This field is one of the largest noncommercial radio services. Amateur radio operators, or "hams', usually build and operate their own transmitters and receivers to call one another in one of the assigned bands. A popular band is 7 to 7.3 MHz. Their main organization is the American Radio Relay League (ARRL), Newington, Connecticut.
Read More......2/23/2009
Television Broadcasting
Television is just another application of wireless radio communications, but with picture information in addition to the sound signal. Two separate carrier waves are transmitted by the station in its assigned channel. One carrier is an AM picture signal, modulated by video signal with the picture information. The other carrier is an FM sound signal modulated by the audio.
A television channel is 6 MHz wide to include both the picture and sound signals for each broadcast station. Channel 2, for instance is 54 to 60 MHz.
FM Radio Broadcast Band
This band is 88 to 108 MHz, with the stations assigned every 200 kHz or 0.2 MHz. The system of frequency modulation reduces static and interference. For this reason, the FM band is used for broadcasting high-fidelity audio signals.
Read More......Standard AM Radio Broadcast Band
This service is the original system of broadcasting for what we generally call radio. Amplitude modulation in transmission of the assigned RF carrier wave. Stations are assigned every 10 kHz in the band of 540 to 1600 kHz. The last digit is usually omitted from this numbers on the tuning dial for the carrier frequencies of different stations.
Read More......Radio Broadcast Service
Radio is an abbreviated form of radiotelegraph and radiotelephone. The word radio means radiation for wireless transmission. At first, communication was by radiotelegraph, using short dots and longer dashes in the Morse code. Now radiotelephone is used more for voice and music programs for entertainment. This application can be considered as communications electronics, which includes television. Practically all radio and television receivers are now solid-state, with transistors and integrated circuits instead of vacuum-tube amplifiers. See figure for a comparison of the old and the new in radio.
The transmission distance for wireless communication can be less than a mile or as much as 5000 miles, depending on the type of service, there are many different uses, such as radio broadcasting of voice and music, television broadcasting, amateur radio, and citizen's band (CB) radio. In addition, radio communication is used for specific services, such as police radio. Finally, radio navigation for ships and planes is another important application.
All radio services in the United States are regulated by the Federal Communication Commission (FCC). The FCC assigns the RF carrier frequencies for transmission and monitors use of the airwaves.
Wireless Broadcasting
Broadcasting means to send out in all directions. A radio broadcasting system is illustrated in Figure. The transmitter sends out electromagnetic radio waves radiated from its antenna. Receivers can pick up transmitted radio signal by means of receiving antenna or aerial. The receiver reproduces the desired signal transmitted by the broadcast station. There are many radio signals in space from different transmitters, but the receiver can be tuned to the frequency of the station we want.
In Figure, the electromagnetic wave shown is a radio-frequency (RF) carrier signal with amplitude modulation (AM). The Amplitude or strength of the RF carrier varies in step with variations in desired voice of music information, which is the audio signal. This technique of modulating a carrier wave is necessary because the audio signal itself cannot be used for wireless transmission. The variations are too slow for effective radiation from an antenna. A higher-frequency carrier wave is chosen for the best radio transmission. Its modulation provides the desired signal information.
In the method of frequency modulation (FM), the modulating signal varies the frequency of the RF carrier wave. Either AM or FM can be used for any type of modulating signal.
Frequency is an important characteristic of any varying voltage or current, to specify how fast the amplitudes change. A complete set of changes in one cycle. The number of cycles repeated in a second is the frequency. The unit for frequency is the hertz (Hz), equal to one cycle per second (cps). As an example, the 60 cycle ac power line has a frequency of 60 Hz.
Radio frequencies are generally considered to be about 30,000 Hz and above. Radio-frequency carrier frequencies for wireless transmission are usually specified in kilohertz (kHz), equal to 1000 Hz, and megahertz (MHz), where 1 MHz = 1,000,000 Hz.
Development of Electronics - II
The rapid advances after that are due largely to the introduction and progress of the vacuum tube as an amplifier for electric signals. Dr. Lee DeForest, with his audion tube, invented in 1906, was a leader in this field. As the design of vacuum tubes advanced, radio broadcasting progressed rapidly. Regularly scheduled programs were broadcast in 1920 by station KDKA in the standard amplitude modulation (AM) radio band. The commercial frequency modulation (FM) radio broadcast service was started in 1939. Stereo broadcasting in this band began in 1961.
Commercial television broadcasting was started officially in 1941, but its popular use did not begin until 1945. Our present color television system was adopted in 1953.
Since the invention of transistors in 1948 at Bell Telephone Laboratories, solid-state devices have replaced tubes for most uses in electronics, radio, and television. The transistor is an application of controlled electron flow in solid semiconductor materials such as silicon (Si) and Germanium (Ge). Transistors and tubes have similar uses for the control of electron flow and amplification of signals. The transistor is much smaller, however, and more efficient, as it does not have the heater used in tubes.
Solid-state electronic devices include transistors, diodes, and integrated circuits. A diode is not amplifier but is used as a one-way conductor to convert alternating current to direct current. Solid-state devices have made new application practicable because of their small size and the economy of ICs packages. One example is the rapid growth of digital electronics for electronic calculators, personal computers, and many other uses.
2/04/2009
Development of Electronics - I
History shows that electronics started in the pioneer days of radio communications. Television developed from radio. Furthermore, radio itself is based on earlier experiments in electricity and magnetism. The start of wireless transmission for radio communications can be taken from the work of Heinrich Hertz, a German physicist. In 1887, he was the first demonstrate the effect of electromagnetic radiation through space. The distance of transmission was only a few feet. However, the experiment demonstrated that radio waves could travel from one place to another without the need for any connecting wires between the transmitting and receiving equipment.
Hertz proved that radio waves, although invisible, travel with the same velocity as light waves. In fact, radio waves and light waves are two examples of electromagnetic radiation. This form of energy combines the effects of electricity ad magnetism. An electromagnetic wave transmits electric energy through space.
The work of Hertz followed earlier experiments on electricity and magnetism. In 1820,a Danish physicist, H. C. Oersted, showed that an electric current produces magnetic effects. Then, in 1831, a British physicist, Michael Faraday, discovered that a magnet in motion can generate electricity. The motion provides the requirement of a change in the magnetic field. In 1864, the British physicist James Clark Maxwell, on the basis of earlier work in electricity and magnetism, predicted the electromagnetic waves demonstrated later by Hertz.
The importance of this work can be judge by the fact that basic units are named after this scientists. The Maxwell (Mx) and the Oersted (Oe) are units of magnetism. The Hertz (Hz) unit is equal to one cycle per second, which is the measure for the frequency of any alternating voltage or current. The Farad (F) is the unit of capacitance, indicating how much electric charge can be stored in a capacitor.
In 1895, Guglielmo Marconi used a long wire as an antenna to develop a practical radio system for long distance. The antenna is needed for efficient radiation. He succeeded in producing wireless communication across Atlantic Ocean in 1901.
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