Questions - Answers

Very Short Answer type Questions

1. What are the basic blocks of a communication system?

A: 1) Transmitter

2) Transmission Channel

3) Receiver
 

2. What is 'World Wide Web' (www)?

A: It is a encyclopedia of knowledge accessible to everyone round the clock throughout the year.
 

3. Mention the frequency range of speech signals.

A: The frequency range of speech signals is from 300 Hz to 3100 Hz.
 

4. What is sky wave propagation?

A: The waves in the high frequency range (few MHz to 30 MHz) are reflected by the ionosphere of the atmosphere. This made of propagation is known as sky wave propagation.
 

5. Mention the various parts of the ionosphere?

A: 1) Part of stratasphere (D)   2) Part of stratosphere (E)

      3) Part of meosphere (F₁)    4) Thermosphere (F₂)

6. Define modulation. Why is it necessary?

A: The process of combining audio frequency signal with high frequency signal (or carrier wave) is called modulation.

Necessity:

1) Low frequencies cannot be transmitted to long distances. Hence the modulation is necessary.

2) To reduce the size of antenna.

3) To avoid mixing up of signals from different transmitters.
 

7. Mention the basic methods of modulation.

A: 1) Amplitude Modulation (AM)

2) Frequency Modulation (FM)

3) Phase Modulation (PM)
 

8. Which type of communication is employed in Mobile Phones?

A: Space wave communication employed in Mobile Phones.

Short Answer type Questions

1. What do you understand by Modulation? Explain the need for modulation.
A: Modulation:
1) The process of combining the low frequency signal with a carrier wave (high frequency signal) is called modulation.
2) Modulating signal may be represented as
Y_m = A_m sin (ω_mt + φ) → (1)
Where A_m = Amplitude of Modulation wave.
ω_m = modulating angular frequency = 2πf_m
φ = Phase constant
3) The high frequency signal is generally called as carrier signal and it is represented by
Y_c = A_c sin (ω_ct + φ) → (2)
Where A_c = amplitude of carrier signal
ω_c = Carrier signal frequency = 2πf_c
Need for Modulation:
1) Low frequencies cannot be transmitted to long distances. Hence the modulation is necessary.
2) To reduce size of antenna
3) To increase effective radiated power by the antenna.
4) To avoid mixing up of signals from different transmitters.

2. What is space wave communication? Explain
A: Space wave communication:
1) The radiowave which travels in a straight line from transmitting antenna to the receiving antenna is called space waves.
2) Communication done by using the space wave is known as space wave communication.
3) space wave is used for Line-of-sight (LoS) communication.
4) TV signals cannot be transmitted via either ground wave or skyvwave because of frequencies greater than 40 MHz.

 

5) To achieve larger TV coverage, transmission of TV signals is done from a tall antenna.
6) If the transmitting antenna is at a height h_T, the distance to the harizon d_T is

 

9) Television broadcast, microwave links and satellite communication are some examples of communication systems that use space wave mode of propagation.

3. Explain Amplitude Modulation.
A: Amplitude Modulation (AM):
1) In AM the amplitude of the carrier is varied in accordance with the information signal. The amplitude modulation process using a sinusoidal signal as the modulation signal.
2) Let C(t) = A_c sin ω_ct represent carrier wave and m(t) = A_m sin ω_mt represent the message or the modulating signal. Where ω_m = 2πf_m.
3) The modulated signal C_m(t) can be written as,

C_m(t) = A_c Sin ω_ct + µ A_c Sin ω_mt sin ω_ct
here µ = A_m/ A_c is the modulation index.
In practice, µ is kept ≤ 1 to avoid distortion.
4) The modulation signal now contains the message signal, which is shown below.

 

Modulation of a carrier wave 1 (a) a sinusoidal carrier waves (b) a modulating(information mesage) signal (c) amplitude modulation:

4. Draw the block diagram of a generalized communication system and explain it briefly.
A: The exchange of information between two persons located at two different places is called communication. It consists of the following parts.
1) Transmitter
2) Communication Channel
3) Receiver

 

1) Information Source: Information Source supplies the message signal. This message signal is generally a speech, music or information, data to be communicated to others.
2) Transmitter: It will convert the message signal converted by source of information into a form that is suitable for transmission through the channel.
3) Communication Channel: A transmitted signal will propagate along the channel. During propagation loss of strength of signal and distortion of signal will take place. It means noise may be developed.
4) Receiver: Receiver extracts the desired message signal from the transmitted signal which are travelling through the channel. It finally gives out message signal as output.
5) User of information: The received information supplied from source of information through receiver. This information is utilised by the user as per the user requirement.
Explanation with example: The microphone of telephone at the speaker's end acts as the input transducer. It converts sound signal into electrical form. When a transmitted signal propagates along the channel, it may be distarted due to channel imperfection. Receiver receives a corrupted version of transmitted signal for example when the message signal reaches the telephone at the receivers end, then the loud speaker of the telephone at the receiver's end converts the message signal into the original signal. The original signal so recovered is called as output signal and the loudspeaker of the telephone at the receiver end is called output transducer i.e, receiver reconstructs a recognizable form of the original message signal for delivery it to the user information.

6. What is a Ground Wave? When it is used for communication?
A: Ground wave: The propagation of electromagnetic wave along the surface of earth is called ground wave.

 

To radiate these signals with high efficiency, the antennas should have a size comparable to the wavelength (λ) of the signal. At longer wave length the antenna have large physical size and they are located on (or) near to the ground. In standard AM broadcast, ground based vertical towers are generally used as transmitting antennas. For these type antennas ground has a strong influence on the propagation of the signal.

7. What are Sky waves? Explain Sky wave propagation.
A: Long distance communication is achieved by reflection of electro magnetic waves by Ionosphere. Such waves are known as skywaves.
Propagation: These waves can be used in long distances communication and the frequency ranges, from a few MHz upto about 30 MHz. This mode of propagation is called as sky wave propagation. The ionospheric layer acts as a reflecter for a certain range of frequencies (3 to 30 MHz). Electro magnetic waves of frequencies higher than 30 MHz penetrate the ionsphere and escape. The phenomenon of bending of electro magnetic waves is such that they are divided towards the earth which is similar to total internal reflection in optics.

 

8. What should be the size of the antenna or aerial? How the power is related to length of the antenna and wavelength?
A: Size of Antenna (or) Aerial: For transmitting a signal, we need an antenna (or) aerial. This antenna should have a size comparable to the wavelength of the signal. So that the antenna properly senses the time variation of the signal. For an electromagnetic wave of frequency 20 KHz, the wavelength is λ is 15 km.
Effective Power radiated by an Antena: A theoritical study of radiation from a linear antena shows that the power radiated is proportional to (lλ²). This implies that for the same antenna length, the power radiated increases with decreasingly λ i.e. increasing frequency.