أجهزة الاستقبال - التلفزيون - الالكترونيات .. الفيزياء

Receivers - Television - Electronics .. Physics

Receivers

The radio receiver performs two essential functions: selecting the required carrier wave from among all the radiations present above the Earth's surface, and extracting the modified wave from the radio wave that carries it so that the audio frequency component is easily extracted from it.

The wave is selected using a resonant circuit placed in a receiver that works according to the principle of capacitive modulation. Then the circuit is tuned so that it oscillates at the required frequency of the carrier wave, which enhances its value without the rest of the waves - the rest of the radiations that the antenna receives are not taken into account among what it receives. The resonant circuit is tuned in most devices by changing the capacitance of the antenna capacitor that makes it up.

Then follows the process of extracting the modified wave from the carrier wave. This is done by using a triode for the second time, its network is fed by the modified carrier wave coming from the resonant circuit, so half of the wave passes through it while the negative half disappears, then the valve output is taken from the elevator circuit and used to feed the headphones, amplifiers or loudspeakers.

Television

In addition to using frequency modulation waves in the process of broadcasting radio signals, they are also used in broadcasting television images by feeding the transmitter from the camera output and then sending and broadcasting them outside.

The simplest type of cameras used in television is the type called videocon, and the length of the imaging channel is about 15 cm and its diameter is 2.5 cm. This camera contains an optical device that focuses the image at a focus on a transparent conductive plate coated on the back with a layer of antimony sulphate, which acts as a light transmitter whose resistance value decreases when light is shone on it. Then an electron beam scans this conductive layer and gives a signal with a specific voltage if it falls on a bright patch of the conductive plate.

The electron beam scans the entire plate using 625 horizontal lines that together form a single structural shape that is repeated 30 times every second.
In each structural shape, the single lines are scanned first, followed by the scanning of the double lines. Then, a short, negative voltage pulse is added to the end of each line, and to the end of the whole structure, one pulse longer than the first pulses. It is worth noting that these lines are not parallel and that there are very small angles between them that always move the beam downward throughout the electronic scanning process.

(Top image) The carrier wave is extracted by cutting the negative part of the modulated wave.

(Lower image) A simplified receiver.

The signal received by the television receiver is used to change the intensity of the electron beam used to scan the cathode ray tube screen, which consists of an electron gun that generates the electron beam, two focusing paths, and magnetic coils surrounding the neck of the tube whose job is to deflect the beam to facilitate the process of scanning the screen.

As for the inner face of the screen, it is coated with zinc sulphate, which glows white when it collides with the electrons. The same method is followed in the process of scanning the screen as used inside the camera panel, but the pulses applied at the end of each line and the end of each structural shape, their task is to keep the camera and the receiver ready to work, each in its designated stage.

In the camera called Videocon (above), the electron beam falls on a light-transmitting plate in which pulses are generated and sent to the cathode ray tube where the image is formed on its front panel (lower image).

(To the right) The electron beam falls on the front of the cathode ray tube in synchrony with the camera beam and the signals sent control the intensity of the beam.