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(i) μ is kept less than 1 so that the noise level can be kept small in the signal.

(ii) μ = a(max) = a(min) / a (max) = 18 / 12 = 9 /6 = 3 /2 = 1.5

(iii) Fading of a signal is prominent in case of amplitude modulation and hence noise level is more in AM than FM.

Ground Wave Propagation: Ground wave propagation is one in which electromagnetic waves glide on the surface of earth between two antennas on the ground.

If the phase of the carrier wave changes in accordance with the phase of the message signal, then the modulation is known as phase modulation. 

Basic terminology used in electronic communication systems : 

(a) Transducer : 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.

(b) Signal : Information converted in electrical form and suitable for transmission is called a signal. Signals can be either analog or digital. Analog signals are continuous variations of voltage or current. They are essentially single-valued functions of time. Sine wave is a fundamental analog signal. All other analog signals can be fully understood in terms of their sine wave components. Sound and picture signals in TV are analog in nature. Digital signals are those which can take only discrete step wise values. Binary system that is extensively used in digital electronics employs just two levels of a signal. ‘0’ corresponds to a low level and ‘1’ corresponds to a high level of voltage/current. There are several coding schemes useful for digital communication. They employ suitable combinations of number systems such as the binary coded decimal (BCD). American Standard Code for Information Interchange (ASCII) is a universally popular digital code to represent numbers, letters and certain characters.

(c) Noise : Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system. The source generating the noise may be located inside or outside the system. 

(d) Transmitter : A transmitter processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception. 

(e) Receiver : A receiver extracts the desired message signals from the received signals at the channel output. 

(f) Attenuation : The loss of strength of a signal while propagating through a medium is known as attenuation. 

(g) Amplification : It is the process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit called the amplifier. Amplification is necessary to compensate for the attenuation of the signal in communication systems. The energy needed for additional signal strength is obtained from a DC power source. Amplification is done at a place between the source and the destination wherever signal strength becomes weaker than the required strength. 

(h) Range : It is the largest distance between a source and a destination up to which the signal is received with sufficient strength. 

(i) Bandwidth : Bandwidth refers to the frequency range over which an equipment operates or the portion of the spectrum occupied by the signal. 

(j) 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. 

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

(l) Repeater : 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. 

(j) Modem : An electronic device used for modulation and /or demodulation of electric signal is called modem. 

Space Wave Propagation: It is the straight line propagation of electromagnetic wave from ransmitting antenna to receiving antenna both installed on the ground. 

In electronic communication system a transduces is a device that converts signals (emw) to electrical form or vice-versa.

It is the process of raising the strength of a signal. 

The answer is (D) Between 10^-13 and 0.1m

In communication using radio waves, an antenna at the transmitter radiates the Electromagnetic waves (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 influence 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. 

 The three basic units of communication are transmitter, medium /channel and receives.

Skywave propagation is a mode of propagation in which communication of radiowaves (in the frequency range 30 MHz–40 MHz) takes place due to reflection from the ionosphere.

 A transmitter processes the incoming message signal, makes it suitable for transmission.

Attenuation  is the loss of strength of a singal, while propagating through a medium.

(a). Point-to-point Communication and 

(b). Broadcast. 

TV signals being of high frequency are not reflected by ionosphere. Hence satellites are used.

a. Ground waves. 

b. Sky waves 

c. Space waves  

The three different modulation are : Amplitude modulation ,Frequency modulation ,Phase modulation

When the waves propagates near to the surface, the waves glide over the surface of the earth, they are called ground waves. The maximum range of coverage depends on the transmitted power and frequency. 

Microwaves are electromagnetic waves of wavelength range 1 mm to 3 cm, they are used in space-communication. 

If the signal (transmitted wave) travels the distance between the transmitter and receiver antenna in a straight line, then such a type of communication is known as LOS Communication. 

The loss of strength of a signal white propagating through a medium is known as attenuation. 

When the signal is in the form of continuous variation of amplitude and can be writes in the sinusoidal form, it is called a sinusoidal signal. When the signal is in the form of discrete variations or pulse, it is called a pulse signal.

Radiowaves having high frequencies are basically called as space waves. 

Ionosphere reflects Radio waves back to Earth.

The length of the antenna should be at least π/4

For a carrier frequency 5×10⁸Hz

λ = c/v = (3 x10⁸/5 x 10⁸) = 0.6 m

Therefore Length of antenna = 0.6/4 = 0.15 m

For high efficiency of signal radiation, the antennas should have a size of at least π/4.

