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# Communication Systems NCERT Solutions - Physics Class 12 - NEET

NCERT QUESTIONS

(Communication System)

Ques 15.1: Which of the following frequencies will be suitable for beyond-the-horizon communication using sky waves?

(a) 10 kHz
(b) 10 MHz
(c) 1 GHz
(d) 1000 GHz

Ans: (b)  10 MHz
For beyond-the-horizon communication, it is necessary for the signal waves to travel a large distance. 10 KHz signals cannot be radiated efficiently because of the antenna size. The high energy signal waves (1GHz − 1000 GHz) penetrate the ionosphere. 10 MHz frequencies get reflected easily from the ionosphere. Hence, signal waves of such frequencies are suitable for beyond-the-horizon communication.

Ques  15.2: Frequencies in the UHF range normally propagate by means of
(a) Ground waves.
(b) Sky waves.
(c) Surface waves.
(d) Space waves.
Ans: (d)  Space waves
Owing to its high frequency, an ultra high frequency (UHF) wave can neither travel along the trajectory of the ground nor can it get reflected by the ionosphere. The signals having UHF are propagated through line-of-sight communication, which is nothing but space wave propagation.

Ques  15.3: Digital signals
(i) Do not provide a continuous set of values,
(ii) Represent values as discrete steps,
(iii) Can utilize binary system, and
(iv) Can utilize decimal as well as binary systems.
Which of the above statements are true?
(a) (i) and (ii) only
(b) (ii) and (iii) only
(c) (i), (ii) and (iii) but not (iv)
(d) All of (i), (ii), (iii) and (iv).
Ans: (c)
A digital signal uses the binary (0 and 1) system for transferring message signals. Such a system cannot utilize the decimal system (which corresponds to analogue signals). Digital signals represent discontinuous values.

Ques  15.4: Is it necessary for a transmitting antenna to be at the same height as that of the receiving antenna for line-of-sight communication? A TV transmitting antenna is 81 m tall. How much service area can it cover if the receiving antenna is at the ground level?
Ans: Line-of-sight communication means that there is no physical obstruction between the transmitter and the receiver. In such communications it is not necessary for the transmitting and receiving antennas to be at the same height.

Height of the given antenna, h = 81 m

Radius of earth, R = 6.4 × 106 m

For range, d = (2Rh)½, the service area of the antenna is given by the relation:

A = πd2

= π (2Rh)
= 3.14 × 2 × 6.4 × 106 × 81
= 3255.55 × 106 m2
= 3255.55
∼ 3256 km2

Ques  15.5: A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75%?
Ans: Amplitude of the carrier wave, Ac = 12 V
Modulation index, m = 75% = 0.75
Amplitude of the modulating wave = Am
Using the relation for modulation index:

Ques  15.6: A modulating signal is a square wave, as shown in Fig. 15.14.

The carrier wave is given by

(i) Sketch the amplitude modulated waveform
(ii) What is the modulation index?
Ans: It can be observed from the given modulating signal that the amplitude of the modulating signal, Am = 1 V
It is given that the carrier wave c (t) = 2 sin (8πt)
∴ Amplitude of the carrier wave, Ac = 2 V
Time period of the modulating signal Tm = 1 s
The angular frequency of the modulating signal is calculated as:

The angular frequency of the carrier signal is calculated as:

From equations (i) and (ii), we get:

The amplitude modulated waveform of the modulating signal is shown in the following figure.

(ii)Modulation index,

Ques  15.7: For an amplitude modulated wave, the maximum amplitude is found to be 10 V while the minimum amplitude is found to be 2 V. Determine the modulation index μ. What would be the value of μ if the minimum amplitude is zero volt?
Ans: Maximum amplitude, Amax = 10 V
Minimum amplitude, Amin = 2 V
Modulation index μ, is given by the relation:

Ques  15.8: Due to economic reasons, only the upper side band of an AM wave is transmitted, but at the receiving station, there is a facility for generating the carrier. Show that if a device is available which can multiply two signals, then it is possible to recover the modulating signal at the receiver station.
Ans: Let ωc and ωs be the respective frequencies of the carrier and signal waves.

Signal received at the receiving station, V = V1 cos (ωc   ωs)t

Instantaneous voltage of the carrier wave, Vin = Vc cos ωct

At the receiving station, the low-pass filter allows only high frequency signals to pass through it. It obstructs the low frequency signal ωs. Thus, at the receiving station, one can

record the modulating signal  , which is the signal frequency.

The document Communication Systems NCERT Solutions | Physics Class 12 - NEET is a part of the NEET Course Physics Class 12.
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## FAQs on Communication Systems NCERT Solutions - Physics Class 12 - NEET

 1. What is a communication system?
Ans. A communication system is a network of devices and processes that enable the transmission of information from one place to another. It involves the use of a transmitter, a medium for signal transmission, and a receiver to decode and interpret the transmitted information.
 2. How does modulation work in a communication system?
Ans. Modulation is the process of altering the characteristics of a message signal to enable its transmission over a medium. It involves combining the message signal with a carrier signal of higher frequency. The resulting modulated signal carries the information of the message signal and can be transmitted efficiently through the medium.
 3. What are the different types of communication systems?
Ans. There are various types of communication systems, including analog and digital systems. Analog systems transmit continuous signals, while digital systems transmit discrete signals. Some common communication systems include radio communication, telephone systems, television broadcasting, and internet communication.
 4. What is the role of a transmitter in a communication system?
Ans. The transmitter is a crucial component of a communication system. It is responsible for converting the message signal into a form suitable for transmission over the medium. The transmitter modulates the message signal with a carrier signal, amplifies it, and transmits it through the medium to reach the receiver.
 5. How does the receiver decode the transmitted signal in a communication system?
Ans. The receiver plays a vital role in decoding and interpreting the transmitted signal in a communication system. It receives the modulated signal from the medium, demodulates it to recover the original message signal, and then processes it for its intended purpose. The receiver may also involve amplification and filtering processes to enhance the quality of the received signal.

## Physics Class 12

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## Physics Class 12

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