CH-3 Electronic Communication Marks: 15 (25)
Introduction:
Communication can be defined as the process of exchange of information through means such as words, actions, signs, etc., between two or more individuals.
Communication is the sharing of thoughts with one another. Communication can be defined as the basic process of exchanging information.
The information may be oral or in written or printed form.
In practice communication involves the conversion of message which may be in the form of words or coded symbols into electrical voltage or current and is used to carry information from one point to another.
Elements of communication system:
The basic elements of a communication system are a transmitter, a communication channel or medium and a receiver. The message input is called Information or Intelligence. It is in the form of voice, code, picture, data etc.
Transmitter: The information signal is given to the transmitter. The transmitter consists of electronic components and circuits designed to convert the information into a signal suitable for transmission over a given communication channel. It may be simple microphone.
Communication channel: The electronic signal is sent from one place to another place by the communication channel or medium. The three major types of communication channels are wire, radio and fiber optic cable. The medium transmits the information. But it attenuates and degrades the signal which causes decrease in its amplitude. Therefore, for successful communication amplification is necessary at transmitter and receiver.
Receiver: The receiver accepts the transmitted message from the channel and converts it back into a form understandable by human. It may be simple earphone.
Simplex or duplex transmission:
(A)Simplex: One way communication is called simplex. The information travels in one direction only. Here one station transmits and other receives. There is no provision for feedback from distant end.
Applications:
(a) Radio broadcasting
(b) TV broadcasting
(c) Cable television.
(d) Wireless remote control
(e) Paging
(f) Telemetry (lt transmits information about physical status of satellite).
(B)Duplex: Two way communication is called duplex.
Half duplex: The type of communication in which one party transmits at a time is called ‘half duplex’. In this type two subscribers are assigned two different frequencies, so at a time one can communicate with other subscriber. When 1st subscriber stops sending message, 2nd can start communicating.
e. g. Radio communication used in the military, fire, police, citizens band (CB) radio etc.
Full duplex: Simultaneous two way communication is called ‘full duplex’. e.g. in telephone system, each can transmit and hear simultaneously.
Applications:
(a) Telephones
(b) RADAR
(c) SONAR
(d) Amateur radio
(e) CB radio
(f) Local area network (LAN)
(g)Two way radio used in aircraft, space applications.
Analog Communication:
A continuous time varying signal, which represents a time varying quantity is called an Analog Signal.
The analog signals are sinusoidal in nature represent the variation of physical quantity. e. g. telephone signals, TV picture signals, radio broadcast signals etc.
The communication based on analog signals and analog values is known as Analog Communication.
Analog Communication is a data transmitting technique in which information signal is transmitted in analog nature. T
his always utilizes continuous signals to transmit data which may obtained from audio, image, video etc All AM, FM audio transmission and T.V. transmission are the examples of analog communication.
Modulation:
The term modulation is related to communication i.e. sending information through transmitter and receiving it through receiver. The information may be sound (audio) or picture (video). It is transmitted in the form of electromagnetic waves. But it is not so easy. Certain process is necessary that is called modulation.
Necessity of modulation:
Q. Why audio waves can’t be transmitted directly? OR What are the practical difficulties in sending audio waves directly?
Ans: Audio signal whose freq range is 20Hz to 20kHz belongs to low: freq. Several difficulties are involved in the direct transmission of these low freq audio signals or video signals as follows-
Length of antenna: For efficient radiation of electromagnetic waves, antenna should have length about quarter wavelength of frequencies used.
e.g. for audio signal of l5kHz,
wavelength, λ = c/f, c-velocity of radio waves = 3x108 m/s
=
=20km
λ /4 = 20/4 = 5km. This much length of antenna is practically not possible. But if we calculate the length of antenna for 1MHz, it will about 75meters which is possible.
Interference: More than one transmitting stations in particular city transmit the signals would be mixed up. All signals from different sources would be mixed as they occupy same frequency range. It will be impossible to recover (select) the desired signal at the receiver.
To avoid this, each station should transmit separate frequency signal.
Poor reception/transmission: Low frequency audio signals can’t travel over a long distance with sufficient strength. They get attenuated due to absorption of energy from them by earth and other intervening objects like trees, buildings, hills etc. i.e. these signals die over a long distance. .
