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# NCERT Exemplar Class 12 Physics Chapter 15 Solutions: Communication Systems

NCERT Exemplar Class 12 Physics Solutions Chapter 15 talks about communication systems which are means to transmit, connect and interconnect data. These systems are used in media and have applications in many Technologies. Based on where they are used, they are categorized in different segments. Some of the examples of communication systems parts are sensors, amplifiers, emitters, etc which are used in emails, computers, radio, and the likes. This chapter of NCERT Exemplar Class 12 Physics will take you through elements of a communication system and basic technologies used, signals, transmission medium, and their bandwidths, modulation, antenna, and aerial size, mixing signals from different transmitters, propagation of electromagnetic waves, and amplitude modulated wave characteristics.

To solve exemplar problems of NCERT Class 12 Physics, students are presented with a total of 30 questions divided into 5 sections. The questions are designed in a variety of ways like very short answers, long answers, objective types with single and multiple correct answers, and short answers.

Our subject matter experts are aware of the stress that you go through while dealing with complex topics on NCERT Exemplar Problems Class 12 Physics. Hence they have provided a step-by-step solution of every topic in an easily comprehensible manner. Students not only get solutions to the problems but are also able to get clarity on the subject with the way these problems are solved.

## Important Topics for NCERT Science Exemplar Solutions Class 12 Chapter 15

• Basics of Communication system – The act of transmitting and receiving information forms a communication system. A communication system is made up of the following major parts:
• Source of informationSource is the point where signals are produced which need to be communicated.
• SignalAny information which is in electrical form, fit for transmission is a signal.
• TransmitterOne needs to convert the source signal into a form which is suitable for transmission along the channel that is the job of a transmitter. Modulators and transmitting antennas are part of this.
• ChannelA physical medium which connects the transmitter and the receiver is the channel. Many forms of a channel are wires, cables, etc.
• Noise If there are imperfections in the channel, the signal can get distorted. That unwanted signal is called noise.
• ReceiverThe signal might get distorted due to noise when it reaches the receiver. The receiver then needs to reconstruct the signal which can be in a recognizable format.
• TransducerA device that converts one form of energy into another is a transducer. E.g.: A microphone which converts sound energy into electrical signals
• Analog signalsWhen there is continuous variation of voltage or current it forms an analog signal
• Digital signalsSignals which have only discrete step-wise values are digital signals.
• RepeaterThe combination of transmitter and receiver is a repeater. Its job is to pick the signal at the transmitter end, amplify and retransmit it to the receiver end.
• AntennaA device which acts as a medium to emit or receive electromagnetic waves is an antenna. It is made of metal in the form of a collection of wires.
• Bandwidth of channel – Range of frequencies that a channel can transmit is called its bandwidth.

Nc =Bc/Bandwidth needed per channel

Nc – number of channels

Bc – total bandwidth of channel

• Bandwidth of signals – In a signal, the difference of the uppermost and lowermost frequency is its bandwidth. Few examples are:
1. Speech signal – freq range is 300-3100 Hz, bandwidth = 2800 Hz
2. Music signal – freq range is 20-20000 Hz, bandwidth – 20KHz
• Bandwidth of transmission medium – Different material used as a transmission medium can have different bandwidths. Some commonly used mediums are free space, wire, fiber optic cable. The range of bandwidth is subdivided and allocated to various services. Few of them are:
1. Cellular – freq bands (896-901 MHz)
2. FM – freq bands (88-108 MHz)
• Modes of communication system – There are 2 modes:
1. Point to point – In this mode, there is a single transmitter and single receiver and communication happens over a link. E.g.: Telephone.
2. Broadcast – In this mode, there is a single transmitter and a large number of receivers. E.g.: Radio.
• Modulation and demodulation
1. A low-frequency wave, which cannot be transmitted to long distances, is superimposed over a high-frequency wave at the transmitter end; this process is modulation.
2. The reverse of modulation where information is retrieved from the modulated wave at the receiver end is demodulation.
• Propagation of electromagnetic waves – Radio waves are transmitted from the transmitter end and travel in free space to reach the receiver antenna. These waves have the speed of light and travel in transverse form.

c= v.λ

Here c- velocity of the wave

v – Frequency of the wave

λ – Wavelength of the wave

Their propagation is of 3 types:

• Ground propagation – When the troposphere is used for transmitting low-frequency waves, usually less than 1 MHz, it is called ground propagation. Large antennas of the order of wavelength of the waves are used for this and such propagation cannot travel large distances. Satellite, TV, etc are examples of ground propagation.
• Skywave propagationWhen the ionosphere is used for transmitting medium frequency waves, usually in the range of 3 MHz-30 MHz, it is called sky wave propagation. The presence of charged ions in this region acts as a reflecting medium for the waves. Here the angle of transmission has to be at a critical angle for total reflection to the ground.
• Space wave propagation – It is also called LoS or line of sight communication. Used by very high-frequency waves which are > 30 MHz, these waves travel in a straight line from the transmitter to the antenna.

Da = (2RHt) -1/2 + (2RHr) -1/2

Here Da – distance between 2 antennas

R – radius of the earth = 6400 Km

Ht – Height of transmitting antenna

Hr – Height of receiving antenna

Range of transmission for an antenna of height Ht is given by:

Dt = (2RHt) -1/2

• Amplitude modulation – The carrier wave’s amplitude is modulated based on the amplitude of the information signal.
• For a sinusoidal wave – m(t) = Amod sinωm
• For a carrier wave – Cm(t) = Acar sinωct

Here Amod – amplitude of modulating signal

Acar – amplitude of carrier signal

ωm or ωc = טc angular frequency of either the modulating or the

carrier wave.

## Discussion of Exercises of NCERT Exemplar Class 12 Physics Solutions Chapter 15

1. The first set of questions, MCQ1, is objective type questions which have only one correct answer. It has 9 questions. The topics discussed in this section are the basics of transmission and antenna, attenuation, and amplitude modulation.
2. The second set of questions is MCQ2, which are again objective type questions where there may be more than 1 correct answer and you must choose all the correct answers for the given problem statement. It has 5 questions based on the size of the antenna required for transmission, amplitude modulation, and band frequencies.
3. The third set of questions requires very short type answers which are based on concepts of analog and digital signals, propagation of electromagnetic waves, modulation, and amplifiers. It has 6 questions.
4. The fourth set of questions requires short type answers where you need to apply knowledge of some basic concepts of transmission and noise, antenna height, propagation of electromagnetic waves, to solve the 5 questions provided.
5. The fifth set of questions involves long answers where you need to show expertise on solving problems around the propagation of waves and amplitude modulation in solving the 5 questions presented.
6. The NCERT Exemplar Problems for all chapters of Physics are very beneficial for students as they are modeled in CBSE as well as questions that come in competitive exams. Solving these problems gives a good base to students for taking up any kind of exam.

## Why Use NCERT Science Exemplar Solutions Class 12 Chapter 15 by Instasolv?

NCERT Exemplar Class 12 Problems with Solutions provided by Instasolv gives the right tools and techniques to students for effectively learning any topic. The simplicity with which our experts solve all the questions is easily comprehensible by students. Students get a better grasp of the topic discussed and with such a rigorous practice they are better at dealing with many questions independently. Instasolv is free of cost and a great resource for students who are appearing in CBSE Board exams.

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