A Radio Communication System

1. A typical radio communication system has two main components, a transmitter and a receiver. The transmitter generates electrical oscillations at a radio frequency called the carrier frequency. Either the amplitude or the frequency itself may be modulated to vary the carrier wave. An amplitude-modulated signal consists of the carrier frequency plus two sidebands resulting from the modulation. Frequency modulation produces more than one pair of sidebands for each modulation frequency. These produce the complex variations that emerge as speech or other sound in radio broadcasting, and in the alterations of light and darkness in television broadcasting.

2. Essential components of a radio transmitter include an oscillation generator for converting commercial electric power into oscillations of a predetermined radio frequency; amplifiers for increasing the intensity of these oscillations while retaining the desired frequency; and a transducer for converting the information to be transmitted into a varying electrical voltage proportional to each successive instantaneous intensity. For sound transmission a microphone is the transducer; for picture transmission the transducer is a photoelectric device.

3. Other important components of the radio transmitter are the modulator, which uses these proportionate voltages to control the variations in the oscillation intensity or the instantaneous frequency of the carrier, and the antenna, which radiates a similarly modulated carrier wave. Every antenna has some directional properties, that is, it radiates more energy in some directions than in others, but the antenna can be modified so that the radiation pattern varies from a comparatively narrow beam to a comparatively even distribution in all directions; the latter type of radiation is employed in broadcasting.

4. The particular method of designing and arranging the various components depends on the effects desired. The principal criteria of a radio in a commercial or military airplane, for example, are light weight and intelligibility; cost is a secondary consideration, and fidelity of reproduction is entirely unimportant. In a commercial broadcasting station, on the other hand, size and weight are of comparatively little importance; cost is off some importance; and fidelity is off the utmost importance, particularly for FM stations; rigid control of frequency is an absolute necessity. In the U.S., for example, a typical commercial station broadcasting on 1000 kHz is assigned a bandwidth of 10 kHz by the Federal Communications Commission, but this width may be used only for modulation; the carrier frequency itself must be kept precisely at 1000 kHz, for a deviation of one hundredth of 1 per cent would cause serious interference with even distant stations on the same frequency.

5. In a typical commercial broadcasting station the carrier frequency is generated by carefully controlled quartz-crystal oscillator. The fundamental method of controlling frequencies in most radio work is by means of tank circuits, or tuned circuits, that have specific values of inductance and capacitance, and that therefore favor the production of alternating currents of a particular frequency and discourage the flow of currents of the frequencies. In cases where the frequency must be extremely stable, however, a quartz crystal with a definite natural frequency of electrical oscillation is used to stabilize the oscillations. The oscillations are actually generated at low power by an electron tube and are amplified in a series of power amplifiers that act as buffers to prevent interaction of the oscillator with the other components of the transmitter, because such interaction would alter the frequency. The crystal is shaped accurately to the dimension required to give the desired frequency, which may then be modified slightly by adding a condenser to the circuit to give the exact frequency desired. In a well designed circuit, such an oscillator does not vary by more than one-hundredth of 1 per cent in frequency.

III. After-text Exercises

1. Give equivalents of the following words and expressions:

Основні компоненти; електричні коливання; підсилювач; інтенсивність; передача звуку; передача зображення; миттєвий; точність відтворення; суворий (жорсткий) контроль; взаємодія.

2. Say whether the following statements are true or false:

a) A typical radio communication system has three main components.

b) Essential components of a radio transmitter include an oscillation generator, amplifiers and a transducer.

c) The principal criteria of a radio in a commercial or military airplane, for example, are large weight and intelligibility.

d) The carrier frequency is generated by a carefully controlled quartz-crystal oscillator.

e) The oscillations are actually generated at high power by an electron tube.

3. Answer the following questions on paragraph 5:

a) What is the carrier frequency generated by in a typical commercial broadcasting station?

b) What is the fundamental method of controlling frequencies realized by?

c) What is used to stabilize the oscillations?

d) Are the oscillations actually generated at low power or at high power?

e) What dimensions are the crystal shaped to?