Communication Systems Class 12 Important Questions NEET Physics [Free PDF Download]

1. Think about communication using optical fibres. Which of the ensuing assertions is false?

(A) they can have refractive indexes that are graded.

(B) Optical fibres can have a homogeneous core with an appropriate cladding.

(C) Electromagnetic interference from the outside can interfere with optical fibres. 

(D) Optical fibres have a very low transmission loss.

2. A telephone link operating at a central frequency of 10GHz is established if 1% this available when how many telephone channels can be simultaneously given when each telephone covering a band width of 5kHz.

(A) $2 \times 10^{4}$

(B) $2 \times 10^{6}$

(C) $5 \times 10^{4}$

(D) $5 \times 10^{6}$

3. In an amplitude modulated wave for audio frequency of $500 \mathrm{cycle} / \mathrm{second}$, the appropriate carrier frequency will be

(A) $50 \mathrm{cycles} / \mathrm{sec}$

(B) $100 \mathrm{cycles} / \mathrm{sec}$

(C) $500 \mathrm{cycles} / \mathrm{sec}$

(D) 50, $000 \mathrm{cycles} / \mathrm{sec}$

4. Range of frequencies allotted for commercial FM radio broadcast is

(A) 88 to $108 \mathrm{MHz}$

(B) 88 to $108 \mathrm{kHz}$

(C) 8 to $88 \mathrm{MHz}$

(D) 88 to $108 \mathrm{GHz}$

5. The process of superimposing signal frequency (i.e. audio wave) on the carrier wave is known as

(A) Transmission

(B) Reception

(C) Modulation

(D) Detection

6. The characteristic impedance of a coaxial cable is of the order of

(A) $50 \Omega$

(B) $200 \Omega$

(C) $270 \Omega$

(D) None of these

7. If $\mu_{1}$ and $\mu_{2}$ are the refractive indices of the materials of core and cladding of an optical fibre, then the loss of light due to its leakage can be minimised by having

(A) $\mu_{1}>\mu_{2}$

(B) $\mu_{1}<\mu_{2}$

(C) $\mu_{1}=\mu_{2}$

(D) None of these

8. Maximum usable frequency (MUF) in $F$-region layer is $x$, when the critical frequency is $60 \mathrm{MHz}$ and the angle of incidence is $70^{\circ}$. Then $x$ is $\left(\cos 70^{\circ}=0.34\right)$

(A) $150 \mathrm{MHz}$

(B) $170 \mathrm{MHz}$

(C) $175 \mathrm{MHz}$

(D) $190 \mathrm{MHz}$

9. A laser is a coherent source because it contains

(A) many wavelengths

(B) uncoordinated wave of a particular wavelength

(C) coordinated wave of many wavelengths

(D) coordinated waves of a particular wavelength

10. A laser beam is used for carrying out surgery because it

(A) is highly monochromatic

(B) is highly coherent

(C) is highly directional

(D) can be sharply focussed

11. In amplitude modulated waves, maximum amplitude is $30 \mathrm{mV}$ and minimum is $5 \mathrm{mV}$, then find out the modulating index.

(A) 7

(B) 3

(C) $0.7$

(D) $0.45$

12. Voltage of modulating wave of $5 \mathrm{~V}$ with $10 \mathrm{MHz}$ frequency, it was superimposed on carrier wave of frequency $20 \mathrm{MHz}, \& 20 \mathrm{~V}$ then the modulation index:

(A) $0.25$

(B) $1.25$

(C) $2.43$

(D) $64.0$

13. Find the area covered by a transmitting antenna of height $50 \mathrm{~m}$: [AIIMS 2011]

(A) $320 \pi \mathrm{km}^{2}$

(B) $1440 \mathrm{~km}^{2}$

(C) $640 \pi \mathrm{km}^{2}$

(D) $120 \pi \mathrm{km}^{2}$

14. For satellite communication which wave is used:

[AIIMS 2013]

(A) Space wave

(B) Sky wave

(C) Ground wave

(D) Microwave

15. If the modulation index is $1 / 2$ and power of carrier wave is 2 units, then what will be the total power of side band :

(A) $0.5$

(B) 1

(C) $2.25$

(D) $0.75$

16. Assertion:  Microwave communication is preferred over optical communication.

Reason: Microwaves provide large number of channels and band width compared to optical signals.

