Английский язык: Методические указания и задания к контрольным работам для студентов специальности «Техническая эксплуатация транспортного оборудования», страница 20

Задание 4. Переведите предложения.

1. What is known as modulation? 2. Can voice or music be transmitted without modulation? 3. What does a simple radiotelephone transmitter consist of? 4. What is termed the carrier frequency current? 5. By what parameters is the carrier frequency current characterized? 6. What types of modulation do you know? 7. What type of modulation is most widely used in radio telephone service, i.e. in the transmission of audio signals?

Задание 5. Переведите предложения.

1. Несущая частота без модуляции – это режим молчания (silence mode). 2. Существует амплитудная модуляция (АМ), частотная модуляция (ЧМ) и фазовая модуляция (ФМ). 3. При (in the case of) амплитудной модуляции амплитуда высокочастотного тока меняется в соответствии с управляющим сигналом. 4. Частотная модуляция (ЧМ) – это изменение частоты высокочастотного колебания в соответствии с колебаниями передаваемого (модулирующего) сигнала более низкой частоты. 5. Режим (mode), имеющий максимальную мощность, называется максимальным или телеграфным режимом. 6. Средняя (average) мощность за период низкой частоты модуляции не равна (is not equal to) мощности несущей частоты и называется телефонной мощностью. 7. Радиотелефонный передатчик должен обеспечивать ( provide) как кратковременную (short-term) максимальную мощность, так и телефонную в течение длительного времени.

Вариант 3

Задание 1. Прочитайте и переведите текст письменно.

The Receiver

     The simplest receiver consists of a receiving antenna, a tuned circuit, a detector, a bettery (if needed), and a pair of headphones. The receiving antenna intercepts the electromagnetic waves which produce a radio-frequency electrical signal in the antenna circuit.

     The tuned circuit enables the listener to select the desired signal from all the radio signals which are picked up by the antenna. It usually consists of a fixed coil or inductor (L) and a variable capacitor (C). The circuit may be tuned to produce resonance by turning the rotor of the variable capacitor.

     Tuned circuits sometimes contain fixed capacitors and variable inductors. The inductance of a coil may be increased by inserting a powdered-iron slug.

     The detector demodulated the selected radio waves, rectifying it and separating the useful audio frequency (a-f) component from the radio-frequency (r-f) component. The radio frequency component of the signal, after performing its mission of bringing the audio frequency component to the antenna of the receiver, is then discarded. The headphones convert the audio frequency signal into sound waves of the same frequency and amplitude as those which entered the microphone at the transmitting station.

     This receiver may be improved by using a power supply, adding a radio-frequency amplifier stage before the detector to strengthen the incoming signal before it is demodulated, and by placing an audio-frequency amplifier stage following the detector to strengthen the a-f signal before it is converted into sound.

If these additional stages of amplification have strengthened the signal sufficiently,

the headphones may then be replaced by a loudspeaker.

Superheterodyne Receiver

     In order to convert the simple radio receiver into a superhet receiver it is necessary to add stages which will lower the carrier frequencies. This may be done by passing the carrier frequency through a mixer into which a “local oscillator” is also passing a constant frequency. This is the heterodyne principle. Hence the new block diagram for the superhet receiver is the same as for the simple radio receiver except for the mixer (which also has an oscillator  connected to it) and an amplifier all placed between the radio frequency amplifier and the detector.

     It is not always necessary to make each stage of a set completely separate, with a different valve for each stage. Usually one valve can do two jobs, which means that the set can be made more cheaply and also can be made smaller. In most superhets the local oscillator valve and the mixer valve are contained in the same envelope. The compositive valve is then known as a “frequency changer.”