Electronic ray tube (CRT) - an electronic device that has the shape of a tube, elongated (often with a conical extension) in the direction of the axis of the electron beam, which is formed in the CRT. A CRT consists of an electron-optical system, a deflection system, and a fluorescent screen or target. TV repair in Butovo, contact us for help.
CRT classification
Classification of CRTs is extremely difficult, which is explained by their extreme
about wide application in science and technology and the possibility of modifying the design in order to obtain technical parameters, which are necessary for the implementation of a specific technical idea.
The dependences on the method of controlling the electron beam of the CRT are divided into:
electrostatic (with an electrostatic beam deflection system);
electromagnetic (with an electromagnetic beam deflection system).
Depending on the purpose, CRTs are divided into:
electron graphic tubes (receiving tubes, television tubes, oscilloscope tubes, indicator tubes, television sign tubes, encoding tubes, etc.)
optical-electronic converting tubes (transmitting television tubes, electron-optical converters, etc.)
cathode beam switches (switches);
other CRTs.
Electron Graphics CRTs
Electron graphic CRTs are a group of cathode ray tubes used in various fields of technology to convert electrical signals into optical ones (signal-to-light conversion).
Electronic graphic CRTs are divided into:
Depending on the application:
television reception (picture tubes, ultra-high resolution CRTs for special television systems, etc.)
receiving oscillographic (low-frequency, high-frequency, ultra-high-frequency, high-voltage pulse, etc.)
reception indicator;
remembering;
signs;
coding;
other CRTs.
Structure and operation of a CRT with an electrostatic beam deflection system
The cathode ray tube consists of a cathode (1), anode (2), a leveling cylinder (3), a screen (4), plane regulators (5) and height regulators (6).
Under the influence of photo- or thermal emission, electrons are knocked out of the cathode metal (a thin conductor spiral). Since a voltage (potential difference) of several kilovolts is maintained between the anode and cathode, these electrons, aligned with the cylinder, move in the direction of the anode (hollow cylinder). Flying through the anode, electrons reach the plane controllers. Each regulator is two metal plates, oppositely charged. If the left plate is charged negatively and the right plate positively, then the electrons passing through them will be deflected to the right, and vice versa. The height regulators operate in a similar way. If alternating current is applied to these plates, it will be possible to control the flow of electrons in both horizontal and vertical planes. At the end of its path, the stream of electrons hits a screen where it can produce images.
Phosphors are applied to the screen of a cathode ray tube in the form of tiny dots, and these dots are collected in groups of three; in each three, or triad, there is one red, one blue and one green dot. In the figure I showed you several such triads. In total, there are about 500 thousand triads on the tube screen. The picture you see on TV consists entirely of luminous dots. Where image details are lighter, more electrons hit the dots and they glow brighter. Accordingly, fewer electrons fall into the dark areas of the image. If there is a white detail in a color image, then everywhere within that detail all three points in each triad glow with the same brightness. On the contrary, if there is a red detail in a color image, then everywhere within this detail only the red dots of each triad glow, and the green and blue dots do not glow at all.
Do you understand what it means to create a color image on a TV screen? This is, firstly, to force electrons to fall into the right places, that is, to those phosphor points that should glow, and not to fall into other places, that is, to those points that should not glow. Secondly, the electrons must go to the right places in the right time. After all, the image on the screen is constantly changing, and where at some point, for example, there was a bright orange spot, a moment later a dark purple spot should appear. Finally, thirdly, in Right place and must arrive at the right time required quantity electrons. More - where the glow should be brighter, and less - where the glow is darker.
Since there are almost one and a half million phosphor dots on the screen, the task at first glance seems extremely difficult. In fact - nothing complicated. First of all, a cathode ray tube has not one, but three separate heated cathodes. Exactly the same as in a regular vacuum tube. Each cathode emits electrons and creates an electron cloud around it. Near each cathode there is a grid and an anode. The number of electrons passing through the grid to the anode depends on the voltage across the grid. So far everything is happening as in a regular three-electrode lamp - triode.
What's the difference? The anode here is not solid, but with a hole in the very center. Therefore, most of the electrons moving from the cathode to the anode are not retained at the anode - they fly out through the hole in the form of a round beam. The structure, consisting of a cathode, grid and anode, is called an electron gun. The gun, as it were, shoots a beam of electrons, and the number of electrons in the beam depends on the voltage on the grid.