It refers to loss of strength of a signal during propagation of a signal. 

The unwanted signal is called a noise in a communication system.  

X : Channel

It connects the transmitter to the receiver.

In amplitude modulation, the amplitude of modulated (carrier) wave varies in accordance with amplitude of information (signal) wave. When amplitude of information increases, the amplitude of modulated wave increases and vice versa. In this case the amplitude of modulated wave is not constant.

The size of the dipole antenna should be 1/4^th of the wavelength. 

It extends the range of communication. 

Range of frequencies for T.V. transmission are 54 MHz to 890 MHz The common feature between these waves and light waves is that both travel at the same speed and both are electromagnetic waves.

X represents communication channel. 

Function: It connects the transmitter to the receiver.

Digital Communication 

 A repeater is a combination of a receives and a transmitter. It pickes up signals, amplifies and retransmits it. A satellite also receives signal, amplifies at and retransmit it to ground station. Thus the given statement is justified. 

The answer is (A) 3.

 The modulation is needed due to 

(i) Transmission of audio frequency electrical signals need long impracticable antenna. 

(ii) The power radiated at audio frequency is quite small, hence transmission is quite lossy. 

(iii) The various information signals transmitted at low frequency get mixed and hence can not be distinguished.

Binary coded signals are in the form of pulse.

Correct option : (3)

Explanation: 

A transducer is (usually) a device/setup that converts variations in a given physical variable into their corresponding electrical variations. It thus ‘readies’ the message signal for transmission over large distances. 

For ‘picture’ (variations in light intensity) the relevant transducer is based on the ‘photoelectric effect’. 

For audio musical compositions (variations in sound intensity) the relevant transducer is the microphone. For pressure variation relevant transducer is based on the phenomenon of ‘piezo electric effect’.

Correct option: (4) (B) and (C)

Explanation: 

Statement (A) is incorrect; it is the other way round. 

Statement (D) is incorrect; during FM, the frequency of the carrier wave varies in accordance with the ‘amplitude’ of the message signal. 

Statement (B) and (C) are both correct. During AM, the side band frequency being (ω_c – ω_m) and (ω_c +ω_m), the bandwidth of the signal is 2ω_m. 

During FM, the bandwidth of the signal is much higher than 2ω.

Correct option: (2) (B) and (D) only

Explanation: 

Statement (A) is incorrect; it is the other way round. 

Statement (C) is incorrect; the operating range of FM is higher than that of AM. 

Statement (B) and (D) are both correct statements.

It is the ratio of the change in amplitude of the modulated carrier wave to the amplitude of the original carrier wave.

Modulation index is the ratio of the amplitude of modulating signal to that of carrier wave.

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

Modulation index is kept low in order to avoid distortion. The low frequency modulating signal is mixed with high-frequency carrier wave, the distortion is restricted due the high-frequency carrier wave for modulation index lying between 0 and 1.

Bandpass filter rejects low and high frequencies and allows a band of frequencies to pass through.

Correct option: (3) Total power carried by the sidebands increases

Explanation: 

Amplitude of the central band is independent of the modulation index. 

Fraction of total power carried by the central carrier wave = 2/2+m² 

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

Therefore, as the modulation index increases, the useful power carried by the central band decreases, but the carried by the sidebands increases.

Correct option: (2) Amplitude of the high frequency carrier signal is varied in accordance with the low frequency message signal

Explanation :

We need a high frequency carrier wave for effective transmission of low frequency message signals in amplitude modulation, it is the amplitude of the high frequency carriers wave that is varied in accordance with the low frequency message signal. 

Correct option: (1) Transmitter

Explanation: 

It is necessary to modulate the (usual) low frequency message signals before they can be transmitted for communication over large distances.

(a) frequency varies with time.

In frequency modulated wave, frequency of the carrier wave varies in accordance with the modulating signal

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

Example: home automation using a mobile phone.

Internet is a fast growing technology in the field of communication system with multifaceted tools. It provides new ways and means to interact and connect with people. Internet is the largest computer network recognized globally that connects millions of people through computers. It finds extensive applications in all walks of life.

Applications:

1. Search engine:

The search engine is basically a webbased service tool used to search for information on World Wide Web. 

2. Communication:

It helps millions of people to connect with the use of social networking: emails, instant messaging services and social networking tools.

3. E-Commerce:

Buying and selling of goods and services, transfer of funds are done over an electronic network.