To overcome these three difficulties some carrier signal is required and it must be high freq signal. So that required length of antenna is possible. For each station this freq of carrier will be fixed. Therefore there is not interference. Also high freq carrier can travel over long distance. It needs process of modulation.
Some definitions:
The process in which some characteristics such as amplitude, frequency, phase of high frequency sine wave (carrier) is varied in accordance with amplitude of modulating signal (audio, video) is called modulation.
The information (low frequency) signal which is to be superimposed on a high frequency signal by process of modulation is called ‘modulating signal'
The high frequency signal is called ‘carrier signal' and the resultant signal is called ‘modulated signal’.
Types of analog modulations:
1. Amplitude modulation (AM):
The process in which, the instantaneous amplitude of carrier changes in accordance with the amplitude of information (modulating) signal, is called amplitude modulation.
2. Frequency modulation (FM):
The process in which, the frequency of carrier changes in accordance with the amplitude of modulating signal, is called frequency modulation.
3. Phase modulation (PM):
The process in which, the phase of carrier changes in accordance with the amplitude of modulating signal, is called phase modulation.
Frequency modulation and phase modulations are also called ‘angle modulations’.
Amplitude Modulation (AM):
Basic principle:
AM is used in radio system for sound transmission and in TV system for picture transmission.
In AM, carrier frequency and phase remain constant during modulation process, but its amplitude changes in accordance with amplitude of modulating signal.
When amplitude of modulating signal is increasing in +ve direction then amplitude of carrier signal is also increasing in the same direction.
When it is decreasing in –ve direction, carrier signal is also decreasing in the same direction.
When the signal is absent we get unmodulated carrier.
If we interconnect the positive and negative peaks of modulated carrier waveform with imaginary line, exact shape of modulating signal can be recreated. This imaginary line on carrier is called the envelope.
Waveform of Amplitude Modulation
Carrier wave can be represented as Ec = Vcsin2πfct where Vc is its peak amplitude and fc is frequency.
Similarly modulating signal i.e. audio signal can be represented as Em= Vmsin2πfmt where Vm is peak amplitude and fm is freq of modulating signal.
In AM, Vc of carrier wave varies with above and below by the amount Em= Vmsin2πfmt.
As a result AM wave is given as-
EAM = Vcsin2πfct +m cos2π(fc – fm) t – m cos2π(fc + fm) t
From above equation it is seen that amplitude modulated wave consists of three components-
Original carrier frequency fc with amplitude Vc.
(fc + fm) i.e. sum of carrier and modulating signal frequencies, it is known as upper side band (USB) with amplitude m.
(fc – fm) i.e. difference of carrier and modulating signal frequencies, it is known as lower side band (LSB) with amplitude m.
Bandwidth (BW) of AM:
From frequency spectrum it is seen that bandwidth (BW) for AM is-
BW = USB – LSB
= (fc + fm) – (fc – fm)
= 2fm
i.e. BW is twice the modulating frequency.
Sidebands and their importance:
The new frequencies generated in the process of modulation are called side bands.
AM wave contains two side bands having frequency of (fc+fm) for USB and (fc-fm) for LSB with amplitude m.
The sidebands are very important because it contains information. The carrier is only used for carrying signal to long distances. The sidebands show variation of modulating signal on both sides of carrier.
It limits number of stations that can use a band of frequencies without interference.
Modulation Index:
The relationship between amplitudes of carrier and modulating signal is given by the factor, called as Modulation Index. It is denoted by “m”. It shows the quality of transmitted signal.
Definition: -The ratio of peak amplitude of modulating signal to the peak amplitude of carrier signal is known as “Modulation Index (m)”.
m = =
Modulation index is also called ‘modulation factor’ or ‘degree of modulation’. It should a number between 0 and 1.
Modulation index when multiplied by 100, gives ‘percentage modulation’.
Importance of Modulation Index:
The important feature of AM is depth of modulation i.e. extent to which amplitude of carrier wave is changed by modulating signal. It is given by modulation index. It tells about the strength and quality of transmitted signal. If ‘m’ is small, amount of carrier amplitude variations are small and audio signal to be transmitted will not be strong. Greater the modulation index, stronger and clear will be the audio signal.
Let’s see effect of ‘m’ on AM signal —
If m<1, it means amplitude of modulating signal is less than amplitude of carrier. Resultant signal is undistorted but after detection at receiver, signal strength of audio is weak.