Electron guns aimed so that the beam emitted from the first cannon always hits only the red dots of the triads, the beam from the second cannon only hits the green dots, and the beam from the third cannon only hits the blue dots. In this way, one of the three problems of creating a color image is solved. By applying the required voltages to the grids of each of the three guns, the required intensities of red, green and blue light are set, and therefore provide the desired coloring for each detail of the image.
Used for both transmission and reception, a cathode ray tube is equipped with a device that emits the electron beam, as well as devices that control its intensity, focus and deflection. All these operations are described here. In conclusion, Professor Radiol looks into the future of television.
So, my dear Neznaykin, I must explain to you the structure and operating principles of a cathode ray tube, as it is used in television transmitters and receivers.
The cathode ray tube existed long before the advent of television. It was used in oscilloscopes - measuring instruments, allowing you to clearly see the forms of electrical voltages.
Electron gun
A cathode ray tube has a cathode, usually indirectly heated, which emits electrons (Fig. 176). The latter are attracted by the anode, which has a potential positive relative to the cathode. The intensity of the electron flow is controlled by the potential of another electrode installed between the cathode and anode. This electrode is called a modulator, has the shape of a cylinder, partially enclosing the cathode, and in its bottom there is a hole through which electrons pass.
Rice. 176. A cathode ray tube gun emitting a beam of electrons. I am the filament; K - cathode; M - modulator; A - anode.
I feel that you are now experiencing a certain dissatisfaction with me. "Why didn't he tell me it was just a triode?!" - perhaps, you think. In fact, the modulator plays the same role as the grid in the triode. And all these three electrodes together form an electric gun. Why? Does she shoot anything? Yes. A hole is made in the anode through which a significant part of the electrons attracted by the anode flies.
In the transmitter, the electron beam “looks” various elements images by running across a light-sensitive surface onto which the image is projected. At the receiver, the beam creates an image on a fluorescent screen.
We'll look at these features in more detail a little later. Now I must explain to you two main problems: how the beam of electrons is concentrated and how it is made to deflect to ensure that all elements of the image are viewed.
Focusing methods
Focusing is necessary so that the cross-section of the beam at the point of contact with the screen does not exceed the size of the image element. The beam at this point of contact is usually called the spot.
In order for the spot to be small enough, the beam must be passed through an electron lens. This is the name for a device that uses electric or magnetic fields and affects an electron beam in the same way as a biconvex glass lens affects light rays.
Rice. 177. Thanks to the action of several anodes, the electron beam is focused to one point on the screen.
Rice. 178. Focusing of the electron beam is ensured by a magnetic field created by a coil to which a constant voltage is applied.
Rice. 179. Deflection of an electron beam by an alternating field.
Rice. 180. Two pairs of plates allow you to deflect the electron beam in the vertical and horizontal directions.
Rice. 181. A sine wave on the screen of an electronic oscilloscope, in which an alternating voltage is applied to the horizontal deflection plates, and a linear voltage of the same frequency is applied to the vertical plates.
Focusing is carried out by electric power lines, for which a second one (also equipped with a hole) is installed behind the first anode, to which a higher potential is applied. You can also install a third one behind the second anode and apply an even higher potential to it than to the second one. The potential difference between the anodes through which the electron beam passes affects the electrons like electric lines of force running from one anode to another. And this effect tends to direct all electrons whose trajectory has deviated to the axis of the beam (Fig. 177).
The anode potentials in cathode ray tubes used in television often reach several tens of thousands of volts. The magnitude of the anode currents, on the contrary, is very small.
From what has been said, you should understand that the power that needs to be given out in the tube is nothing supernatural.
The beam can also be focused by influencing the flow of electrons with a magnetic field created by the current flowing through the coil (Fig. 178).
Deflection by electric fields
So, we managed to focus the beam so much that its spot on the screen is tiny. However, a fixed spot in the center of the screen does not provide any practical benefit. You need to make the spot run along the alternating lines of both half-frames, as Lyuboznaykin explained to you during your last conversation.
