CBSE NOTES CLASS 12 PHYSICS

CHAPTER 15 COMMUNICATION SYSTEMS

Communication systems

Communication is the act of transmission and reception of information.

Every communication system has three essential elements-transmitter, medium or channel and receiver.

Block diagram of communication system

Modes of communication

Point-to-point communication

Communication takes place over a link between a single transmitter and a receiver. Example - telephony.

There are a large number of receivers corresponding to a single transmitter. Examples - radio and television.

Terms related to communication

Transmitter

A transmitter processes the original message signal so as to make it suitable for transmission through a channel and subsequent reception.

When a transmitted signal propagates along the channel it may get distorted due to channel imperfection and noise.

The receiver reconstructs a recognisable form of the original message signal at the channel output for delivering it to the user of information.

Communication channel

The transmitter is located at one place, the receiver is located at some other place (far or near) separate from the transmitter.

The channel is the physical medium that connects the transmitter and receiver.

A channel may be

• wires or cables connecting the transmitter and the receiver, or

• wireless.

Transducer

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.

Any device that converts one form of energy into another can be termed as a transducer.

An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature, sound etc) into corresponding variations in the electrical signal at its output.

Attenuation

The loss of strength of a signal while propagating through a medium is known as attenuation.

Amplification

It is the process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit called the amplifier. Amplification is necessary to compensate for the attenuation of the signal in communication systems. The energy needed for additional signal strength is obtained from a DC power source. Amplification is done at a place between the source and the destination wherever signal strength becomes weaker than the required strength.

Repeater

A repeater is a combination of a receiver and a transmitter. A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency. Repeaters are used to extend the range of a communication system. A communication satellite is a repeater station in space.

Range of communication

It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.

Different types of signals

Electrical signal

Information converted in electrical form and suitable for transmission is called a signal.

Noise signal

Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system.

Analog signal

Analog signals are continuous variations of voltage or current. They are single-valued functions of time. Sine wave is the fundamental analog signal. All other analog signals can be understood in terms of their sine wave components. Examples - sound and picture signals in TV.

Digital signal

Digital signals are those which can take only discrete stepwise values. Binary system used in digital electronics uses just two levels of a signal. ‘0’ corresponds to a low level and ‘1’ corresponds to a high level of voltage/current.

Coding systems

The codes used to represent numbers, letters and certain characters constitue coding system. BCD (binary coded decimal), ASCII (American Standard Code for Information Interchange), EBCDIC (Extended Binary Coded Decimal Interchange Code) etc. are examples of coding systems.

Bandwidth

Bandwidth refers to the frequency range over which, an equipment operates or the portion of the spectrum occupied by the signal.

Bandwidth of some media types

 Media type Bandwidth Twisted-pair for analog voice applications 1MHz Coaxial cable 1GHz Microwave 100GHz Satellite 100GHz Fiber 75THz

Some wireless frequency bands

 Service Frequency bands Comments Standard AM broadcast 540-1600 kHz FM broadcast 88-108 MHz Television 54-72 MHz 76-88 MHz 174-216 MHz 420-890 MHz VHF (very high frequencies) TV UHF (ultra high frequencies) TV Cellular Mobile Radio 896-901 MHz 840-935 MHz Mobile to base station Base station to mobile Satellite Communication 5.925-6.425 GHz 3.7-4.2 GHz Uplink Downlink

Rectangular waves

Digital signals are in the form of rectangular waves. This rectangular wave can be decomposed into a superposition of sinusoidal waves of frequencies ν0, 2ν0, 3ν0, 4ν0 ... nν0, where n is an integer extending to infinity and ν 0 is fundamental frequency equal to $\frac{1}{{\mathrm{T}}_{\mathrm{o}}}$.

To reproduce the rectangular wave shape exactly we need to superimpose all the harmonics ν0, 2ν0, 3ν0, 4ν0 .., which implies an infinite bandwidth.

However, for practical purposes, the contribution from higher harmonics can be neglected, thus limiting the bandwidth.

Received waves are a distorted version of the transmitted one. 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.

Propagation of electromagnetic waves

The strength of the wave keeps on decreasing with distance travelled. Several factors influence the propagation of e-m waves and the path they follow.

The earth’s atmosphere plays a vital role in the propagation of e-m waves.

Different types of waves used in communication

Ground waves

A ground wave is a surface wave that propagates close to the surface of the Earth. The mode of propagation is called surface wave propagation and the wave glides over the surface of the earth. The ground has a strong influence on the propagation of the signal.

A wave induces current in the ground over which it passes and it is attenuated as a result of absorption of energy by the earth. The attenuation of surface waves increases very rapidly with increase in frequency. The maximum range of coverage depends on the transmitted power and frequency.