Under Modulation
If m=1, after detection we get maximum strength of audio signal because it varies between (Ec + Em) to (Ec – Em).
Perfect modulation
If m>1, amplitude of modulating signal is more than carrier amplitude. Resultant signal produces distortion in the signal. It is known as over modulation which is undesirable. Therefore, m is always less than or equal to 1.
. Over modulation
Frequency Modulation (FM):
FM is used for sound transmission in TV transmission and in FM radio broadcasting system.
Basic principle:
It is the process of modulation in which the frequency of high freq carrier changes in accordance with instantaneous amplitude of modulating signal.
Carrier amplitude remains constant. FM is use in radio transmitter and TV transmitter for transmission of sound.
In FM, frequency of carrier changes in accordance with amplitude of audio signal. In this amplitude of carrier signal remains constant. As amplitude of modulating signal increases, frequency of carrier signal increases.
When amplitude of modulating signal becomes zero, freq of FM wave is same as freq of carrier.
As amplitude of modulating signal decreases, freq of carrier in FM decreases. For maximum +ve amplitude of modulating signal, freq of carrier is maximum and for maximum –ve amplitude, freq of carrier in FM is minimum
Some definitions:
Frequency Deviation: The change in carrier frequency in accordance with amplitude of modulating signal is called freq deviation.
Frequency deviation, ∆f = fmax – fc OR
= fc – fmin
Carrier Swing: The total variation in carrier freq from lowest to highest is called carrier swing. Carrier swing = fmax – fmin
= (fmax – fc) – (fc – fmin)
= ∆f + ∆f
= 2∆f
Modulation Index: It is the ratio of frequency deviation to modulating signal frequency.
Modulation Index
In FM, m can be greater than 1 without causing distortion.
Percentage modulation: In FM, percentage modulation is the ratio of actual frequency deviation to the maximum carrier deviation allowed.
Maximum frequency deviation allowed is 75kHz for FM radio broadcast and 25 kHz for TV audio broadcast.
Bandwidth for FM:
For FM broadcast, a band of frequency from 88MHz to 108MHz is used.
A maximum frequency deviation of 75kHz is allowed for commercial FM broadcast stations.
Therefore FM channel width is 2x75=l50kHz. A guard band of 25kHz is allowed on both sides.
So total channel width becomes 2(75+25)=200kHz. Guard band is to prevent the interference between adjacent channels
Sidebands in FM:
In FM, theoretically infinite numbers of pairs of upper and lower sidebands are generated.
Typically any sideband whose amplitude is at least one percent of unmodulated carrier is called ‘significant sideband’.
The number of significant sidebands and their amplitudes depend upon the amount of frequency deviation and modulating frequency.
If modulation index is known, the number and amplitudes of significant sidebands can be calculated.
B W = 2 N (maximum modulating frequency) OR
BW = 2 (maximum frequency deviation + maximum modulating frequency)
Comparison between AM and FM:
Satellite Communication:
In nature the earth and other planets are satellites which revolve around the sun.
A satellite is a physical object that orbits or revolves around some celestial body.
A balance between the inertia of the revolving satellite and the gravitational pull keeps the satellite in orbit.
Artificial satellites can be launched into orbit for communication and other purposes.
A communication satellite is basically RF repeater (transponder) station which has made broadband long distance communication easier.
A satellite revolves around the earth in either a circular or elliptical Path (orbit), A satellite revolves in an obit that forms a plane which passes through centre of gravity of earth or the geocentre.
If direction of satellite’s revolution is same as the earth’s rotation, the orbit is said to be posigrade and if the direction of satellite’s revolution is against direction of earth’s rotation, it is retrograde.
The location of a satellite is specified in terms of latitude and longitude.
The satellite location can be specified by a point on the surface of the earth directly below the satellite.
This point is known subsattellite point (SSP). The SSP is located using latitude and longitude.
Necessity/Concept of geosynchronous satellite:
To use a satellite for communication relay or repeater purposes, the ground station antenna must be able to follow or track the satellite as it passes overhead.
Depending upon the height and speed of the satellite, the earth station will only be able to use it for communication purpose for that short period of time when it is visible.
Intermittent communication is undesirable in communication. Therefore, a geosynchronous satellite can be used.
A satellite which orbits the earth around the equator at a distance of 35, 860 km is called "geosynchronous or geostationary satellite.
