Saturday, June 27, 2009

10. Communication Systems

10. Communication Systems
1.What are the different ways of propagation of radio waves?
Radio wave is propagated from the transmitting to the receiving
antenna mainly in three different ways depending on the frequency of
the wave. They are :
(i) Ground (surface) wave propagation
(ii) Space wave propagation
(iii) Sky wave (or) ionospheric propagation
2.Define: Ground or surface wave propagation
Ground or surface waves are the radio waves which travel along the surface of the earth. Ground wave propagation takes place when the transmitting and receiving antennas are close to the ground. Ground wave propagation is of prime importance only for medium and long wave signals. All medium wave signals received during the daytime use surface wave propagation.
3.Define Space wave propagation
Radio waves propagated through the troposphere of the Earth are
known as space waves. Troposphere is the portion of the Earth’s
atmosphere which extends upto 15 km from the surface of the Earth.
Space wave usually consists of two components :(i) A component which travels straight from the transmitter to the receiver.
(ii) A component which reaches the receiver after reflection from
the surface of the Earth. Space wave propagation is particularly suitable for the waves having frequency above 30 MHz.
4.How ionospheric layer formed.
The ionosphere is the upper portion of the atmosphere, which absorbs large quantities of radiant energy like ultra violet rays, cosmic rays etc., from the sun, becoming heated and ionised. This ionized region contains free electrons, positive and negative ions. The ionosphere is the upper portion of the atmosphere, which absorbs large quantities of radiant energy like ultra violet rays, cosmic rays etc., from the sun, becoming heated and ionised. This ionized region contains free electrons, positive and negative ions.
5.Define Sky wave (or) ionospheric propagation
Radio waves in the short wave band, radiated from an antenna at large angles with ground, travel through the atmosphere and encounters the ionised region in the upper atmosphere. Under favourable circumstances, the radiowaves get bent downwards due to refraction from the different parts of the ionised region and again reach the earth at a far distant point. Such a radio wave is called the sky wave and such a propagation of radio wave is known as sky wave propagation or ionospheric propagation. Long distance radio communication is thus possible through the sky wave propagation.
6.Define :Skip distance and skip zone
In the skywave propagation, for a fixed frequency, the shortest
distance between the point of transmission and the point of reception
along the surface is known as the skip distance.The minimum distance between the transmitter and the ray which strikes the ground (ie)Earth is called as the skip distance.
The region between the point where there is no reception of
ground waves and the point where the sky wave is received first is
known as skip zone. In the skip zone, there is no reception at all.
7.What is Modulation and Demodulation?
This process of changing of amplitude or frequency or phase of the carrier wave in accordance with the intensity of the signal is known as modulation.. Some of the modulation process namely,(i) amplitude modulation, (ii) frequency modulation and (iii) phase modulation are discussed.
At the receiver end, the audio signal is extracted from the modulated wave by the process called demodulation.
8.Define Amplitude modulation (AM)
When the amplitude of high frequency carrier wave is changed in
accordance with the intensity of the signal, the process is called
amplitude modulation.
9.Define Modulation factor
Modulation factor is defined as the ratio of the change
of amplitude in carrier wave after modulation to the amplitude of the
unmodulated carrier wave.
i.e. modulation factor, m = Amplitude change of carrierwave after modulation/Amplitude of carrier wave before modulation
m =Signal amplitude/Carrier amplitude
10.Define channel width.
The channel width is given by the difference between extreme
frequencies i.e. between maximum frequency of USB and minimum
frequency of LSB.
Channel width = 2 × maximum frequency of the modulating signal;
= 2 × (fs)max
11.State the Advantages and limitations of amplitude modulation.
(i) Easy transmission and reception
(ii) Lesser bandwidth requirements
(iii) Low cost
Noisy reception .Low efficiency . Small operating range

