At frequencies above 40 MHz, communication is essentially limited to line-of-sight
paths. At these frequencies, the antennas are relatively smaller and can be placed at heights of many wavelengths
above the ground. Because of line-of-sight nature of propagation, direct waves get blocked at some point
by the curvature of the earth as illustrated in figure-5 below
As already mentioned, 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. No signal, in general, is a single frequency
sinusoid, but it spreads over a range of frequencies called the signal bandwidth
For transmitting a signal, we need an antenna or an aerial. This antenna should have a size comparable to the
wavelength of the signal (at least ?/4 in dimension) so that the antenna properly senses the time variation of the
signal. For an electromagnetic wave of frequency 20 kHz. the wavelength ? is 15 km
A theoretical study of radiation from a linear antenna
Another important argument against transmitting basband signals directly is more practical in nature. Suppose
many people are talking at the same time or many transmitters are transmitting baseband information signals
simultaneously. As these signals will get mixed up and there is no simple way to distinguish between them. This
points out towards a possible solution by using communication at high frequencies and allotting a band of
frequencies to each message signal for its transmission.
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
Side band frequencies The AM wave contains three frequency
Amplitude modulation can be produced by a variety of methods. A conceptually simple method is shown in the
block diagram of figure–10
The transmitted message gets attenuated in propagating through the channel. The receiving antenna is therefore
to be followed by an amplifier and a detector. 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 figure-12.
The transmitted message gets attenuated in propagating through the channel. The receiving antenna is therefore
to be followed by an amplifier and a detector. 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 figure-12.
It is the second type of continuous wave or sinusoidal wave modulation.
In this mode of modulation, the frequency of the carrier signal varies in accordance with the modulating signal.
The amplitude of the carrier wave is fixed while its frequency is changing.
In frequency modulation, the frequency of the carrier wave is modified in accordance with the amplitude of the
modulating wave.
Various electrical machines and noises cause amplitude disturbance in the transmission
of amplitude-modulated wave. This makes the reception noisy. So, there is
a need for different type of modulation which
Frequency modulation (FM) gives better quality transmission and has a larger bandwidth. In
FM signals, the intelligence (information or message signal) is in the form of frequency variations
and, therefore, the atmospheric or man-made noises (which are generally amplitude
The process of changing the frequency of a carrier wave in accordance with the audio frequency signal is
known as frequency modulation
The TV signals are frequency-modulated. Their transmission cannot be obtained by ground wave propagation.
This is because the signals get absorbed by ground due to their high frequency. The transmission via sky wave
propagation is also not desirable. This is becasuse the ionosphere is unable to reflect radio waves of frequencies
greater than 40 MHz
The pulse amplitude, pulse width and pulse position modulations not completely digital.
A completely digital modulation is obtained by pulse code modulation (PCM).
An analog signal is pulse code modulated by following three operation.
The process of extracting the audio signal from the modulated wave is known as
demodulation or detection
A diode can be used to detect or demodulate an amplitude modulated (AM) wave. A diode basically acts as
a rectifier i.e. it reduces the modulated carrier wave into positive envelope only.
The term data is applied to a representation of facts, concepts or instructions suitable for communication
interpretation or processing by human beings or by automatic means. Data in most cases consists of
pulse type of signals