A satellite at that distance revolves around the earth in exactly 24 hours (1 cycle/day). It revolves in exact synchronism with the earth's rotation.
Special earth station tracking antennas are not required. The antenna can simply be pointed at the satellite and remain in a fixed position.
Approximately 40 percent of the earth's surface can be accessed from such a satellite.
Three Such satellites can cover the entire earth's surface excepting small areas near north and south poles.
Only geostationary satellites have a fixed subsatellite point (SSP) on the surface of earth.
Such satellite does not remain stationary even when placed at the correct height because of equatorial ellipticity and the effects of gravitational pull of the sun and the moon.
Corrections to the height and the velocity of the satellite are done at regular intervals throughout the life of the satellite.
Merits of geosynchronous satellite communication satellite):
It requires a limited earth station antenna tracking since the satellite remains fixed in the position relative to earth.
It is capable of providing continuous and uninterrupted communication over the desired area
Satellite communication links are not affected by Doppler frequency shift.
Demerits:
A costly launch vehicle is required.
Regions near the north and south poles are not covered.
There is a time delay of about 300 msec between a transmitted and received signal.
Use of satellite (communication) as a relay station:
Communication satellites are used as relay station for other sources. If a transmitting station cannot communicate directly with other receiving stations of the line of sight restrictions, then a satellite is used as repeater. The transmitting station sends the information to the satellite which retransmits it to the receiving station
A transmitting station (earth station) transmits information to the satellite. A receiver in the satellite picks up the transmitted signal amplifies it and changes it to another frequency. The original signal being transmitted to the satellite from the earth station is called uplink.
In C band uplink frequencies are in the 5.925 to 6.425 GHz range. The retransmitted signal from the satellite to the receiving station is called downlink. Usually the downlink frequency is lower than the up-link frequency. It is in the range of 3.7 o 4.2 GHz. Generally uplink and downlink frequencies are referred to by 6 GHz/ 4 GHz.
Transponder:
Block Diagram:
The transmitter- receiver combination in the satellite is known as "Transponder'. A transponder consists of receiving antenna, low-noise amplifier (L.NA), mixer, local oscillator, power amplifier and transmitting antenna.
Signals received from the earth Station are amplified by a low-noise amplifier. A frequency converter is formed by a mixer and local oscillator (LO).
The uplink frequencies (6 GHz) are converted to downlink frequencies (4 GH2) by frequency Conversion. This is necessary because the transponder cannot transmit and receive the same frequency.
Otherwise the transmitter's strong signal would overload the receiver and block out the very small up-link signal, thereby stopping any communication. The interference can be avoided by using widely spaced transmit and receive frequencies.
After frequency conversion, the power level of the signal is increased by using power amplifier. Transmitting antenna transmits the downlink signal to the receiving station on earth.
The transponder has a wide bandwidth of 500 MHz. But it is divided into 12 separate channels, each 36 MHz wide
A typical communication satellite has 12, 24, or more transponders. Thus, transponder is the basic communication electronics unit on a satellite.
Applications of satellite communication:
In communication (TV and telephony etc).
Surveillance
Observation Meteorological)
Monitoring earth's resources
Navigation using Global Positioning System (GPS).
Use of satellites in communication (TV and telephony):
Most communication satellites operate in the microwave frequency range in C band. The 500 MHz band width is divided into 12 separate channels each 36 MHz wide.
For each channel a separate transponder is used. A satellite has 12.24, or more transponders. The up-link frequencies are the 5.925 to 6.425 GHz range and the down-link frequencies are in the 3,7 to 4.2 GHz range. One typical transponder can handle upto 1000 one-way analog telephone channels as well as one full colour TV channel.
A transponder can handle digital data at rates upto 60 Mbits/second. Frequency reuse and spatial isolation techniques can be used to increase the bandwidth and signal carrying capacity of the satellite.
Use of satellites in surveillance:
In military satellites, board film cameras take photographs which are received on the earth. Television cameras can take pictures and send them back to earth as electrical signals.
Infrared sensors detect heat sources, and small radars can profile earth features. Intelligence satellites collect information about enemies.
They allow monitoring for the purpose of proving implementation with nuclear test ban and missile treaties etc.
Uses of satellites for observations and monitoring earth's resources:
Meteorogical satellites are used for studying the weather conditions. The satellites take photographs of cloud cover and the pictures are sent back to earth.
