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(a) Mobile phone

A mobile phone is a full duplex device by which two persons can talk and hear each other at the same time.

(i) Agriculture:

The implementation of information and communication technology (ICT) in agriculture sector enhances the productivity, improves the living standards of farmers and overcomes the challenges and risk factors. 

(a) ICT is widely used in increasing food productivity and farm management. 

(b) It helps to optimize the use of water, seeds and fertilizers etc. 

(c) Sophisticated technologies that include robots, temperature and moisture sensors, aerial images, and GPS technology can be used. 

(d) Geographic information systems are extensively used in farming to decide the suitable place for the species to be planted.

(ii) Mining:

(a) ICT in mining improves operational efficiency, remote monitoring and disaster locating system. 

(b) Information and communication technology provides audio-visual warning to the trapped underground miners. 

(c) It helps to connect remote sites.

Mobile communication is used to communicate with others in different locations without the use of any physical connection like wires or cables. It allows the transmission over a wide range of area without the use of the physical link. It enables the people to communicate with each other regardless of a particular location like office, house, etc. It also provides communication access to remote areas.

It provides the facility of roaming: 

that is. the user may move from one place to another without the need of compromising on the communication. The maintenance and cost of installation of this communication network are also cheap.

Applications:

1. It is used for personal communication and cellular phones offer voice and data connectivity with high speed. 

2. Transmission of news across the globe is done within a few seconds. 

3. Using Internet of Things (IoT), it is made possible to control various devices from a single device. 

Example: home automation using a mobile phone. 

4. It enables smart classrooms, online availability of notes, monitoring student activities etc. in the field of education.

(a) fidelity

If a radio receiver amplifies all the signal frequencies equally well, it is said to have high fidelity

Correct answer is (c) In the channel.

Correct answer is (a) 75 kHz.

(i) Modulation index is a measure of  extent of modulation done on a carrier signal. In Amplitude modulation, it is defined as the ratio of the amplitude of modulating signal to that of the carrier signal.

\(m = \frac{A_m}{A_c}\)

(ii) Keeping the amplitude of modulating signal less than the amplitude of carrier wave helps one to reduce the distortions in the signal.

Correct option: (3) square law device, band pass filter and power amplifier.

Explanation: 

The sequence of (amplitude) modulation, of the carrier wave by the message signal is as per the block diagram shown here.

After adding the message signal m (t) to the carrier wave c (t), the signal x (t) is produced. This is passed into a square law device whose output contains a d.c. term and sinusoidal of frequencies ω_m , 2ω_m, ω_c , 2ω_c, (ω_c –ω_m) and (ω_c +ω_m). The band pass filter than rejects the d.c. term and sinusoids of frequencies ω_c , (ω_c –ω_m) and (ω_c + ω_m). This output is the AM wave which has its central frequency ω_c an its side band frequencies (ω_c –ω_m) and (ω_c +ω_m). 

The output of the band pass filter is followed by a power amplifier. This provides it the necessary power. This power argumentative signal is than fed into an appropriate sized transmitting antenna which than radiates it out into space.

(b) Frequency modulation.

Correct option: (2) 33.3

Explanation: 

Fraction of total power carried by the sidebands = m²/2 + m² 

Therefore, at 100% modulation, sidebands carry only 1/3rd of the total power.

Here, A_max = 10V and A_min = 2 V

Modulation index = A_max -A_min/A_max +A_min = (10-2/10+2) = 8/12 = 0.67

Generally, the modulation index is kept less then one to avoid distortion.

Correct option: (2) 0, 4 units

Explanation: 

V_c = 4 

Modulated Index = m = 0.2 

Minimum amplitude = 4(1 – 0.2) = 3.2 V 

Maximum amplitude = 4(1 + 0.2) = 4.8 V

The refractive index increases with increase in frequency which implies that for higher frequency waves, angle of refraction is less, i.e. bending is less. Hence, the condition of total internal reflection is attained after travelling larger distance (by 3MHz wave).

The correct sequence in which these are arranged in a basic communication system:

(b) BADEC.

(c) {A_c + k₂ v_m(t)} sin (ω_ct + φ ).

The frequency of the AM wave will be  (b) ω_C.

(b) A is transmitted via ground wave, B via sky wave and C via space wave. 

(a) ~ 400 m. 

Key concept: The process of changing the amplitude of a carrier wave in accordance with the amplitude of the audio frequency (AF) signal is known as amplitude modulation (AM).
In AM, frequency of the carrier wave remains unchanged.
Side band frequencies: The AM wave contains three frequencies f_c, (f_c + f_m) and (f_c -f_m),fc is called carrier frequency, (f_c +f_m) and (f_c -f_m) are called side band frequencies.
(f_c +f_m): Upper side band (USB) frequency
(f_c -f_m): Lower side band (LSB) frequency
Side band frequencies are generally close to the carrier frequency.
According to the problem, frequency of carrier signal is f_c = 1 MHz and frequency of speech signal = 3 kHz
= 3 x 10^-3 MHz
= 0.003 MHz
We know that, Frequencies of side bands = (f_c ± f_m) = (1 + 0.003) and (1 – 0.003)
So, side band frequencies are 1.003 MHz and 0.997 MHz.