(A) Both assertion and reason are true and the reason is correct explanation of assertion.

(B) Both assertion and reason are true and the reason is not the correct explanation of assertion.

(C) Assertion is true but reason is false.

(D) Both assertion and reason are false.

17. Assertion:  Television signals are received through sky-wave propagation.

Reason:  The ionosphere reflects electromagnetic waves of frequencies greater than a certain critical frequency.

(A) Both assertion and reason are true and the reason is correct explanation of assertion.

(B) Both assertion and reason are true and the reason is not the correct explanation of assertion.

(C) Assertion is true but reason is false.

(D) Both assertion and reason are false.

18. Assertion: Electromagnetic waves with frequencies smaller than the critical frequency of ionosphere cannot be used for communication using sky wave propagation.

Reason: The refractive index of the ionosphere becomes very high for frequencies higher than the critical frequency.

(A) Both assertion and reason are true and the reason is correct explanation of assertion.

(B) Both assertion and reason are true and the reason is not the correct explanation of assertion.

(C) Assertion is true but reason is false.

(D) Both assertion and reason are false.

19. Assertion:  A Communication satellite is essentially a repeater in space.

Reason:  It reflects the signals from transmitter to receiver.

[AIIMS 2010]

(A) Both assertion and reason are true and the reason is correct explanation of assertion.

(B) Both assertion and reason are true and the reason is not the correct explanation of assertion.

(C) Assertion is true but reason is false.

(D) Both assertion and reason are false.

20. Assertion:  In the transmission of long distance radio signals, short wave band is used. 

Reason:  In shorter wavelength, Attenuation is very less.

[AIIMS 2011]

(A) Both assertion and reason are true and the reason is correct explanation of assertion.

(B) Both assertion and reason are true and the reason is not the correct explanation of assertion.

(C) Assertion is true but reason is false.

(D) Both assertion and reason are false.

21. Assertion:  In a communication system based on amplitude modulation the modulation index is kept $<1$.

Reason: It ensures minimum distortion of signal.

(A) Both assertion and reason are true and the reason is correct explanation of assertion.

(B) Both assertion and reason are true and the reason is not the correct explanation of assertion.

(C) Assertion is true but reason is false.

(D) Both assertion and reason are false.

22. The maximum distance upto which TV transmission from a TV tower of height $\mathrm{h}$ can be received is proportional to:

[AIIMS-2003]

(A) $\mathrm{h}^{1 / 2}$

(B) $\mathrm{h}$

(C) $\mathrm{h}^{3 / 2}$

(D) $h^{2}$

23. In short wave communication waves of which of the following frequencies will be reflected back by the ionospheric layer having electron density $10^{11}$ per $\mathrm{m}^{-3}$?

[AIIMS-2003]

(A) $2 \mathrm{MHz}$

(B) $10 \mathrm{MHz}$

(C) $12 \mathrm{MHz}$

(D) $18 \mathrm{MHz}$

24. For skywave propagation of a $10 \mathrm{MHz}$ signal, what should be the minimum electron density in ionosphere: 

[AIIMS-2005]

(A) $\sim 1.2 \times 10^{12} \mathrm{~m}^{-3}$

(B) $\sim 10^{6} \mathrm{~m}^{-3}$

(C) $\sim 10^{14} \mathrm{~m}^{-3}$

(D) $\sim 10^{22} \mathrm{~m}^{-3}$

25. Given below is a circuit diagram of an AM demodulator.

[AIIMS-2006]