These pictures are used for determining and predicting the weather. More accurate and more-detailed maps can be created by using observation satellites.
Satellites can monitor the status of the earth's resources like land and oceans. They can observe crops, forests, lakes and rivers. They can spot diseased crop areas, mineral resources and sources of pollution etc.
Global positioning system (GPS):
This system is a network of 24 low-earth-orbit satellites spaced equally around the world in overlapping patterns. Each satellite transmits a signal back to earth on low microwave frequencies.
Receivers on the earth pick up transmissions from four satellites simultaneously. In a microprocessor the exact position of the receiver on the earth is computed. The receiver output is a display giving the latitude, longitude and latitude of the receiver.
GPS can be used by military individuals and industry for general navigation purposes. We can know our position on earth at any time by using a low-cost hand-held microwave GPS receiver. The accuracy is about 100 meters from the actual location.
Digital Communication:
Digital communications is the transmission of binary or digital information from one point to another. Data communications systems permit the transfer of information between computers and also permit the remote operation of a computer from a terminal.
Digital signals are binary pulses that have two distinct states, each represented by a voltage level. The pulses switch rapidly between these two levels. One level is referred to as a binary 0 or low, and the other as a binary 1 or high.
The binary 0 level might be 0V or ground and the binary 1 level might be +5V.
Binary signals are easy to generate and process with electronic circuits. Usually, the binary signals represent data or information. Specifically, the binary signals are codes made up of groups or patterns of 0s and 1s. Each pattern represents a numerical value, a letter of the alphabet, or some special symbol.
Codes used in digital communication-
Telegraph continuous wave: The first type of electronic communication in use was the telegraph. When telegraph key is depressed, current flows through coil that attract armature and makes click. When key is opened the armature makes another click
Baudot code: It is used in teletype machines. A teletype machine is a teletypewriter like device that is used to send and receive coded signals over a communication link. It is a 5-bit code which represents 32 different symbols.
Morse code: It was used in telegraph. It is a series of dots and dashes that represents letters of alphabet, number and punctuation marks.
American Standard Code for Information Interchange (ASCII): This is 7-bit binary code for representing 128 numbers, letters, punctuation marks and other symbolist has sufficient number of code combination to represent both upper and lower case letters of alphabets.
Extended Binary Coded Decimal Interchange Code (EBCDIC): It is 8-bit code developed by IBM. It represents 256 characters.
Audio and video signals are analog signals. They are converted into digital signals by means of A to D converter. These messages are then composed using any of above codes.
Fiber Optic Communication:
An optical fiber is a flexible, transparent fiber made of glass (silica) or plastic, slightly thicker than a human hair. The field of engineering concerned with the design and application of optical fibers is known as fiber optics.
Fiber optic communication is a light wave communication. A light signal is passed through a fiber optic cable known as light pipe or light guide. The light signal propagates through fiber cable by multiple reflections in zigzag path.
Some part of the signal is lost due to internal absorption. Light is an electromagnetic wave like radio wave. The light pipe carries many signals like many telephone channels through the cable without any interference.
Construction of FOC:
Optical fibers typically include a transparent core surrounded by a transparent cladding material with a lower index of refraction.
Light is kept in the core by total internal reflection. This causes the fiber to act as a waveguide.
Block Diagram of Fiber Optic Communication System:
Working:
Transmitter:
The main function of this section is to transmit the information signal like voice, picture or computer data in the form of light flash.
The information is converted into digital signal by using an encoder circuit. It converts analog signal into pulses. It uses analog to digital converter or ADC circuit.
If the input is computer data is directly connected to a light source transmitter.
The light source transmitter is a powerful light source. It is light emitting diode (LED) or injection laser diode (ILD) to generate a low intensity light beam.
(LASER- Light Amplification by Stimulated Emission of Radiation)
Fiber optic cable:
Light beam pulses are then fed into a fiber optic cable.
Fiber optic cable is used to carry these optical signals over long distances. It is made of glass or plastic.
Fiber optic cable provides wide band width. So information carrying capacity is large.
It has very less attenuation than electric cables. Also it is smaller, lighter and stronger than electric cable.
Receiver section:
The light signal is detected by using a photo sensor like photocell or photo diode. It converts light pulses into an electrical signal.