(a) 1.003 MHz and 0.997 MHz.

The power of the signal received is (b) 100 W.

(i) Device used for communication which transmits the signal by amplifying it.

(ii) Device which merges the high frequency carrier wave and low frequency base-band signal.

Identification: 

A is the square law device. 

B is the bandpass filter.

Functions:

Square law device is a non linear device and produces the output. Bandpass filter rejects dc and sinusoidal frequencies,ω_m, 2ω_m,2ω_c and gives the AM wave as its output.

TV signals have high frequency range 100 to 200 MHz. Ionospheric layers do not reflect back such high frequency signals. Hence, sky waves cannot be used for transmission of TV signals.

The correct option is (B) the frequency of modulated wave varies as amplitude of modulating wave.

Explanation:

In FM, frequency of modulated wave varies as amplitude of modulating wave.

(c) less than one 

Ionosphere is the upper most layer of earth’s atmosphere having veiy low density. Its refractive index is less than one.

Correct option: (4)

Explanation:

The solar radiation is intense at great heights; the molecular concentration (of air molecules) is very high near the surface of the earth. At intermediate heights, both these terms are reasonably high; this results in a peak of ionisation density only at intermediate heights. 

The frequency range over which the baseband signals or the information signals such as voice, music, picture, etc. is transmitted is known as bandwidth.

The shortest distance between the transmitter and the point of reception of the sky wave along the surface is called as the skip distance.

The range of frequencies required to transmit a piece of specified information in a particular channel is called channel bandwidth or the bandwidth of the transmission system.

(a) total internal reflection

Signals propagate through an optical fibre by suffering repeated total internal reflections.

An electronic device used for modulation and /or demodulation of electric signal is called modem. 

A repeater is a combination of a receiver and a transmitter. A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency. Repeaters are used to extend the range of a communication system.  

(b), (d)

(b) The bandwidth required for amplitude modulation is 6kHz. 

(d) The side band frequencies are 1503 kHz and 1497 kHz.

(a) \(\sqrt{2r}\)

Initial range, r – \(\sqrt{2Rh}\)

When height fo antenna is doubled r' = \(\sqrt{2R \times 2h} - \sqrt{2r}\)

Correct option: (4) F2 layer

Explanation: 

The approximate height of the four layers is as follows. 

D layer – 65–75 km

E layer – 100 km 

F1 layer – 170–190 km 

F2 layer – 300 km (at night) and 250–400 km (during day)

Correct option: (3) Ionosphere

Explanation:

Ionosphere (having a lot of air molecules ionised) reflects the radio waves back to the earth’s surface). 

Correct option: (4)

Explanation: 

The density of air molecules, as well as temperature of the air, have lower values at higher altitudes. 

(i) X : IF stage 

Y : Amplifier 14

(ii) The carrier frequency is changed to a lower frequency by intermediate frequency (IF) stage preceding the detection.

An amplifier increases the strenglh of the detected signal.

Any device that converts one form of energy into another can be termed as a transducer. In electronic communication systems, we usually come across devices that have either their inputs or outputs in the electrical form. An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature, etc) into corresponding variations in the electrical signal at its output. 

Modulation : The original low frequency message/information signal cannot be transmitted to long distances. Therefore, at the transmitter, information contained in the low frequency message signal is superimposed on a high frequency wave, which acts as a carrier of the information. This process is known as modulation. There are several types of modulation, abbreviated as AM, FM and PM.

Demodulation : The process of retrieval of information from the carrier wave at the receiver is termed demodulation. This is the reverse process of modulation. 

Space wave propagation

UHF (> 40MHz) 

μ = A_m / A_c 

A_m = Amplitude of modulating wave. 

A_c = Amplitude of Carrier wave. 

They are relatively Noise-free and error free. 

Oscillator is generate Radiowaves of constant amplitude.

 a. Two wire transmission lines. 

b. Coaxial cables 

c. Optical fibers. 

It is a discontinuous and discrete signal having binary variations 1 and 0 with time. 

It is the largest distance between the source and the destination upto which the signal is received with sufficient strength. 

It’s an electrical signal which varies continuously with time. 

(a) less than 3 x 10^-7 m

Ozone layer blocks ultraviolet radiation from the sun for this radiation λ < 3 x 10^-7 m.

(a) stops ultraviolet radiation