How to ensure that the spot deviates, firstly, horizontally, so that it quickly runs along the lines, and, secondly, vertically, so that the spot moves from one odd line to the next odd one, or from one even to the next even one? In addition, it is necessary to ensure a very fast return from the end of one line to the beginning of the one that the spot has to run through. When the spot finishes the last line of one half-frame, it should very quickly rise upward and take its original position at the beginning of the first line of the next half-frame.
In this case, the deflection of the electron beam can also be achieved by changing the electric or magnetic fields. Later you will learn what shape the voltages or currents that control the sweep should have and how to obtain them. Now let's see how the tubes are arranged, the deflection in which is carried out by electric fields.
These fields are created by applying a potential difference between two metal plates located on one side or the other of the beam. We can say that the plates represent the plates of the capacitor. The plate that has become positive attracts electrons, and the plate that has become negative repels them (Fig. 179).
You will easily understand that two horizontal plates determine the vertical deflection of the electron beam. To move the beam horizontally, you need to use two plates located vertically (Fig. 180).
Oscilloscopes use this method of deflection; Both horizontal and vertical plates are installed there. The first ones are subjected to periodic voltages, the shape of which can be determined - these voltages deflect the spot vertically. A voltage is applied to the vertical plates, deflecting the spot horizontally at a constant speed and almost instantly returning it to the beginning of the line.
In this case, the curve that appears on the screen displays the shape of the change in the voltage being studied. As the spot moves from left to right, the voltage in question causes it to rise or fall depending on its instantaneous values. If you look at the AC voltage in this way, you will see a beautiful sinusoidal curve on the screen of the cathode ray tube (Fig. 181).
Screen fluorescence
Now it’s time to explain to you that the inside of the cathode ray tube screen is covered with a layer of fluorescent substance. This is the name given to a substance that glows under the influence of electron strikes. The more powerful these impacts, the higher the brightness they cause.
Don't confuse fluorescence with phosphorescence. The latter is inherent in a substance that, under the influence daylight or the light of electric lamps itself becomes luminous. This is exactly how the hands of your alarm clock glow at night.
Televisions are equipped with cathode ray tubes, the screen of which is made of a translucent fluorescent layer. Under the influence of electron beams, this layer becomes luminous. In black-and-white televisions, the light produced in this way is white. As for color TVs, their fluorescent layer consists of 1,500,000 elements, one third of which emit red light, the other third emit blue light, and the last third emit green light.
Rice. 182. Under the influence magnetic field magnet (thin arrows), electrons are deflected in a direction perpendicular to it (thick arrows).
Rice. 183. Coils that create magnetic fields provide deflection of the electron beam.
Rice. 184. As the deflection angle increases, the tube is made shorter.
Rice. 185. Placement of the conductive layer necessary for the removal of primary and secondary electrons from the screen into the external circuit.
Later they will explain to you how combinations of these three colors make it possible to obtain the entire gamut of a wide variety of colors, including white light.
Magnetic deviation
Let's return to the problem of electron beam deflection. I described to you a method based on changing electric fields. Currently, television cathode ray tubes use beam deflection by magnetic fields. These fields are created by electromagnets located outside the tube.
Let me remind you that magnetic power lines tend to deflect electrons in a direction that forms a right angle with them. Consequently, if the magnetization poles are located to the left and right of the electron beam, then the field lines go in the horizontal direction and deflect the electrons from top to bottom.
And the poles located above and below the tube shift the electron beam horizontally (Fig. 182). Passing through such magnets alternating currents corresponding shape, force the beam to complete the required path of complete image scanning.
So, as you can see, the cathode ray tube is surrounded by a considerable number of coils. Around it there is a solenoid that ensures focusing of the electron beam. And the deflection of this beam is controlled by two pairs of coils: in one the turns are located in the horizontal plane, and in the other in the vertical plane. The first pair of coils deflects electrons from right to left, the second - up and down (Fig. 183).
Previously, the angle of beam deviation from the tube axis did not exceed , but the total beam deviation was 90°. Nowadays, tubes are manufactured with a total beam deflection of up to 110°. Thanks to this, the length of the tube decreased, which made it possible to produce televisions of smaller volume, since the depth of their case decreased (Fig. 184).