Since the signal received through the fiber optic cable is slightly distorted and weak in amplitude; it is amplified by an amplifier and reshaped by using a shaper circuit.
In this way this section obtains digital pulses. Finally, it is converted in to its original form by using a decoder circuit like digital to analog converter (DAC).
Benefits /Advantages of fiber optic communication:
Widest bandwidth: Fiber optics cables can carry much information at a time.
Small in size: Many cables can be crowded to carry more information in a small space.
High speed: It is fastest communication with maximum bandwidth.
Very simple: The transmitters and receives are very simple and provide reliable communications.
Low loss of signal: Signal can be transmitted over long distance with negligible attenuation.
Light weight: These are lightweight cables than conventional copper cables.
Greater safety: They do not carry electric current so, provide shockproof communications.
Noiseless communication: They are free from electrical interference resulting in noiseless communications.
Secured communication: Fiber optic cable cannot be tapped like cable TV signals and also they do not radiate signals.
More strength: As compared to copper cable, fiber optic cables are very strong,.
Do not show corrosion due to water or chemicals.
Applications of fiber optic communication:
National telephones systems.
Inter connection from TV studio to main transmitter.
Closed circuit TV system.
Secured communication systems at military bases.
Aircraft communications and aircraft controls.
In Computer network like WAN, LAN.
Ship-board communications
In railway communications and controls.
In private organization for internal communications systems.
Nuclear plant interconnections.
Cell / Mobile Phone
Basic concept of cell phone:
A mobile phone (also known as a wireless phone, cell phone or cellular telephone) is a short range, electronic device used for and mobile voice or data communication over a network of specialized base stations known as cell sites.
The main concept behind the cellular radio system involves dividing a large geographical service area / city of any size into many small areas called “Cells".
Each cell has the equipment to switch, transmit and receive calls.
The cell covers only limited area with its own low power transmitter and receiver. The cell is designed to provide better service for vehicle in its small area.
The main advantage of dividing in geographical area into cells is avoiding the need of high power transmitter.
The cell structure, shape and division of area into number of cells depend on the surrounding area, the total geographical shape and physical structure like hills, tall buildings etc.
Practically they are not hexagonal in shape they are irregularly shaped.
In city area they may be put on top of buildings, from where the best radio coverage can be obtained.
The cell site are interconnected and controlled by this central ‘mobile telecommunication switching office’ (MTSO).
It contains basic telephone switching systems as well as some additional digital equipment programmed for cellular control.
Principle of cellular phone:
In cellular system, instead of transmitting a dial tone signals, a visual display is normally used to indicate that the phone is ready and the required number can be dialed.
User can dial the required number. Once the cell site receives the incoming dial tones it interprets it and sends a dialed number to the MTSO. This will make the local exchange to connect to proper MTSO.
The MTSO will order each cell under its control to transmit the necessary code number for cellular unit.
When required cellular telephone recognizes the unique code transmitted by the cell station, it will automatically seize setup channel of nearest cell and acknowledge that it is ready.
The cell control circuit will next select available voice channel and order the cellular unit to switch over.
Once the voice channel link is established, the cell will ring the telephone to inform that there is incoming call.
The cellular unit user can take telephone off hook for normal conversation.
Block diagram of Cell phone:
Working:
1. Antenna: Antenna used for cell phone is duplexer which is used for transmission and reception.
2. Transmitter: Transmitter is operated within range 825 to 845 MHz with low power output. This band of 20 MHz has 666 channels separated by 30 kHz. Analog signal uses FM modulator and digital signal uses phase modulator. From frequency synthesizer RF carrier is used to obtain FM or PM signal. Output of transmitter is 3 W which depends upon size of cell. Power control circuit is used to keep sensing of power level of signal as per input from MTSO.
3. Receiver: RF signal from space is intercepted by antenna. This signal is amplified by RF amplifier. Receiver has two frequency convertors. First convertor converts received signal to 82.2 MHz and second convertor converts it to 10.7 MHz which is IF of FM. The FM detector removes RF signal and the output is fed to speaker where we can listen to caller. Frequency for mixer is derived by frequency synthesizer. Demodulated signal is monitored by MTSO for cell switching.
4. Frequency synthesizer: It is crystal oscillator which produces respective carrier signal for transmitter and receiver. MTSO decides which frequency signal is to be generated for transmission and reception.
