Text 6. Radio Receivers (I)

Transmission of intelligence by radio is based on modulation, this being a process by which the message to be transmitted is superim­posed at the sending end of a radio link as a modulating signal on a strong carrier wave, thereby changing the latter’s amplitude, frequen­cy or phase. The modulated carrier is radiated by a transmitting aerial as a wave of electromagnetic energy which propagates through space at the velocity of light. At the point of reception the modulated wave is picked up by a receiving aerial and is fed to the receiver input. In the receiver the signal is separated from the radio-frequency carrier and drives the receiver load, which may be a speaker, a recorder, a cathode-ray tube, etc. As an electromagnetic wave travels away from the transmitter it is weakened or attenuated. This is why radio receiv­ers should be capable of picking up relatively weak signals.

Radio serves a variety of purposes such as communication, broad­casting, navigation, radar and telecontrol.

Radio communication is the transmission and reception of mes­sages without wires or waveguides. It includes communication by radio telegraph, radio-telephone, radio teletypewriter, radio facsimi­le and television. It is the only method of communication between stationary and mobile objects (e. g. from ship to shore, from ground to aircraft, from ground to spaceships, etc.).

Radio broadcasting is radio transmission for general reception, including speech, music and commercial television.

Radio navigation is the use of radio facilities for determining the position and direction of ships and planes.

Radar (which is an acronym for Radio Detection and Ranging) is a technique for determining the range and bearings of objects (targets) by transmitting beamed high-power signals against reflective tar­gets, the reception of the reflected signals and the presentation of the resultant data on a dial or a cathode ray display.

Telecontrol is a technique for control of machinery by radio.

There exist two classes of receivers: communication and broad­cast receivers, the former being used in point-to-point radio tele­phone and telegraph service while the latter are designed for the reception of sound and visual programmes.

Notes

receiver [ri'siiva]		(радіо) приймач
transmission [trænzJmiJYi]		передача (no радіо)
intelligence [inltelid33ns]		відомість
to superimpose [^juiparini'pouz]		— накладати
thereby ['ôea'bai]		— таким чином
to attenuate [a'tenjueit]	— розчиняти
facsimile [fsek'simili]	— факсиміле
mobile ['moubail]	— рухливий
target ['ta:git]	— ціль

Text 8. Oscillators

An oscillator is a device which produces a regularly-recurrent waveform of voltage or current and it comprises two basic ele­ments — an amplifier and a frequency determining circuit.

The most common oscillator produces an output waveform, the latter approaching the sinusoidal very closely and comprises a valve and a tuned circuit. An amplifier valve with a parallel tuned circuit in its anode lead is known to give an amplification A at a resonance frequency of the tuned circuit, at this frequency the anode voltage being in phase opposition to the grid voltage. If a fraction l/A of the anode voltage is taken, reversed in phase, and fed to the grid, the grid voltage is obtained from the anode voltage. The attenuation and phase reversal can be obtained by including a small coil in the grid circuit and coupling it to the anode coil.

With such a circuit, oscillation starts up at a small amplitude and builds up until the non-linearities of the valve limit its growth. These non-linearities are found to play an essential part, one of the more important of them coming into action through the use of an automat­ic grid-bias current. The amplitude of the oscillation increasing, so does the grid voltage and hence the grid bias. The increasing bias reduces the mutual conductance of the valve and so the amplifica­tion, and an equilibrium condition is at length reached at which a steady amplitude of oscillation is generated/

Notes

	oscillator ['osileits]	— осцилятор; вібратор
	to comprise [kam'praiz]	— охоплювати
	amplifitr ['æmplifaia]	— підсилювач
	anode [a'noud]	— анод
	A - amplification	— посилення
	resonance ['rezsnans]	— резонанс
	grid	— сітка
	1A — anode current	— анодний струм
	attenuation [a^enju'ei/n]	— розчинення
	non-linearity	— нелінійність
	... coming into action	— починає діяти
grid bias ['baias]			— зміщення сітки
an equilibrium condition is at length reached			— врешті решт досягається рівновага