Return of electrons
You may be asking yourself what is the final path of the electrons hitting the fluorescent layer of the screen. So know that this path ends with an impact causing the emission of secondary electrons. It is completely unacceptable for the screen to accumulate primary and secondary electrons, since their mass would create a negative charge, which would repel other electrons emitted by the electron gun.
To prevent such electron accumulation outer walls The flasks from the screen to the anode are covered with a conductive layer. Thus, electrons arriving at the fluorescent layer are attracted by the anode, which has a very high positive potential, and are absorbed (Fig. 185).
The anode contact is brought out onto the side wall of the tube, while all other electrodes are connected to the pins of the base located at the end of the tube opposite to the screen.
Is there a danger of explosion?
Another question is undoubtedly rising in your mind. You must be asking yourself how much force the atmosphere puts on those big vacuum tubes that are installed in TVs. You know what's on the level earth's surface Atmosphere pressure is about . The area of the screen, the diagonal of which is 61 cm, is . This means that the air presses on this screen with a force. If we take into account the rest of the surface of the flask in its conical and cylindrical parts, then we can say that the tube can withstand a total pressure exceeding 39-103 N.
Convex sections of the tube are easier than flat ones and can withstand high pressure. Therefore, in the past, tubes were made with a very convex screen. Nowadays, we have learned to make screens strong enough that even with flat shape they successfully withstood air pressure. Therefore, there is no risk of an explosion directed inwards. I deliberately said an explosion directed inward, and not just an explosion, because if a cathode ray tube ruptures, then its fragments rush inward.
In old TVs, as a precaution, a thick layer was installed in front of the screen. protective glass. Currently they are doing without it.
Flat screen of the future
You are young, Neznaykin. The future opens before you; you will see the evolution and progress of electronics in all areas. In television, there will undoubtedly come a day when the cathode ray tube in the television will be replaced by a flat screen. Such a screen will be hung on the wall like a simple picture. And all the electrical circuits of the TV, thanks to microminiaturization, will be placed in the frame of this picture.
The use of integrated circuits will make it possible to reduce to a minimum the size of the numerous circuits that make up the electrical part of the TV. The use of integrated circuits has already become widespread.
Finally, if all the TV controls and buttons have to be placed on the frame surrounding the screen, then it is most likely that remote control devices will be used to adjust the TV. Without rising from his chair, the viewer will be able to switch the TV from one program to another, change the brightness and contrast of the image and volume soundtrack. For this purpose he will have at hand a small box emitting electromagnetic waves or ultrasounds, which will force the TV to make all the specified switching and adjustments. However, such devices already exist, but have not yet become widespread...
Now let's go back from the future to the present. I leave it to Lyuboznaykin to explain to you how cathode ray tubes are currently used to transmit and receive television images.
The operating principle of a cathode ray tube is based on the emission of electrons from a negatively charged thermionic cathode, which are then attracted by a positively charged anode and collected on it. This is the operating principle of an old thermionic tube.
In a CRT, high-speed electrons are emitted from an electron gun (Figure 17.1). They are focused by an electron lens and directed towards a screen, which behaves like a positively charged anode. The screen is coated on the inside with fluorescent powder, which begins to glow when struck by fast electrons. The electron beam (beam) emitted by the electron gun creates a stationary spot on the screen. In order for the electron beam to leave a trace (line) on the screen, it must be deflected in both the horizontal and vertical directions - X and Y.
Rice. 17.1.
Beam deflection methods
There are two methods for deflecting an electron beam in a CRT. IN electrostatic The method uses two parallel plates, between which an electrical potential difference is created (Fig. 17.2(a)). The electrostatic field generated between the plates deflects electrons entering the field's area of action. IN electromagnetic method, a beam of electrons is controlled by a magnetic field created electric shock flowing through the coil. At the same time, as shown in Fig. 17.2(b), two sets of control coils are used (in TVs they are called deflection coils). Both methods provide linear deviation.
Rice. 17.2. Electrostatic (a) and electromagnetic (b)
electron beam deflection methods.
However, the electrostatic deflection method has a wider frequency range, which is why it is used in oscilloscopes. Electromagnetic deflection is better suited for high-voltage tubes (picture tubes) operating in televisions, and is also more compact in implementation, since both coils are located in the same place along the neck of the television tube.