12.Explain limitation or demerits in A.M.wave.
In an AM wave, the signal appears in the
amplitude variations of the carrier. Practically, all the natural and man
made noises consists of electrical amplitude disturbances. As a radio
receiver cannot distinguish between amplitude variation that represent
noise and those that contain the desired signal, the reception is
generally noisy.
(ii) Low efficiency : In AM, useful power is available in the side
bands, since they contain signals. The sideband power for an AM wave
is low. Hence the efficiency of AM is low.
(iii) Small operating range : Due to low efficiency of amplitude
modulation, transmitters employing this method have a small operating
range i.e. the messages cannot be transmitted over long distances.
13.Define Frequency modulation (FM)
When the frequency of carrier wave is changed in accordance with
the intensity of the signal, the process is called frequency modulation.
In frequency modula-tion, the amplitude and phase of the carrier
wave remains constant. Only, the frequency of the carrier wave is
changed in accordance with the signal.
14.Define resting frequency and carrier swing.
The frequency of an FM transmitter without signal input is called the resting
frequency or centre frequency (fo ) and this is the allotted frequency of the transmitter. When the signal is applied, the carrier frequency deviates up and down from its resting value fo.
The change or shift either above or below the resting frequency is
called frequency deviation (Δf). The total variation in frequency from the
lowest to the highest is called carrier swing (CS). Hence,
Carrier swing = 2 × frequency deviation = 2 × Δf
15.What are the advantages of F.M.waves?
(i) It gives noiseless reception. Noise is a form of amplitude
variation and a FM receiver will reject such noise signals.
(ii) The operating range is quite large.
(iii) The efficiency of transmission is very high.
(i) A much wider channel is required by FM.
(ii) FM transmitting and receiving equipments tends to be more complex.
16.Define : Phase modulation (PM)
In phase modulation, the phase of the carrier wave is varied in
accordance with the amplitude of the modulating signal and the rate of
variation is proportional to the signal frequency. The waveform of the
phase modulated wave is similar to that of FM wave. The phase
modulation, generally uses a smaller bandwidth than FM.
17.What is an antenna?
An antenna is a long conductor (wire and rod) that acts as a conversion device. It converts an electrical signal into electromagnetic energy when used as a transmitting antenna. In receiving antenna, the electromagnetic energy is converted into an electrical signal. An antenna is characterised by an important parameter,directivity.
18.Define directivity of an antenna.
Directivity is the ability of the antenna to concentrate the
electromagnetic waves in the most desired directions (during
transmission) or to have maximum reception from most preferred
directions (during reception).
19.Define Scanning .
Scanning is the process by which an electron beam spot is made to
move across a rectangular area, so as to cover it completely. This
rectangular area may be the target surface in a television camera or the
screen of a picture tube in a television receiver.
For scanning the picture elements, saw tooth potentials can be
used. Saw tooth potentials are produced by using a unijunction
transistor and a R-C network. Saw tooth potentials are applied to
horizontal and vertical deflector plates in a TV camera.
20.Define Interlaced scanning
In this scanning, the total number lines are divided into two groups called fields. During the presentation of the first field, only the odd numbered lines are scanned, while during the second field all the even numbered lines are scanned. Half way along the bottom of the first field, the vertical retrace returns the scanning beam to the top of the image and completes the unfinished lines. (i.e) The remaining even numbered lines are then scanned during second field. This method of scanning is known as interlaced scanning.
21.What is RADAR?State the principle.
The term RADAR is an acronym for RAdio Detection And
Ranging. It is a system which uses radio waves to detect and to fix the
position of targets at a distance
Principle of radar
Radar works on the principle of ‘radio echoes’. The transmitter in a radar, radiates the high power electrical pulses into space. When these pulses are incident on any distant target such as a mountain, ship or aircraft, they get scattered in all directions. The transmitter antenna receives a part of the scattered energy. This transmitter antenna also acts as receiving antenna for the receiving pulse.
22.State the applications of radar.
(i) Air and sea navigation is made entirely safe, with radar installations. High flying planes and ship at sea, can get detailed reports of mountains, ice bergs, rivers, lakes, shore lines etc., which they can avoid.
(ii) Radar systems are used for the safe landing of air crafts. On approaching the air field, the pilot is guided by signals from a radar set,
so that it flies along the line of the runway and lands safely, whatever
be the visibility.
(iii) Rain drops may reflect suitable radar signals and thus enable
meteorologists to measure the distance of the clouds, with great
accuracy for forecasting.
(iv) The pulses can be used for discovering the position of buried
metals, oils and ores.
23.Define : Analog communication
In analog communication, analog signals are used. An analog signal is a continuously varying voltage or current. Traditionally, in telephone and radio systems, the messages consisted of information conveyed by voice. The voice signal is an analog signal and so could take on any value within the overall range allowed. For example, if the telephone system were set up to handle voice signals, which ranged from 0 to 1 volt, the values transmitted at any instant could be 0.345 V, 0.179 V, and so on. This is called analog signal
communication, because the signal can be any value within the range.
24.Define : Digital Communication
A digital communication system offers many advantages to the
user, that cannot be achieved with an analog system. Digital
communications system may make use of analog links and concepts.
A digital system is a more general case of a binary system. In
binary system, only two signal values can exist. They are often called
0 and 1, but these names represent specific voltages.
25.List the advantages & disadvantages of digital communication.
(i) The transmission quality is high and almost independent of
the distance between the terminals.
(ii) The capacity of the transmission system can be increased.
(iii) The newer types of transmission media such as light beams
in optical fibers and wave guides operating in the microwave frequency
extensively use digital communication.
(i) A digital system requires larger bandwidth.
(ii) It is very difficult to gradually change over from analog to
digital transmission.
26.What is Modem?
The name modem is the abbreviation of the term Modulator and
Demodulator. As the name implies, both functions are included in a
modem. A modem is used to convert digital signals into analog signals
capable of being transmitted over telephone lines. At the receiving end
of the system, modem is used to demodulate the analog signals and
reconstruct the equivalent digital output. Modems are placed at both
ends of the communication circuit
27. Fiber optical communication
Coherent light can be generated with laser or light emitting diodes and may be detected by photo-diodes. Optical fibers are used for transmission of light. An optical fiber is a thin transparent rod, usually made of glass or plastic, through which light can propagate. The light signals travel through the rod from the transmitter to the receiver and can be easily detected at the receiving end of the optical fiber. The principle of total internal reflection is used for the transmission of light signals through the optical fiber.
28.State the advantages& applications of fibre optical communication.
(i) Transmission loss is low. (ii) Fiber is lighter and less bulky than equivalent copper cable. (iii) More information can be carried by each fiber than by
equivalent copper cables.(iv) There is no interference in the transmission of light from electrical disturbances or electrical noise.
The various applications of fiber in communication area are,
voice telephones, video phones, message services, data network etc.
29.State the merits of satellite communication.
(i) Mobile communication can be easily established by satellite
(ii) Satellite communication is economical compared with
terrestrial communication particularly where long distances are
(iii) Compared to the optical fiber communication, satellite
communication has the advantages that, quality of transmitted signal
and location of sending and receiving stations are independent of
(iv) For thin traffic remote areas like north east regions in India,
Ladakh etc., satellite communication is most economical.
(v) For search, rescue and navigation, satellite communication is
far superior and economical compared to other systems.
30.State the demerits of satellite communication.
(i) Between talks there is a time gap which becomes quite
annoying. This time delay also reduces the efficiency of satellite in data
(ii) An imperfect impedance match may cause echo, received
back after a delay. Echo suppressor has to be used.
(iii) Repair of satellite is almost impossible, once it has been

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