15. COMMUNICATION SYSTEMS (Long Answer Type Question)

Q.1. With the help of block diagram explain the working of a fibre optic communication link.

Ans ⇒ In principal Optical libre communication is similar to microwave link. Moderní optical bre system use special equipments similar to that in use in the microwave radio system.

                     1. Light sources (LD or LED) : They are basically p-n junctions. Light from LED is produced by spontaneous emission whereas light from an LD is made by stimulated emission. The laser outputs coherent, strictly monochromatic and unidirectional and therefore it has a very narrow spectrum, whereas LED has an incorents output and wide spectrum. But LED has higher reliability, simpler drive circuit, lower temperature sensitivity, immunity to reflected light and low cost. LED is suitable for short to medium distance application. Injection Laser diode (ILD) has high output power, high coupling efficiency, wide bandwidth and narrow spectrum.

2. Amplifiers : Semiconductor amplifiers or doped fibre amplifiers are used to limit fibre losses and to increase repeater spacing. They are also known as repeaters because they can directly boost the signal just prior to detection.

A typical transmitter and a receiver of optic fibre system is shown in figure.

                       3. Modulators : Modulation of a light source is done by direct modulation of direct current supplying the source of by using an external modulator following the source. The simplest form of direct modulation is to change the light biasing current above and below the threshold value to turn the laser on and off to obtain digitalisation (0 and 1 configuration).

4. Filters : They are essential components to minimise the cross talk between channels.

5. Light Detectors : Light emerging from the end of an optical fibre link must be detected and converted into electronic pulses for further processing so that the transmitted information can be received. Avlanche Photodiodes are used as detector.

Q.2. Explain briefly the principle of transition through satellite communication. State main two advantages of using a satellite for transmitting signals.

Ans ⇒ Satellite communication is like the line-of sight microwave transmission. In this case, a beam of modulated microwaves is projected towards the satellite and the satellite returns these signals to earth. The role of a satellite is that of ionosphere in case of radio wave.
The signals are received by a device known as transponder fitted on the satellite. This device retransmit these signals after amplification towards the earth. These signals are received by the receiving stations on the earth. The signals received are very weak, so they are amplified at the receiving stations and then they are broadcasted/telecasted. Thus, a communication satellite acts as a big microwave repeater in the sky. The satellite used as a repeater must be at rest with respect to the earth’s transmission station.
A satellite which is at rest with respect to the earth is known as geo-stationary satellite. A satellite can be geo-stationary, if it moves around the earth in an orbit such that (i) its direction of motion is same as that of the earth and (ii) the time taken by the satellite to complete one orbit is equal to the time period of the rotation of the earth about its axis. The orbit in which the geo-stationary satellite moves around the earth is known as geo-synchronous orbit and hence the geo-stationary satellite is also known as geo-synchronous satellite. The distance of the geo synchronous orbit is about 36,000 km above the equator.
Satellite communication from one position to another on the surface of the earth. A geo synchronous satellite can establish a communication link over a large part of the earth but a single satellite cannot cover the whole part of the earth because, the curvature of the earth keeps a large part of the earth out of sight. At least three satellites put in synchronous orbit are required to provide the communication link over the whole part of the earth (Figure B). Each geo-synchronous satellite is 120° apart from each other.

Advantage of Satellite Communication :
First communication satellite (a geostationary satellite i.e. geosynchronous satellite) was launched in 1958.

(i) A single relay station can cover a very large part of the earth through satellite communication. This reduces the large requirement of repeater stations, which is costlier and needs constants maintenance.

(ii) Satellite communication is ultimately economical. The cost of communication through satellite is independent of the distance between any two points on the earth. The cost of a telephone call across the ocean will be same as that of a telephone call across few km.

Q.3. What is meant by the term modulation ? Explain with the help of a block diagram, how the process of modulation is carried out in radio broadcasts. What are limitations of Amplitude Modulation ?

Ans ⇒ Modulation is the process of variation of some characteristic of a high frequency wave (i.e., carrier wave) in accordance with the instantaneous value of an audio signal i.e., modulating signal. Amplititude, angular frequency or angular velocity and phase angle are the characteristics or parameters of a high frequency carrier wave. Any of these parameters can be changed in accordance with that of an information signal. The modulation is named according to the name according to the name of the parameter changed i.e., amplitude modulation, frequency modulation and phase modulation.

Audio wave (or modulating wave), carrier wave (or high frequency wave) and amplitude modulated waves are shown in figure.
The process of changing the amplitude of a carrier wave in accordance with the amplitude of the audio frequency signal (AF) is known as amplitude modulation (A.M.).

Class 12th physics Long Type question in English