5. Logic unit: It intercepts serial data from cell site and MTSO. It consists of ROM, RAM, PROM and few digital circuits for controlling transmitter and receiver for detecting incoming call for identifying number.
6. Control unit: It consists of a handset in which earpiece and mouth piece are fixed. It also consists of tone dialing circuit which selects transmitter while making call and receiver while receiving call. It also has LCD display and indicator.
Advantages of cell phone:
It keeps you in constant contact with people you consider important.
It can help you seek help immediately during emergency cases.
It can take photos.
Mobile phone also gives us easier access on the internet.
You can carry it anywhere.
It has a lot of useful function like calendar, making notes, alarm clock, timer, phone book and calculator.
You can listen to music, text, and play games when you're bored.
keep in touch with friends and family
Disadvantages of cell phone:
Mobile phones can be expensive.
They can damage your ear.
Sometimes the reception is poor in some areas, limiting your connectivity.
People use the phone while they are driving, and this can cause problems.
They can limit your face to face time with friends and family
They can get you in trouble at school/college.
People spend lots and lots of money buying the latest model
Affects our body because of radiation it produces
Easily broken
Advantages of cell phone over landline:
Using cell phones means that no new telephone cables and wires need to be laid.
Telephone poles aren't required.
The primary advantage of cell phones over landlines is 'mobility'.
One can communicate from anywhere at any time with cell phone,
They are light in weight, portable, connectible to internet.
A mobile handset is equivalent to a watch, calculator, organizer, notepad, camera, MP3 Player and other media devices like alarm clock, telephone book.
Operating systems of cell phones: Android, Bada, Symbian, iPhone, Blackberry
Questions
Marks wise Questions: 1M(5), 3M(2), 4M(2), 6M(1)
1. Fill in the blanks-
Undesirable interference in communication is --------- when is added in communication channel.
(i) Noise (ii) Distortion (iii) Reactance (iv) None
The spectrum space occupied by a signal is called ----------.
(i)Electromagnetic spectrum (ii) Bandwidth (iii) Wavelength (iv) frequency
In FM the frequency of --------- is changed in accordance with modulating signal.
(i) Carrier signal (ii) Modulating wave (iii) Audio signal (iv) Video signal [O-09, M17]
Bandwidth of AM receiver is -----------.
(i)5kHz (ii) 10kHz (iii) 15kHz (iv) 40kHz
For sound transmission in TV ---------- is used.
(i)Frequency modulation (ii) Amplitude modulation
(iii)Phase modulation (iv) Pulse modulation
The range of FM band in radio is ------------.
(i) 88kHz to l08kHz (ii) 20Hz to 20kHz
(iii) 550kHz to 1630kHz (iv) 88MHz to 108MHz
Maximum BW of FM is ------------
(i)10kHz (ii) 100kHZ (iii) l500kHz (iv) 200kHz
The amount of frequency shift in FM is directly proportional to the --------- of modulation frequency.
(i) Amplitude (ii) Frequency (iii) Phase (iv) None
In AM broadcasting, the maximum modulating signal frequency is -----------kHz. [M-09]
(i) 10 (ii) 20 (ii)5
In AM ------------ of carrier is changed in accordance with modulating signal. [M-10]
T. V. transmission is an example of __________ communication. [M-11]
In FM, the frequency of carrier is changed in accordance with --------- of modulating signal.
(i) Amplitude (ii) harmonic (iii) Phase [O-11]
In FM Modulator -------------- diode is used. [M-12]
Fiber optic cable carries---------.
(i) Electric signal (ii) Light signal (iii) Sound signal [M-15]
In AM useful power is carried by --------.
(i) Carrier signal (ii) Modulating signal (iii) Side bands
Geosymchronous satellite has angular frequency of -------------.
(i) 1 cycle/month (ii) 1 cycle/day (iii) 24 cycles/day
-------- is used in FM to avoid interference with neighboring channel
(i) Guard band (ii) Side band (iii) Carrier
Most widely used data communication code is -----------.
(i) Baudat` (ii) ASCII (iii) EBCDIC
As height of satellite orbit goes lower, speed of satellite ----------.
(i) Increases (ii) Deceases (iii) Remains same
ASCII code used in digital communication system is -------bit code
(i) 16 (ii) 8 (iii) 7
--------- communication in digital communication is fastest.
i) Serial (ii) Parallel (ii) Both
Cell phone operates in the range ---------.
