3.6 RADIO
Introduction : In a basic sense, communication means transmission of information from one person to another. (Anything having some meaning or sense is called ‘information’. A written message, a spoken word, a picture are all examples of information). In a technical sense, communication refers to the sending, receiving and processing of information by ‘electrical means’.
The different types of communication systems are telegraph, telephone, radio, television, radar and so on, have been developed depending upon the information to be sent and received.
Know more about transistors
The transistor described in the above section is a
‘bipolar junction transistor’.
Field Effect Transistor(FET),
Metal Oxide Semiconductor Field Effect Transistor(MOSFET) and Junction Effect Transistor are some of the other types.
Radio communication : Radio communication is the process of sending information or intelligence (music or speech in the form of words) from one place and receiving it at another place, Radio waves are spherical and spread in all directions without using connecting wires.
There are two major parts in radio broadcasting system namely a transmitter and a receiver. Transmission of speech or music over long distances, using radio waves is called radio broadcasting. Radio signals sent from the transmitter travel through space and are received by the receiver.
Note
Guglielmo Marconi (1874-1937), an Italian engineer, invented and developed telegraph in 1890’s. In 1901, he transmitted a message across the Atlantic.
On Christmas Eve in 1906, Reginald Fessendeu transmitted both speech and music over several hundred miles.
Dr. S K Mitra(1890-1963), an Indian physicist had constructed a radio transmitted in 1923, about four years before radio broadcasting started in our country.
Radio transmitter:
The block diagram of a radio transmitter is shown in fig. The audio(sound) information is changed into corresponding electrical signal using a transducer. Transducer is a device that converts information energy to be transmitted into electrical signal. For sound transmission, the transducer is microphone (1). This signal called audio frequency (AF) signal, being weak, is amplified by a suitable amplifier called AF amplifier(2).
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| Radio transmitter |
As AF signals, have low energy, they cannot be transmitted over long distances. Therefore radio waves of suitable frequency are generated in an RF oscillator (3). These radio waves are mixed with the amplified AF signal in the mixer stage(4). In this stage, the AF signal is superposed on the RF wave. The RF wave is said to be modulated. The RF wave that carries the AF signal, is called carrier frequency. The modulated signal is amplified using an RF amplifier (5) and then sent to the antenna(6). The antenna radiates the RF signal into space, in the form of spherical radio waves.
Know this
Radio transmission in AM and FM.
You have read in dailies, about radio programmes given in AM and FM. What is the about? The mixing of audio and carrier frequencies tha is modulation can be done in two ways. If the amplitude of the carrier is varied in accordance with the AF signal, then the modulation is called amplitude modulation(AM). If the frequency of the carrier is varied in accordance with the AF signal, then the modulation is called frequency modulation(FM). AM radio stations have carrier frequencies from about 530 to 1600 kHz, FM radio stations have much higher carrier frequencies, between 88 and 108 MHz. Markings on radio dial corresponds to carrier frequencies.
Radio Receiver :
Block diagram of a simple radio receiver is shown in fig . \radio waves sent by all stations, are received by the antenna(1) of the receiver. A particular station is ‘tuned in’ and the RF signal amplified (2). The signal goes to the detector or demodulator(3). Here the AF signal is separated from the carrier. The audio signal is then amplified by AF amplifier (4)and sent to a loud speaker or head phones (5).
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| Radio receiver |
3.7 TELEVISION
Television is one of the marvels of electronics. The word television, the combination of two words ‘tele’ and ‘vision’ literally means seeing at a distance. Television broadcasting is the process of transmission of transient visual image of an actual or recorded scene, along with audio signal to a distance using radio waves.
A television transmitter works similar to that of a radio transmitter. At the TV station, a pick up instrument (TV camera) explores (scans) different portions of the picture or scene to be transmitted, in a particular order. The camera converts the variations in brightness in the various portions into electrical impulses (photoelectric device is used as transducer). These electrical impulses constitute the video signal (picture signal). This video signal is amplified and is used to amplitude modulate an RF carrier.
The sound picked up be a microphone is amplified and is used to frequency modulate and RF carrier (The frequency of this carrier is 5.5MHz higher that that of the video carrier). Carrier frequencies in TV for various channels lie in the range from about 40 MHz to 600 MHz.
The two modulated carriers(Video and Audio signals) are amplified and transmitted through the same transmitting antenna.
As the receiving end, RF signals are collected by the receiving antenna and the desired station is selected. The output corresponding to the video signal is used to control the intensity of light in a cathode ray tube; to obtain the picture transmitted. An F.M. receiver is used to separate the audio signal and reproduce sound.
Activity
Make a list of a some AM and FM radio stations to which you listen.
How are their wavelengths and frequencies related?
Do you know?
The first demonstration of television, was given during 1927 by J.L. Barid (UK) and C.F. Jenkins (USA). The actual TV came into existence in 1930, with the invention of cathode ray tube and TV camera.
POINTS TO REMEMBER
Electronics is a field of pure and applied science dealing with the design and application of electronic devices.
Substances whose conductivity lies in between that of conductors and insulators are called semiconductors.
Ex: Silicon and Germanium
When an electron is removed from a covalent bond, it leaves a vacancy, equivalent to positive charge. An electron from a neighbouring atom can drop into this vacancy, leaving the neighbour with a vacancy. Such a vacancy is usually called a hole and it is considered as an apparent charge carrier.
Doping is the process of adding minute traces of impurities to enhance the conductivity of semiconductors.
The traces of impurities added fordoping are called dopants.
Pentavalent impurity is a donor impurity. On addition of a donor impurity the pure semiconductor becomes n-type.
Trivalent impurity is a acceptor impurity. On addition of an acceptor impurity the pure semiconductor becomes p-type.
Diode is a p-n junction. It acts as a conductor when it is forward biased and acts as a resistor when it is reversed biased.
Transistor is a specially constructed three terminal semiconductor device.
In an npn transistor p-type semiconductor is sandwiched between two n-type semiconductors.
In a pnp transistor n-type semiconductor is sandwiched between two p-type semiconductors.
Three regions of a transistor are base, emitter and collector.
In a radio transmitter microphone acts as a transducer. RF oscillator produces carrier waves and transmitting antenna transmits the radio waves in the form of spherical waves.
The mixer in a radio transmitter modulates audio frequencies and radio frequencies. Audio frequencies and modulated frequencies are amplified by respective amplifiers.
A radio receiver (radio) receives the signals of a tuned radio station by receiver antenna. Audio signals and carrier waves are separated by demodulator. Audio frequencies are amplified and the loud speaker reproduces the sound.
Television broadcasting is the process of transmission of transient visual image of an actual or recorded scene along with audio signal to a distance using radio waves.
Camera acts as a transducer in the television transmission.
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