# Class 12th Physics – 15 Communication Systems MCQs

#### With respect to digital signals, distortion in the received waves are due to removal of ____

Correct! Wrong!

The rectangular wave can be decomposed into a superposition of sinusoidal waves of frequencies ν0, 2ν0, 3ν0, ... nν0, where n tends to infinity.
To reproduce the rectangular wave shape exactly we need to superimpose all the harmonics, which implies as infinite bandwidth. However, for practical purposes, the contribution from higher harmonics can be neglected, thus limiting the bandwidth. As a result, received waves are a distorted version of the transmitted waves.

#### The unit of bandwidth is ____

Correct! Wrong!

Bandwidth is the range of frequencies occupied by the signal. So, the unit of bandwidth from among the given options is kHz. km, kN and second are units of distance, force and time respectively.

#### Which of the following statements is wrong?

Correct! Wrong!

The attenuation of surface waves increases very rapidly with increase in frequency. The maximum range of coverage depends on the transmitted power and frequency (less than a few MHz). So option "A" is true.
In case of option "B" the frequency range is that of AM broadcast, which happens through ground wave propagation. So, this is true.
"D" is true as the given range is that of Sky wave propagation.
Option "C" is wrong since satellite communication is used for frequencies above 40 MHz

#### Which of the following is not an example of broadcast communication system ____. a. Television b. Fax c. Radio d. Telephone

Correct! Wrong!

The two main types of communication systems are point to point and broadcast. Telephone and Fax are examples of point to point communication system, because communication takes place over a link between a single transmitter and receiver. In case of Radio and Television, a large number of receivers correspond to a single transmitter.

#### A digital signal which is a rectangular waveform is more or less recovered

Correct! Wrong!

If the bandwidth is large enough to accommodate a few harmonics, the information is not lost and the rectangular signal is more or less recovered. This is because the higher the harmonic, less is its contribution to the wave form.

#### The Envelope detector provides a signal at a frequency equal to the ____.

Correct! Wrong!

Detection is the process of recovering the message signal from the modulated carrier. The modulated signal is passed through a rectifier. The rectified output is passed through an envelope detector which may be an RC circuit. This gives out the envelope of the message signal, which will be of message frequency.

#### Which Indian Physicist made significant contribution to the field of radio transmission ____.

Correct! Wrong!

Jagdish Chandra Bose developed an apparatus for generating ultra-short electromagnetic waves. J C Bose's invention was published in the 'Proceedings of The Royal Society' on 27 April 1899, much before Marconi's first wireless communication on 13 December 1901.

#### Which of the following is not a part of the ionosphere?

Correct! Wrong!

The Mesosphere, stratosphere and the thermosphere are parts of the ionosphere

#### Communication is

Correct! Wrong!

Communication comprises transmission, processing and reception of the information at the user end.

#### Fibre optic cable, co-axial cable, twisted pair cables are examples of ____.

Correct! Wrong!

Transmitter converts the message signal produced by the source of information into a form suitable for transmission through the channel - which may be wires, cables or wireless. The receiver reconstructs a recognizable form of the original message signal for delivering it to the end user. Transducer is a device that converts one form of energy to another.

#### Which of the following is the channel used in Radio communication ____

Correct! Wrong!

Radio communication is broadcast mode of communication, and the channel used is Free space.

#### Which of the following does not use the space wave mode of propagation?

Correct! Wrong!

Television, Microwave links and satellite communication use space wave mode of propagation. Telephony is point to point communication that usually happens through twisted pair cables.

#### Communication signals can be transmitted via ____. a. Air b. Optical fiber c. Coaxial cables d. Vaccum

Correct! Wrong!

Communication signals are electromagnetic signals. Therefore they can propagate without a medium.

#### The principle used in the transmission of signals through a fibre optic cable is ____.

Correct! Wrong!

The principle used in the transmission of signals through a fibre optic cable is Total internal reflection.

#### A repeater is a combination of a. Transmitter b. Oscillator c. Amplifier d. Receiver e. Transducer

Correct! Wrong!

The main blocks of a repeater are transmitter, amplifier (which increases signal strength) and receiver. The other two options: transducer is an energy converter and oscillator is a frequency generator.

#### Repeaters are a. used for extension of bandwidth b. combination of transmitter and receiver c. used for extension of range d. used for modulation

Correct! Wrong!

A repeater is a combination of a receiver and a transmitter. It picks up the signal from the transmitter, amplifies and retransmits the signal to the receiver. Repeaters are used to extend the range of a communication system

#### Loss of signal strength while propagating through the medium is known as ____ .

Correct! Wrong!