The antennas should have a size comparable to the wavelength λ of the signal (at least ~ λ/4). At longer wavelengths (i.e., at lower frequencies), the antennas have large physical size and they are located on or very near to the ground. Ground based vertical towers are generally used as transmitting antennas.

Sky waves

Sky wave propagation is a method of radio frequency propagation that uses the area between the surface of the earth and the ionosphere for transmission.

In this case ionospheric reflection of radio waves is used in long distance communication. This mode of propagation is called sky wave propagation and is used by short wave broadcast services.

The ionosphere has a large number of ions or charged particles. It extends from a height of ~ 65 Km to about 400 Km above the earth’s surface. Ionisation occurs due to the absorption of the ultraviolet and other high-energy radiation coming from the sun by air molecules.

The ionosphere is further subdivided into several layers.

Different layers of atmosphere and their interaction with the propagating electromagnetic waves

 Name of the stratum (layer) Approximate height Exists during Frequencies most affected Troposphere 10 km Day and night VHF (up to several GHz) D (part of stratosphere) Part of Ionosphere 65-75 km Day only Reflects LF, absorbs MF and HF to some degree E (part of stratosphere) 100 km Day only Helps surface waves, reflects HF F1 (Part of Mesosphere) 170-190 km Daytime, merges with F2 at night Partially absorbs HF waves yet allowing them to reach F2 F2(Thermosphere) Daytime:250-400 km Night time: 300 km Day and night Efficiently reflects HF waves, particularly at night

The degree of ionisation varies with the height. The density of atmosphere decreases with height. At great heights the solar radiation is intense but there are few molecules to be ionised.

Close to the earth, even though the molecular concentration is very high, the radiation intensity received is low so that the ionisation is again low.

At some intermediate height, ionisation density is maximum.

The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz). Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape.

The phenomenon of bending of e-m waves so that they are diverted back towards the earth is similar to total internal reflection in optics.

Space waves

A space wave travels in a straight line from transmitting antenna to the receiving antenna.

Space waves are used for line-of-sight (LOS) communication as well as satellite communication for frequencies > 40 MHz. The frequencies of these waves are very high.

The antennas are relatively smaller and can be placed at heights of many wavelengths above the ground. Because of line-of-sight nature of propagation, direct waves get blocked at some point by the curvature of the earth. If the signal is to be received beyond the horizon then the receiving antenna must be high enough to intercept the line-of-sight waves.

If the transmitting antenna is at a height hT, then the distance to the horizon dT is given as , where R is the radius of the earth (approximately 6400 km). dT is also called the radio horizon of the transmitting antenna.

The maximum line-of-sight distance dM between the two antennas having heights hT and hR above the earth is given by

where hR is the height of receiving antenna.

Television broadcast, microwave links and satellite communication use space wave mode of propagation.

Modulation

The original low frequency message/information signal cannot be transmitted to long distances because of various reasons.

Therefore, at the transmitter, information contained in the low frequency message signal is superimposed on a high frequency wave, which acts as a carrier of the information. This process is known as modulation. There are several types of modulation, e.g. AM, FM and PM.

Demodulation

The process of retrieval of information from the carrier wave at the receiver is called demodulation. This is the reverse process of modulation.

Baseband signals

Signal representing the band of frequencies of the original signal, as delivered by the source of information, is called baseband signal. The range of frequencies is called the signal bandwidth.

Need for modulation

1. Size of the antenna or aerial

The antenna should have a size comparable to the wavelength of the signal (at least λ/4 in dimension) to sense the time variation of the signal. For an electromagnetic wave of frequency 20 kHz, the wavelength λ is 15 km. Hence direct transmission of such baseband signals is not practical due to large antenna requirement. We can transmit with reasonable antenna lengths if transmission frequency is high (for example, if ν is 1 MHz, then λ is 300 m). Therefore, there is a need of translating the information contained in the original low frequency baseband signal into high or radio frequencies before transmission.

2. Effective power radiated by an antenna

Power radiated by an antenna is related to length L of the antenna and wavelength λ as follows,

Therefore, for the same antenna length, the power radiated increases with decreasing λ, i.e., increasing frequency. Hence, the effective power radiated by a long wavelength baseband signal would be small. For a good transmission, we need high powers and hence the need of using high frequency transmission.

1. Mixing up of signals from different transmitters

If many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously, all these signals will get mixed up and there is no simple way to distinguish between them. Using communication at high frequencies and allotting a band of frequencies to each message signal for its transmission can solve the problem.

Mechanism of modulation

The carrier wave may be continuous (sinusoidal) or in the form of pulses.

Sinusoidal carrier

Sinusoidal carrier

A sinusoidal carrier wave can be represented as

where c(t) is the signal strength (voltage or current), Ac is the amplitude, ωc ( = 2πνc) is the angular frequency and φ is the initial phase of the carrier wave.