(i) 1600 to 2400 KHz (ii) 2 to 4GHz (iii) 825 to 845 MHz
In mobile handset ------------ card is important. [O-09]
(i) Play (ii) SIM (iii) Punch
The area to be converted by a cellular telephone system is divided into-----------. [M-11]
In mobile handset, ----------- is an input device. [M-16]
(i) Mouse (ii) Keypad (iii) Both
In AM Demodulation, point contact/germanium/OA79 is used in ------------circuit.
(i) Detector
2. Match the following-
A B
1. Analog signal 1. 10 KHz
2. Digital signal 2. 200 KHz
3. AM BW 3. Light/LASER/LED/broadband communication
4. FM BW 4. Diode detector
5. FM detector/Radio receiver 5. Telephone signals
6. AM detector 6. Telegraph signals
7. SW Band [M-09] 7. Electromagnetic radiation
8. MW Band [O-09] 8. 3MHz to 30MHz
9. Transmitting antenna [M-10] 9. 500kHz to 1600kHz
10. Fiber optic [M-10, 17] 10. Phase discriminator/slope detector [M-12]
11. FM 11. Audio transmission in TV/Infinite sidebands
[M-13, M-15]
3. True or false-
Length of antenna does not depend on frequency of radio waves.
In AM there are only two side bands.
In FM there are infinite numbers of side bands.
In AM, amplitude of carrier remains constant and frequency changes.
In FM, frequency of carrier remains constant and amplitude changes.
There are two sidebands in FM. [M-10]
The superimposing of modulating signal over carrier signal is called modulation. [M-11]
The modulation of carrier signal is done at transmitter. [M-12]
Injection laser diode is used in fiber optic communication.
In TV, frequency modulation is used for picture transmission. [M-14]
4. Answer the following-
Define amplitude modulation and frequency modulation. [O-11]
Explain Amplitude Modulation. [M-12]
Explain process of AM with necessary waveforms. [M-10,14, 16,17]
State expression of AM wave and explain USB and LSB frequencies. [O-09]
Explain importance of sidebands in A. M. [M-11, M-12, M-13]
Explain necessity of modulation OR need of modulation in communication. OR Why can audio signals not be transmitted over long distance? [M-09, O-09, 11,M-10, M-13,14,17]
Explain importance of carrier in modulation.
Explain working of FM detector with circuit diagram.
Compare AM and FM OR differentiate between AM and FM
[O-09, M-11, O-11, M-12, M-13, 14,15]
.Draw ckt diagram of FM discriminator.
Define modulation index and give its importance [O-11].
What is demodulation? Draw circuit of (AM demodulator) diode detector in AM and explain it. [M-09, O-09, M-10, M-11, O-11, M16,17]
What is importance of sidebands in AM?
Explain amplitude modulation with suitable waveforms.
Explain importance of modulation index in AM. Draw suitable waveforms.
Draw frequency spectrum of AM wave.
Explain basic principle FM. Draw waveform. [M-17]
Explain working of F. M. modulator using varactor diode. [M-11, 16, 17]
Define for FM: frequency deviation, carrier swing, and modulation index and percentage modulation. [M-14,16]
What are merits and demerits of FM over AM? [M-10,16]
With circuit diagram explain working of AM modulator. [M-09, 11, M-12, M-13,16,17]
Draw waveforms of AM wave for different values of modulation index. [O-09]
State and explain types of pulse modulation. [M-10]
State advantages/benefits of fiber optic communication. [O-09, M-12, M-13,16]
Draw block diagram of fiber optic communication system and explain working of each block. [M-09, M-10, M-11, O-11, M-12, M-13,14,15, 16 ]
Write applications of fiber optic communications. [M-09, M-11, 14]
Explain working of fiber optic transmitter and receiver with the help of suitable diagram. [O-09]
Explain half duplex and full duplex in communication system. [M-13]
State advantages of cellular phone.
Give any two disadvantages of cellular phone.
State advantages of cell phone over landline.
Write a note on geosynchronous satellite. State its merits.
Explain the working of satellite transponder with block diagram.
Explain use of satellite in communication.
Explain serial and parallel types of data communication.
Explain principle of cell phone.
With block diagram explain cell phone. [M-16]
Distinguish between serial and parallel transmission.
List the applications of satellite.
State and explain different codes used in digital communication.