Amplification is the process of increasing the strength of signal. Modulation and Demodulation are operations done on the message signal for transmission and reception. Attenuation is the loss of signal strength suffered by the signal while transmission through the medium.

#### The frequency band 3.7 - 4.2 GHz corresponds to which of the following wireless communication system

Correct! Wrong!

The frequency band 3.7 - 4.2 GHz corresponds to the uplink band of frequencies for satellite communication. Uplink is the path taken by the signal from an earth station to the satellite.

#### Which of the following is not a transducer? a. Loudspeaker b. Receiver c. Transmitter d. Microphone

Correct! Wrong!

A loudspeaker converts electrical energy to sound energy. A microphone converts sound energy to electrical energy. Receiver and transmitter are elements of communication system which the process may employ use of transducers.

#### In the process of demodulation, the carrier waves and signal waves are ____.

Correct! Wrong!

Demodulation is the process of retrieval of information from the carrier wave at the receiver. So, it is separation. This is the reverse process of modulation. The process of interference does not occur in demodulation.

#### In the ionosphere, ionization occurs due to ____ of UV and other high energy radiation from the sun by the air molecules.

Correct! Wrong!

The ionosphere has a large number of icons or charged particles. It extends from a height of ~ 65 Km to about 400 Km above the earth's surface. Ionization occurs due to the absorption of the UV rays and other high - energy radiations from the sun by air molecules.

#### The three basic elements of any communication system are ___.

Correct! Wrong!

communication system has three essential elements * Transmitter - which converts the message signal into a form that can be sent over the channel * Channel is the physical medium - wire/ cable/ wireless * Receiver - reconstruct the original form of the message signal to be sent to the user.

#### To reproduce the rectangular wave shape of digital signals practically, the harmonic that can be neglected for limiting the bandwidth are ____

Correct! Wrong!

The rectangular wave can be decomposed into a superposition of sinusoidal waves of frequencies ν0, 2ν0, 3ν0,....nν0, where n tends to infinity.
To reproduce the rectangular wave shape exactly, we need to superimpose all the harmonics, which implies an infinite bandwidth. However, for practical purposes, the contribution from higher harmonics can be neglected, thus limiting the bandwidth

#### The number of harmonics comprising a rectangular signal is ____

Correct! Wrong!

The rectangular wave can be decomposed into a superposition of sinusoidal waves, called harmonics of frequencies ν0, 2ν0, 3ν0,...... nν0 where n tends to infinity.

#### Current or voltage variations (with respect to time) of an analog signal is ____.

Correct! Wrong!

Analog signals are continuous, time varying signals. Digital signals are discrete (discontinuous)

#### The message signal takes which of the following path in a communication system____.

Correct! Wrong!

-# If the output of the information source is a non-electrical signal like a voice signal, a transducer converts it to electrical form before giving it as an input to the transmitter. # A transmitter converts the message signal produced by the source of information into a form suitable for transmission through the channel. # The receiver reconstructs a recognizable form of the original message signal for delivering it to the end user.

#### As the Electromagnetic waves travel away from the transmitter, the strength of the waves ____ .

Correct! Wrong!

In communication using radio waves, an antenna at the transmitter radiates the EM waves, which travel through the space which reaches the receiving antenna at the other end.
As the EM wave travels away from the transmitter, the strength of the wave keeps decreasing. For example, in ground wave propagation, the wave induces current in the ground over which it passes and it is attenuated as a result of absorption of energy by the earth.

#### In an AM modulator, the centre frequency of the band pass filter is ____.

Correct! Wrong!

The band pass filter allows a band of frequencies to pass through. In case of AM modulator, it has to be designed so as to output the frequencies in the range fc - fm to fc + fm.
So, the central frequency of the band pass filter should be fc, the carrier frequency.

#### The largest distance between a source and a destination up to which the signal is received with sufficient strength is called the ____

Correct! Wrong!

Bandwidth refers to the frequency range over which an equipment operates. Skip distance is the smallest distance from a transmitter, measured along the earth’s surface, to the point where the sky wave of a fixed frequency but less than critical frequency returns to the earth after reflection from the ionosphere. Mean free path refers to the distance travelled by a particle between two successive collisions

#### The range of frequencies for speech signals is 300 Hz to 3100 Hz. The bandwidth is ____

Correct! Wrong!

Bandwidth is the difference between the upper and lower frequencies of a given frequency range.
Here, the upper frequency is 3100 Hz = 3.1 kHz and the lower frequency = 300 Hz = 0.3 kHz.
So, the bandwidth = (3.1 - 0.3) kHz = 2.8 kHz

Class 12th Physics - 15 Communication Systems MCQs
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