For good demodulation of AM signal of carrier frequence $f$, the value of $\mathrm{RC}$ should be:-

(A) $\mathrm{RC}=\dfrac{1}{\mathrm{f}}$

(B) $\mathrm{RC}<\dfrac{1}{\mathrm{f}}$

(C) $\mathrm{RC} \geq \dfrac{1}{\mathrm{f}}$

(D) $\mathrm{RC} \gg \dfrac{1}{\mathrm{f}}$

Solution:

1. (C)

Electromagnetic interference, a typical noise in optical fibre connections, is caused by the fundamentals of electromagnetism. When magnetic field lines cut across conductors, an electrical current is produced. 

Since current cuts across the conductor, the flow of electrons in a conductor creates a magnetic field that varies with the current flow. Since signals in fibre optics are delivered as light rather than electricity, they are susceptible to electromagnetism. 

Optical fibre cables are immune to external electromagnetic interference because changes in the generated magnetic field outside a conductor carry the same information as the current flowing through the conductor.

2. (A)

So, first of all we will see the $1 \%$ of $10 \mathrm{GHz}$

i.e. $=10 \times 10^{8} \times \dfrac{1}{100}=10^{8} \mathrm{~Hz}$

Therefore, the number of channels $=\dfrac{10^{8}}{5 \times 10^{3}}$

$=2 \times 10^{4}$

3. (D)

In the question, we have the information as an amplitude modulated wave for an audio frequency of 500 cycles/ second. So by seeing these options, we know that carrier frequency will always be greater than audio frequency. So now, on comparing the options, only option D is the correct one.

Hence, option D is correct.

4. (A)

A maximum frequency deviation of $75 k H z$ is permitted for commercial F M broadcast stations in the 88 to 108 $\mathrm{MHz}$ very high-frequency band.

5. (C)

The modulation process is defined in electronics and telecommunication as varying some or more than one specific property of a waveform that changes periodically. This waveform is known as the carrier signal. The carrier signal and the modulating signal containing the information are transmitted.

Therefore, we can say that in telecommunications, modulation is the process in which a message signal is conveyed. For instance, a digital bit stream or even an analog audio can be physically transmitted by placing them inside another signal.

Hence, we can say that the process of superimposing signal frequency (i.e. audio wave) on the carrier wave is known as modulation.

6. (A)

Coaxial cables, better known as coax, are generally made of copper. These have metal shielding designed to provide immunity against greater bandwidths and noise. These can be used to transmit signals at a higher speed than normal pair cables to large distances. It has an electrical cable surrounded and separated by a dielectric with a concentric conducting shield.

Electrical impedance is the resistance or opposition to current flow offered by a part or whole of the electric circuit when a voltage is applied across it. In a capacitor, this impedance is caused by the presence of an electric field, while in an inductor, it is caused by the magnetic field.

The Impedance of a coaxial cable depends on the cable's dielectric constant and the diameter of the outer and inner conductors. But most coaxial cables have a characteristic impedance of 50 to 90 ohms.

7. (A)

The cladding is made of a material with a slightly lower index of refraction than the core. This difference in the indices causes total internal reflection at the core-cladding boundary along the length of the fiber. Light is transmitted down the fiber and does not escape through the sides of the fiber.

Therefore,  $\mu 1>\mu 2$

8. (C)

Since, we know that maximum usable frequency is given by the formula:

MUF=$\dfrac{f_{c}}{\cos \theta}$

Now on substituting the values, we get

$=\dfrac{60}{\cos 70^{\circ}}$

And on solving it, we get

$=175 M H z$

9. (D)

We know that laser light is coherent because it consists of waves of the same wavelength in phase. Lasers are used in optical disk drives, laser printers, barcode scanners, DNA sequencing instruments, fibre-optic, semiconducting chip manufacturing (photolithography), free-space optical communication, laser surgery and skin treatments cutting and welding materials, and military and law enforcement devices.

Hence, laser light is considered coherent because it consists of coordinated waves of a particular wavelength.

