CBSE Class 12th Physics Notes: Communication Systems (Part – III)

Chapter notes for class 12th Physics are available here. These notes are based on chapter 15 ( Communication Systems) of class 12th Physics NCERT textbook.

Created On: Feb 6, 2017 13:00 IST

CBSE Class 12th Physics Chapter wise Notes based on the chapter Communication Systems (Chapter number 15 of Class 12 Physics NCERT textbook) are available here.  

These notes are important for coming CBSE Class 12 Physics board exam 2017.

These notes are the continuation of CBSE Class 12th Physics Notes: Communication Systems (Part ‒ I) and (Part ‒ II).

In part ‒ I, we have studied about communication system, elements of communication system and basic terminology used in electronic communication systems etc.

Whereas, in part – II, we have gone through terms like bandwidth of signals, bandwidth of transmission medium, propagation of electromagnetic waves, ground wave propagation, sky wave propagation, space wave propagation.

Now in part III, we will study the topics given below


Necessity of Modulation

Types of Modulation

Amplitude modulation

Frequency modulation

Frequency Spectrum of the Amplitude Modulated Signal

Production of Amplitude Modulated Wave

Detection of Amplitude Modulated Wave




World Wide Web




Facsimile (FAX)

The notes of third part of the chapter are given below:

Baseband Signals

Message signals are also called baseband signals, which essentially designate the band of frequencies representing the original signal, as delivered by the source of information.

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It is the phenomena of superimposing the low audio frequency baseband message or information signals or modulating signal on a high-frequency carrier wave. The resultant wave is called the transmitted wave, which is transmitted.

Necessity of Modulation

We cannot transmit an electronic signal in the audio frequency (AF) range [baseband signal frequency less than 20 kHz] over a long distance directly due to the following reasons

• Size of the antenna or aerial

• Effective power radiated by an antenna

• Mixing up of signals from different transmitters

To overcome these limitations, we use the process of modulation.

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Types of Modulation

There are three types of modulation:

Amplitude modulation (AM)

Frequency modulation (FM)

Phase modulation (PM)

Amplitude modulation

In amplitude modulation the amplitude of the carrier is varied in accordance with the information signal.

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Frequency modulation

Let c(t) = Acsin ωct represent carrier wave and m(t) = Amsinωmt represent the message or the modulating signal where ωm = 2πfm is the angular frequency of the message signal.

The modulated signal cm (t) can be written as cm (t) = (Ac + Am sin ωmt) sin ωct

Here, the modulated signal now contains the message signal,

cm (t) = Ac sinωct + μAc sinωmt sinωct …(i)

Here μ = Am/Ac is the modulation index and μ ≤ 1. It should be kept less than 1 to avoid distortion.

On further simplifying equation …(i) we get

cm (t) = Ac sinωct + (μAc/2) cos (ωc ωm) t ‒ (μAc/2) cos (ωc + ωm) t.

Frequency Spectrum of the Amplitude Modulated Signal

Frequency Spectrum of the Amplitude Modulated Signal

Here, ωc – ωm and ωc + ωm are respectively called the lower side and upper side frequencies.

As long as the broadcast frequencies (carrier waves) are sufficiently spaced out so that sidebands do not overlap, different stations can operate without interfering with each other.

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Production of Amplitude Modulated Wave

Block diagram of a simple modulator for obtaining an AM signal is shown in the figure given below

Production of Amplitude Modulated Wave

Here the modulating signal Am sin ωmt is added to the carrier signal Ac sin ωct to produce the signal x (t). This signal x (t) = Am sinωmt + Ac sinωct is passed through a square law device which is a non-linear device which produces an output y (t) = B x (t) + Cx2 (t) where B and C are constants.

The modulated signal cannot be transmitted as such. The modulated signal is fed to an antenna of appropriate size for radiation as shown in figure given below

Block diagram of a transmitter

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Detection of Amplitude Modulated Wave

Detection is the process of recovering the modulating signal from the modulated carrier wave.

The block diagram of a typical receiver is shown in the figure given below:

Block diagram of a receiver

The transmitted message signal gets attenuated while propagating through the channel.

The receiving antenna is, therefore, to be followed by an amplifier & a detector.

In addition, to facilitate further processing, the carrier frequency is usually changed to a lower frequency by what is called an intermediate frequency (IF) stage preceding the detection.

The detected signal may not be strong enough to be made use of and hence is required to be amplified. The block diagram of a detector for AM signal, the quantity on y-axis can be current or voltage

Block diagram of a detector for AM signal

The modulated signal of the form given in (a) of above figure is passed through a rectifier to produce the output shown in (b). This envelope of the signal (b) is message signal. In order to retrieve m(t ), the signal is passed through an envelope detector (which may consist of a simple RC circuit).


It is a system with billions of users worldwide. It permits communication and sharing of all types of information between any two or more computers connected through a large and complex network.

Email (or Electric Mail)

It permits the exchange of text/graphic material (digital data and information) using email software. One can write a letter and send it to the recipient through ISP’s (Internet Service Providers) who work like the dispatching and receiving post offices.

FTP (or File Transfer Protocol)

FTP is a standard network protocol used to transfer computer files from one computer to another connected to the Internet.

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World Wide Web (WWW)

Computers that store specific information for sharing with others provide websites either directly or through web service providers. Companies or individuals can post information about their activities for restricted or free use on their websites. This information becomes accessible to the users. Several search engines like Google, Yahoo! etc. help us in finding information by listing the related websites.


HTML is an acronym for Hyper Text Markup Language. It is the set of markup codes or symbols used for creating web pages.


It is a term used for the use of the Internet to promote a business using electronic means such as using credit cards.
With the help of E-commerce, customers view images and receive all the information about various products or services of companies through their websites. Customers can do on-line shopping from home or office. Goods are dispatched or services are provided by the company through mail or courier.


It is the real-time conversation among people with common interests through typed messages is called chat. Anyone belonging to the chat group gets the message instantaneously and can respond rapidly.

Facsimile (FAX)

It is the electronic reproduction and transmission of a document at a distant place is carried out with the help of FAX.  It scans the contents of a document (as an image) and converts it into electronic signals. These signals are then sent to the destination (another FAX machine) in an orderly manner using telephone lines.
At the destination, the signals are reconverted into a replica of the original document. Note that FAX provides an image of a static document unlike the image provided by television of objects that might be dynamic.

CBSE Class 12th Physics Notes: Communication Systems (Part – I)

CBSE Class 12th Physics Notes: Communication Systems (Part – II)

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