RECEIVERS

 

A radio receiver has two main functions to perform: firstly, it has to select the required radiation from all the other electromagnetic waves that are present over the surface of the Earth, and secondly, it has to separate (or demodulate) the audio-frequency current from the radio-frequency carrier. This process is also called detection.

The selection is effected by making use of a resonant circuit. If a resonant circuit in the receiver is tuned so that it will oscillate at the same frequency as the carrier-wave that is to be received, it is clear that of all the electromagnetic waves picked up by the receiving aerial, the only one that will be selectively strengthened will be the one with a frequency exactly fitting the resonance of the tuned circuit. This aspect of resonance is familiar in connection with sound waves; if a note is played on a piano in a room containing a glass turn with a natural resonant frequency the same as that of the note, the glass will vibrate in resonance with the note, and may even shake.

The receiving aerial, then, collects the modulated carrier-wave and feeds it into a circuit tuned to resonate at the frequency of carrier-wave. This tuning can be effected by altering the capacity of the capacitor of the resonant circuit.

The second function of a receiver is to demodulate the carrier-wave; again, this operation can be performed by using an electro-valve. Valves are not only used for amplification, they also serve another purpose — rectification, that is to say, they will only permit current to flow in one direction. Thus, if a modulated carrier-wave fed into the grid of a triode valve, and the grid is correctly bias; only current flowing in one direction will pass through the valve, bottom half, or negative swing, of the wave will be cut off.

The shape traced by the peaks of the emergent half of the carrier-waves is the same as the original audio-frequency current, so that these half-waves are fed into earphones or at loudspeaker, the original sound wave will be reproduced. The half-waves of the carrier themselves will not, of course, affect the earphones or loudspeaker as their frequency is far too high. Demodulation has therefore b achieved by the use of an electronic valve as a rectifier.

Rectification can also be brought about by the use of certain crystals, which have the property of allowing current to pass in direction only. These crystals were used in the early days of radio crystal sets; transistors and the semiconductor diode are a more modern application of the same principle.

TELEVISION. Television works on a similar principle to radio except that signals have to be transmitted one for the sound and one for the vision. In the receiver these signals, which are close to each other in frequency have to be amplified and separated.

The separator contains two resonant circuits, one tuned to the frequency of the sound carrier-wave. The sound signal is then done with in the same way as a sound receiver. The picture is formed by making a beam of electrons move across the screen of a cathode tube, variation in the intensity of the beam causing areas transmission — these are the number of lines that can be seen making up the picture. With interlaced scanning the whole screen is covered by half the number of lines in 1/50th of a second — this is called a frame. The remaining lines fill the spaces between the first frame during a subsequent period of l/50th of a second, thus the whole process takes l/25th of a second. During this time the spot of light has covered the whole screen twice, but owing to the persistence of the fluorescent coating of the tube, the whole screen is illuminated. To obtain a proper picture, the movement of the electron beam in the receiver must be exactly synchronized with the electron beam in the television camera. This is achieved by transmitting synchronizing pulses with the vision signal.

The cathode ray tube (CRT) consists of an evacuated glass tube containing an electron gun (a heated cathode surrounded by a metal cylinder) producing a beam of electrons that fall on the fluorescent screen. Magnetic deflection coils deflect the beam from side to side producing the lines, and another set of deflection coils give the up and down movement required by the frames.

The television camera consists of an optical lens system similar to the lens used in a photographic camera, the image from which is projected into a camera tube. This consists of a photosensitive mosaic scanned by an electron beam. All are housed in an evacuated glass tube. The output signals of the camera tube are usually preamplified within the body of the camera.

Colour TV works on the same principle, the camera having three separate tubes, each sensitive to one of the three colours red, green, or blue. In the receiver the cathode ray tube has three electron guns, each one is controlled by the appropriate tube in the camera. The screen of the CRT is coated with a mosaic of different colour spots that fluoresce when electrons from a particular gun strike them, all the colours are made up from blends of the three colours.

2. Переведите на русский язык следующие английские словосочетания:

1. audio-frequency current; 6. the frequency of the sound carrier-wave;

2. the radio-frequency carrier; 7. sound receiver;

3. receiving aerial; 8. cathode ray tube;

4. a resonant circuit; 9. evacuated glass tube;

5. a triode valve; 10. photosensitive mosaic

 

3. Найдите в тексте английские эквиваленты следующих словосочетаний:

1 детектирование; 6. ток звуковой частоты;

2. электронно–лучевая трубка; 7. несущая высокой частоты;

3. вакуумная стеклянная трубка; 8. принимающая антенна;

4. светочувствительная мозаика; 9. резонансный контур;

5. звукоприемник; 10. частота звукового носителя волны

4. Найдите в тексте слова, имеющие общий корень с данными словами. Определите, к какой части речи они относятся, и переведите их на русский язык:

1. modulate 6. amplify

2. receive 7. deflect

3. carry 8. conductor

4. rectify 9. move

5. magnet 10. sensitive

 

5. Задайте к выделенному в тексте предложению все типы вопросов (общий, альтернативный, разделительный, специальный: а) к подлежащему, б) к второстепенному члену предложения).

 

6. Выполните анализ данных предложений, обратив внимание на следующие грамматические явления: формы и функции причастия, независимый причастный оборот, формы и функции герундия, герундиальный оборот, конверсия:

1. This consists of a photosensitive mosaic scanned by an electron beam.

2. This is achieved by transmitting synchronizing pulses with the vision signal.

3. A radio receiver has two main functions to perform. A radio receiver functions to perform certain actions.

4. Colour TV works on the same principle, the camera having three separate tubes, each sensitive to one of the three colours red, green, or blue.

5. For obtaining a proper picture, the movement of the electron beam in the receiver must be exactly synchronized with the electron beam in the television camera.

 

7. Ответьте на вопросы по тексту:

1. What are the two main functions of a radio receiver?

2. What are valves used for?

3. How does the receiving aerial work?

4. What principle does television work?

5. What is the picture formed by?

6. What does the cathode ray tube consist of?

7. What does the television camera consist of?