10.  (D)

A laser is a device that emits light through optical amplification based on a type of emission and stimulates emission. The term laser is a short form for light amplification by stimulated emission of radiation. In surgery, the effect of the beam is restricted to the affected area, so a laser beam should be sharply focused.

11.   (C)

Given, $E_{\max }=30 \mathrm{~V}$

$E_{\min }=5 V$

Now, we know that the modulation index

$\left(m_{a}\right)=\dfrac{E_{\max }-E_{\min }}{E_{\max }+E_{\min }}$

$=\dfrac{30-5}{30+5}$

$=\dfrac{25}{35}$

$=0.71$

Therefore, the modulation index will be 0.7.

12.   (A)

Since, $\mathrm{m}=\dfrac{\mathrm{V}_{\mathrm{m}}}{\mathrm{V}_{\mathrm{c}}}$

So by substituting the values given in the options, we get

$\mathrm{m}=\dfrac{5}{20}=0.25$

13.   (C)

Maximum distance upto which signal from a tower of height $\mathrm{h}$ reaches $=\mathrm{d}=$ $\sqrt{2 \mathrm{Rh}}$

where, $\mathrm{R}$ is the radius of earth.

Thus area covered by the antenna $=\pi \mathrm{d}^{2}=\pi(2 \mathrm{Rh})=\pi \times 2 \times 6400 \mathrm{~km} \times 0.05 \mathrm{~km}=$ $640 \pi \mathrm{km}^{2}$

14.   (A)

From the sending antenna to the receiving antenna, a space wave moves directly in a single direction. Both satellite communication and line of sight communication employ space waves.

15.   (C)

As we know that $P_{\mathrm{m}}=\mathrm{P}_{\mathrm{c}}\left(1+\dfrac{\mathrm{m}^{2}}{2}\right)$

So now on substituting the values, we get

$\therefore \mathrm{P}_{\mathrm{m}}=2\left(1+\dfrac{1}{8}\right)$

$=2 \times \dfrac{9}{8}=\dfrac{9}{4}=2.25 \mathrm{~W}$

16.   (D)

In addition to having a wider bandwidth and more channels, optical communication is always preferred to microwave communication.

Consequently, both statements are false.

Hence, option D is the correct answer.

17.    (D)

In sky wave propagation, the radio waves, which have a frequency between $2 M H z$ to $30 M H z$, are reflected to the ground by the ionosphere. But radio waves having a frequency greater than $30 \mathrm{MHz} the ionosphere cannot reflect $ because they penetrate the ionosphere at this frequency. 

It makes the sky wave propagation less reliable for the propagation of TV signal having a frequency greater than $30 \mathrm{MHz}$. Critical frequency is defined as the highest frequency that is returned to the earth by the ionosphere. 

Thus, above this frequency, an electromagnetic wave will penetrate the ionosphere and is not reflected by it.

18.   (D)

The highest frequency above which the ionosphere no longer returns the sky wave back to earth when transmitted in the vertical direction is called critical frequency, so for sky wave communication, the frequency of electromagnetic wave should be less than the critical frequency.

Refractive index of a medium is given by

$\mu=A+\dfrac{B}{\lambda^{2}}+\dfrac{C}{\lambda^{4}}+\ldots$

19.   (A)

Communications satellite:

It is a man-made satellite that serves as a communication route between a source transmitter and a receiver situated at various points on Earth by amplifying and relaying radio telecommunications signals via a transponder.

We can say satellite work as repeater between transmitting and receiving antenna.

Communications satellites are used for television, telephone, radio, internet, and military applications.

20.   (A)

The radio waves utilised for very long-distance radio transmission at medium and high frequencies are known as sky waves (i.e., shortwave band). There is relatively little attenuation at shorter wavelengths.

21.   (A)

The modulating signal will lose information if the modulation index is greater than one. This is because distortions result from giving the carrier wave's contribution more weight than the modulating signal. Hence, option A is the correct answer.