CRT design
In Fig. 17.3 gives a schematic representation internal device cathode ray tube with electrostatic deflection system. Various electrodes and their corresponding potentials are shown. Electrons emitted from the cathode (or electron gun) pass through a small hole (aperture) in the grid. The grid, whose potential is negative with respect to the cathode potential, determines the intensity or number of electrons emitted and thus the brightness of the spot on the screen.
Rice. 17.3.
Rice. 17.4.
The electron beam then passes through an electron lens, which focuses the beam onto a screen. The final anode A 3 has a potential of several kilovolts (relative to the cathode), which corresponds to the ultra-high voltage (UHV) range. Two pairs of deflection plates D 1 and D 2 provide electrostatic deflection of the electron beam in the vertical and horizontal directions, respectively.
Vertical deflection is provided by Y-plates (vertical deflection plates), and horizontal deflection is provided by X-plates (horizontal deflection plates). The input signal is applied to the Y-plates, which deflect the electron beam up and down according to the amplitude of the signal.
The X-plates cause the beam to move horizontally from one edge of the screen to the other (sweep) at a constant speed and then very quickly return to its original position (reverse). On X - The plate is supplied with a sawtooth signal (Fig. 17.4) generated by the generator. This signal is called a timebase signal.
Applying appropriate signals to X - and Y-plate, it is possible to obtain such a displacement of the electron beam that it will be “drawn” on the CRT screen exact shape input signal.
This video explains the basic principles of operation of a cathode ray tube:
Since 1902, Boris Lvovich Rosing has been working with Brown's tube. On July 25, 1907, he filed an application for the invention “Method electric transmission images at a distance." The beam was scanned in the tube by magnetic fields, and the signal was modulated (change in brightness) using a capacitor, which could deflect the beam vertically, thereby changing the number of electrons passing to the screen through the diaphragm. On May 9, 1911, at a meeting of the Russian Technical Society, Rosing demonstrated the transmission of television images of simple geometric shapes and receiving them with playback on a CRT screen.
At the beginning and middle of the 20th century, Vladimir Zvorykin, Allen Dumont and others played a significant role in the development of CRTs.
Classification
According to the method of deflection of the electron beam, all CRTs are divided into two groups: with electromagnetic deflection (indicator CRTs and picture tubes) and with electrostatic deflection (oscillographic CRTs and a very small part of indicator CRTs).
Based on their ability to store a recorded image, CRTs are divided into tubes without memory and tubes with memory (indicator and oscilloscope), the design of which includes special memory elements (units) with the help of which a once recorded image can be reproduced many times.
Based on the color of the screen, CRTs are divided into monochrome and multicolor. Monochrome may have different colour glow: white, green, blue, red and others. Multicolor ones are divided according to the principle of action into two-color and three-color. Two-color - indicator CRTs, the color of the screen glow changes either by switching the high voltage, or by changing the current density of the electron beam. Three-color (based on primary colors) - color picture tubes, the multi-color glow of the screen is ensured by special designs of the electron-optical system, color separation mask and screen.
Oscillographic CRTs are divided into tubes of the low-frequency and microwave ranges. In the designs of the latter, sufficient a complex system deflections of the electron beam.
Picture tubes are divided into television, monitor and projection (used in video projectors). Monitor kinescopes have a smaller mask pitch than television ones, and projection kinescopes have increased screen brightness. They are monochrome and have red, green and Blue colour screen glow.
Design and principle of operation
General principles
Black and white kinescope device
In a cylinder 9 a deep vacuum is created - first the air is pumped out, then all the metal parts of the kinescope are heated by an inductor to release the absorbed gases; a getter is used to gradually absorb the remaining air.
To create an electron beam 2 , a device called an electron gun is used. Cathode 8 , heated by filament 5 , emits electrons. To increase electron emission, the cathode is coated with a substance that has a low work function ( largest producers CRTs use their own patented technologies for this). By changing the voltage on the control electrode ( modulator) 12 you can change the intensity of the electron beam and, accordingly, the brightness of the image (there are also models with cathode control). In addition to the control electrode, the gun of modern CRTs contains a focusing electrode (until 1961, domestic picture tubes used electromagnetic focusing using a focusing coil 3 with core 11 ), designed to focus a spot on the kinescope screen into a point, an accelerating electrode for additional acceleration of electrons within the gun and anode. After leaving the gun, the electrons are accelerated by the anode 14 , which is a metallized coating of the inner surface of the kinescope cone, connected to the gun electrode of the same name. In color picture tubes with an internal electrostatic screen, it is connected to the anode. In a number of picture tubes of early models, such as 43LK3B, the cone was made of metal and represented the anode itself. The voltage at the anode ranges from 7 to 30 kilovolts. In a number of small-sized oscillographic CRTs, the anode is only one of the electrodes of the electron gun and is supplied with voltages of up to several hundred volts.