During the process of modulation, any of the three parameters, that is, Ac, ωc or φ of the carrier wave can be controlled by the message or information signal.

Accordingly there are three types of modulations using a sinusoidal carrier ,

1. Amplitude modulation (AM),

2. Frequency modulation (FM)

3. Phase modulation (PM).

Pulsed shaped carrier

Pulsed shaped carrier

Pulse characteristics which can be modified are, amplitude, duration or width of pulse, position. Different types of pulse modulation are,

1. pulse amplitude modulation (PAM),

2. pulse duration modulation (PDM) or pulse width modulation (PWM),

3. pulse position modulation (PPM)

Amplitude modulation

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

Let c(t) = Ac sin ωct represent carrier wave and m(t) = Am sin ωmt 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,

${\mathrm{C}}_{\mathrm{m}}\left(\mathrm{t}\right)=\left({\mathrm{A}}_{\mathrm{c}}+{\mathrm{A}}_{\mathrm{m}}\mathrm{sin}{\mathrm{\omega }}_{\mathrm{m}}\mathrm{t}\right)\mathrm{sin}{\mathrm{\omega }}_{\mathrm{c}}\mathrm{t}$

Where, μ = $\frac{{\mathrm{A}}_{\mathrm{m}}}{{\mathrm{A}}_{\mathrm{c}}}$ is the modulation index.

In practice, μ is kept ≤ 1 to avoid distortion.

Using the trigonometric relation,

We can write,

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

The modulated signal consists of the carrier wave of frequency ωc plus two sinusoidal waves each with a frequency slightly different from ωc, known as side bands.

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

Production and transmission of amplitude modulated wave

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 sin ωmt + Ac sin ωct is passed through a square law device which is a non-linear device which produces an output y(t) = Bx(t ) + Cx2(t) , where B and C are constants. Thus,

Using the trigonometric relation,

and

we get,

There is a DC term C/2(Am2+Ac2) and sinusoids of frequencies ωm, 2ωm, ωc, 2ωc, ωc – ωm and ωc + ωm. This signal is passed through a band pass filter which rejects DC and the sinusoids of frequencies ωm, 2ωm and 2ωc and retains the frequencies ωc, ωc – ωm and ωc + ωm. The device which only passes frequencies within a certain range and rejects frequencies outside that range is called band pass filter. The output of the band pass filter is of the same form as an AM wave.

The modulator is to be followed by a power amplifier which provides the necessary power and then the modulated signal is fed to an antenna of appropriate size for radiation.

Detection of amplitude modulated wave

The transmitted message gets attenuated in propagating through the channel. The receiving antenna is therefore to be followed by an amplifier and a detector.

To facilitate further processing, the carrier frequency is usually changed to a lower frequency by 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.

Detection is the process of recovering the modulating signal from the modulated carrier wave. The modulated carrier wave contains the frequencies ωc and ωc ± ωm.

Block diagram of AM wave detector

The Internet

It is an interconnection of billions of (users) systems worldwide. It permits communication and sharing of all types of information between any two or more computers connected through a large and complex network. It was started in 1960’s and opened for public use in 1990’s.

Main applications of Internet

1. E mail – It permits exchange of text/graphic material using email software. We 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.

2. File transfer – FTP (File Transfer Protocol) allows transfer of files/software from one computer to another connected to the Internet.

3. World Wide Web (WWW) – Computers that store specific information for sharing with others provide websites either directly or through web service providers. Government departments, companies, NGO’s (Non-Government Organisations) and 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.

4. Hypertext is a powerful feature of the web that automatically links relevant information from one page on the web to another using HTML (hypertext markup language).

5. E-commerce – Use of the Internet to promote business using electronic means such as using credit cards is called E-commerce. Customers view images and receive all the information about various products or services of companies through their websites. They can do on-line shopping from home/office. Goods are dispatched or services are provided by the company through mail/courier.

6. Chat – Real time conversation among people with common interests through typed messages is called chat. Everyone belonging to the chat group gets the message instantaneously and can respond rapidly.

Facsimile (FAX)

It scans the contents of a document (as an image, not text) to create 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 image of a static document unlike the image provided by television of objects that might be dynamic.

Mobile telephony

The concept of mobile telephony was developed first in 1970’s and it was fully implemented in the following decade. The central concept of this system is to divide the service area into a suitable number of cells centred on an office called MTSO (Mobile Telephone Switching Office).

Each cell contains a low-power transmitter called a base station and caters to a large number of mobile receivers (popularly called cell phones). Each cell could have a service area of a few square kilometers or even less depending upon the number of customers. When a mobile receiver crosses the coverage area of one base station, it is necessary for the mobile user to be transferred to another base station. This procedure is called handover or handoff. This process is carried out very rapidly, to the extent that the consumer does not even notice it. Mobile telephones operate typically in the UHF range of frequencies (about 800-950 MHz).