22.   (A)

If $\mathrm{r}$ be the radius of the area upto which TV signal is visible, then $r=\sqrt{2 \mathrm{Rh}}$ where $\mathrm{R}$ is the radius of the earth and $\mathrm{h}$ is the height of the TV tower. 

Therefore from this we can say that, $r \propto \sqrt{h}$

23.   (A)

The critical frequency of a sky wave for reflection from an ionospheric layer of the atmosphere is given by, $\mathbf{U}_{\mathrm{c}}=9 \mathrm{n}^{1 / 2}$

where $n$ is the number density of electrons/m3. It is given that $n=10^{11} / \mathrm{m}^{3}$

$U_{C}=9 \times\left(10^{11}\right)^{1 / 2}=2.8 \times 10^{6} \mathrm{~Hz}=2.8 \mathrm{MHz}$

Hence the wave of frequency $2 \mathrm{MHz}$ will be reflected back.

24.   (A)

The critical frequency of a sky wave for reflection from a layer of atmosphere is given by

$f_{C}=9(N)^{1 / 2}$

where $\mathrm{N}$ is the number density of electron $/ \mathrm{m}^{3}$.

$10 \times 10^{6}=9(N)^{1 / 2}$

$N=\left(\dfrac{10 \times 10^{6}}{9}\right)^{2}$

$\simeq 1.2 \times 10^{12} \mathrm{~m}^{-3}$

25.   (D)

High frequency carrier waves are filtered off to get signal waves during demodulation. We use parallel capacitance, which absorbs the high frequency component of the import modulated signal, to achieve this. So reactance of $\mathrm{C}$ should be small.

$\Rightarrow \dfrac{1}{\omega \mathrm{C}}<<\mathrm{R} \text { or } \dfrac{1}{2 \pi \mathrm{fC}} \leq \mathrm{R}$

$\Rightarrow \mathrm{RC} \gg \dfrac{1}{\mathrm{f}}$

Importance of Communication System

The chapter on Communication Systems will help students understand different concepts about the topic. Communication systems can be defined as the collection of various systems that are used for connection, interconnection, transmission, and communication. These systems are mostly arranged in 3 different sections namely technology, application, and media. 

In the chapter, students will get to learn about different elements of communication systems such as the information source, input transducer, channel, transmitter, receiver, and output transducer. They will be further introduced to concepts such as the Bandwidth of different signals and the transmission medium. They can learn about ground waves, skywaves, and other modes of propagation of electromagnetic waves.

Also, the chapter teaches students about modulation and why it is important. Students can learn about the size that an aerial or an antenna has. They can also get to know about amplitude modulation. The chapter also contains details about the production and detection of Amplitude Modulated Wave. Students can take help from NEET Communication Systems important questions to cross-check their answers and prepare for the NEET examination in the most efficient manner. They will have all the necessary information about the chapter along with different tricks to score high in the exam.

Benefits of Communication Systems Important Questions from Vedantu

• The students who are preparing for the NEET entrance examination and want to complete the revision of this chapter on Communication Systems can take help from the detailed solutions provided along with the questions to cross-check their answers and rectify any mistakes. 

• Students can get a thorough understanding of the topic and all the concepts that are included in the chapter. They can learn various terminologies and definitions that are a part of the chapter. Also, they get information about communication systems and their changes throughout the ages by reading the important questions and answers. 

• Students will be able to tackle different questions from the NEET exam by applying Communication Systems important formulas. This will also help them in handling the numerical problems in the examination. 

• Students can practice drawing diagrams related to communication systems and have a detailed understanding of the chapter.

Download Communication Systems Important Questions Free PDF

Download the Communication System Class 12 important questions PDF right now and start your preparation for the NEET examination in the best way. Learn how different communication systems work and draw diagrams to make your concepts clearer. The important questions and related information will draw you closer to your goal of clearing the NEET entrance test.