Radiotechnics

Radiotechnics . Set of techniques aimed at obtaining various applications of radio waves, including the emission and reception of sounds by radio or hercian waves. and that it is based on complex electronic circuits within which we find; radio communication, broadcasting, television, thermo ion amplifier

Summary

[ hide ]

  • 1 Radio communication
  • 2 Broadcasting
  • 3 Television
  • 4 Thermo ionic amplifier
  • 5 Source

Radiocomunication

Radio communication is the technology that enables signal transmission by modulating electromagnetic waves. These waves do not require a physical means of transport, so they can spread through both air and empty space.

In 1895 , AS Popov informed the Russian Physical-Chemical Society of his discovery of a storm recorder, a device that allowed the recording of electromagnetic waves produced during atmospheric electric shocks. This device was actually the first radio receiver. A year later Popov, in an assembly of the aforementioned society, demonstrated a radio communication session. From one building, a radiogram saying: Heinrich Hertz was transmitted to another, 250 m away. By this time he was also dealing with the problems of radio communication G. Marconi, who did much to introduce the use of radio. Now when radio and television communication is done between Earthand the spaceships near Venus or Mars, the news about a transmission at 250 m or even several kilometers away seems insignificant to us. But it should not be forgotten that these were the first steps of humanity, a new, unexplored branch of science and technology.

At the dawn of radio engineering, high-frequency spark discharge was used to obtain high-frequency oscillations. This procedure had a number of drawbacks. In the first place, all the energy was consumed practically in heat, and not in radiation, so that the efficiency of the generator was very small and the emission range was only a few tens of kilometers. On the other hand, spark discharge did not achieve sine waves, but a series of damped impulses. With them it was only possible to carry out radiotelegraph communication and not the transmission of sound , since the damped impulses cannot be modulated.

Starting in the 1950s, radio lamps began to be replaced in many circuits by semiconductor devices (transistors, which have a number of important advantages. But the replacement of the lamp by the transistor does not change the essence of the phenomenon,

Broadcasting

The base of a transmitter of Radio modern generator is not damped oscillations of lamps or transistors. The generator produces the high-frequency oscillations (carrier frequency ú)

Sound oscillations (penetrate the microphone and transform into electrical oscillations. The process of transforming undamped sinusoidal oscillations into modulated oscillations takes place in the modulator. Once amplified, the modulated oscillations reach the antenna , which serves to radiate the electromagnetic waves . These waves are received by the receiver antenna and produce oscillations in the resonant circuit CR The weak high frequency oscillations enters an amplifier and then the detector . From the detected oscillations the low frequency component is separated ( acoustic ), which after amplification is transmitted to the dynamic speaker.

The receiver’s resonant circuit consists of a coiland a capacitor of variable capacity, which gives the possibility of tuning it to the frequency of the wave emitted by one or the other station. If sine waves were received, it would be convenient to have a resonant circuit with a very large quality factor, which would allow it to increase its selectivity, that is, its ability to distinguish the signals of two transmitters of neighboring carrier frequencies. But the resonant circuit must receive modulated oscillations, which do not have a frequency, but a band of frequencies that fill a certain range of the spectrum. So that the signal is not distorted, it is necessary to reproduce without distortion the entire frequency band, which is achieved with a very flat resonance curve. This is only possible in the case where the quality factor of the circuit is small.

Thus, the conditions of high selectivity and good reproducibility contradict each other and in practice a reasonable compromise has to be reached. We also note that the higher the carrier frequency, the wider the frequency range that can be reproduced without distortion. This is one of the reasons why the short and ultra-short wave range is increasingly used.

TV

The television schemegeneral features coincide with that of broadcasting. The station is distinguished in that here the oscillations of the carrier frequency are modulated not only by the acoustic signal, but also by the image signal, originating from special transmitter tubes (iconoscopes or superorticons). Finally, the modulation volume also includes the signals to synchronize the scanning of the electron beam of the iconoscope, on whose screen the image is reproduced. In the television receiver, the high-frequency signal is again divided into three: the image signal (video signal), sound accompaniment and command signals. Once amplified, these signals go to their respective channels, in which they are used properly. Command signals cause the electron beam to scan horizontally, along the lines, and to go from one line to another. In total, in 1/30 of a second, the beam traces 625 lines that constitute a frame. If during this time there is no image signal, the screen will be illuminated evenly. The amplified video signal is sent to the control electrode of the electronic gun, because the intensity of the electron beam varies and, consequently, the brightness of a given point on the screen. At the expense of these variations the image is produced. Due to the large volume of information that the television signal has to carry, it occupies a frequency band of the order of 4-5 MHz (in broadcast receivers it is around 10 kHz). This means that the carrier frequency of the the beam draws 625 lines that make up a box. If during this time there is no image signal, the screen will be illuminated evenly. The amplified video signal is sent to the control electrode of the electronic gun, because the intensity of the electron beam varies and, consequently, the brightness of a given point on the screen. At the expense of these variations the image is produced. Due to the large volume of information that the television signal must carry, it occupies a frequency band of the order of 4-5 MHz (in broadcast receivers it is about 10 kHz). This means that the carrier frequency of the the beam draws 625 lines that make up a box. If during this time there is no image signal, the screen will be illuminated evenly. The amplified video signal is sent to the control electrode of the electronic gun, because the intensity of the electron beam varies and, consequently, the brightness of a given point on the screen. At the expense of these variations the image is produced. Due to the large volume of information that the television signal must carry, it occupies a frequency band of the order of 4-5 MHz (in broadcast receivers it is about 10 kHz). This means that the carrier frequency of the because the intensity of the electron beam varies and, consequently, the brightness of a given point on the screen. At the expense of these variations the image is produced. Due to the large volume of information that the television signal must carry, it occupies a frequency band of the order of 4-5 MHz (in broadcast receivers it is about 10 kHz). This means that the carrier frequency of the because the intensity of the electron beam varies and, consequently, the brightness of a given point on the screen. At the expense of these variations the image is produced. Due to the large volume of information that the television signal has to carry, it occupies a frequency band of the order of 4-5 MHz (in broadcast receivers it is around 10 kHz). This means that the carrier frequency of theelectromagnetic waves must also be very high. The frequency used is from 50 to 900 MHz (the wavelength, from 6 m to 30 cm). The waves used in broadcasting are much longer and range from 1.5km to a few tens of meters.

Thermo Ion Amplifier

Since the receiver is usually found at very great distances from the transmitter, an insignificant part of the energy radiated by it reaches its antenna . The problem of amplifying the power of weak oscillations arises. This problem is solved using thermionic or transistor lamp amplifiers. The circuit of a very sensitive triode amplifier. Let’s see what the fundamental groups of this circuit are for and how they work in principle.

The weak electrode voltage to be amplified is supplied to the electrodes of the grid. The resistance Rg connects the grid with the minus of the anode battery . As a result, the grid potential fluctuates negligibly from this negative potential.

Resistance R1 and capacitor C1 in the cathode circuit give rise to the so-called “cathodic polarization”, at the expense of which the grid potential turns out to be lower than the cathode potential. Indeed, inside the triode the current goes from the anode to the cathode and then, through resistor R1, at least from the anode battery. Judging by the direction of the current, we convince ourselves that Formula. Depending on the anode voltage and resistance R1, the potential difference between the cathode and the grid varies, allowing you to choose the amplifier “work point” O either in the linear part of the characteristic or in its inflection. capacitor C1, with several tens of microfarads of capacity, accumulates electric charge and with this attenuates the pulsations of the cathode potential.

 

Leave a Comment