The beam then passes through the deflection system 1 , which can change the direction of the beam (the figure shows a magnetic deflection system). Television CRTs use a magnetic deflection system as it provides large deflection angles. Oscillographic CRTs use an electrostatic deflection system as it provides greater performance.
The electron beam hits the screen 10 , coated with phosphor 4 . Bombarded by electrons, the phosphor glows and a rapidly moving spot of variable brightness creates an image on the screen.
The phosphor acquires a negative charge from the electrons, and secondary emission begins - the phosphor itself begins to emit electrons. As a result, the entire tube acquires a negative charge. To prevent this from happening, over the entire surface of the tube there is a layer of aquadag, a conductive mixture based on graphite, connected to the anode ( 6 ).
The kinescope is connected through the leads 13 and high voltage socket 7 .
In black-and-white TVs, the composition of the phosphor is selected so that it glows in a neutral gray color. In video terminals, radars, etc., the phosphor is often made yellow or green to reduce eye fatigue.
Beam angle
The deflection angle of the CRT beam is the maximum angle between two possible positions of the electron beam inside the bulb at which a luminous spot is still visible on the screen. The ratio of the diagonal (diameter) of the screen to the length of the CRT depends on the angle. For oscillographic CRTs, it is usually up to 40°, which is due to the need to increase the sensitivity of the beam to the effects of deflection plates and ensure linearity of the deflection characteristics. For the first Soviet television picture tubes with a round screen, the deflection angle was 50°; for black-and-white picture tubes of later releases it was 70°; starting in the 1960s it increased to 110° (one of the first such picture tubes was 43LK9B). For domestic color picture tubes it is 90°.
As the beam deflection angle increases, the dimensions and weight of the kinescope decrease, however:
- The power consumed by the scanning nodes increases. To solve this problem, the diameter of the kinescope neck was reduced, which, however, required a change in the design of the electron gun.
- the requirements for the accuracy of manufacturing and assembly of the deflection system are increasing, which was realized by assembling the kinescope with the deflection system into a single module and assembling it in the factory.
- the number is increasing necessary elements raster geometry and information settings.
All this has led to the fact that in some areas 70-degree picture tubes are still used. Also, an angle of 70° continues to be used in small-sized black and white picture tubes (for example, 16LK1B), where length does not play such a significant role.
Ion trap
Since it is impossible to create a perfect vacuum inside the CRT, some air molecules remain inside. When colliding with electrons, they form ions, which, having a mass many times greater than the mass of electrons, practically do not deviate, gradually burning out the phosphor in the center of the screen and forming a so-called ion spot. To combat this, until the mid-1960s, the “ion trap” principle was used: the axis of the electron gun was located at a certain angle to the axis of the kinescope, and an adjustable magnet located outside provided a field that turned the flow of electrons towards the axis. Massive ions, moving rectilinearly, fell into the trap itself.
However, this construction forced an increase in the diameter of the kinescope neck, which led to an increase required power in the coils of the deflection system.
In the early 1960s, a new method of protecting the phosphor was developed: aluminizing the screen, which also doubled the maximum brightness of the kinescope, eliminating the need for an ion trap.
Delay in supplying voltage to the anode or modulator
In a TV, the horizontal scanning of which is made using lamps, the voltage at the anode of the kinescope appears only after the output horizontal scanning lamp and the damper diode have warmed up. By this time, the kinescope heat has already warmed up.
The introduction of all-semiconductor circuitry into horizontal scanning units gave rise to the problem of accelerated wear of the kinescope cathodes due to the supply of voltage to the anode of the kinescope simultaneously with switching on. To combat this phenomenon, amateur units have been developed that provide a delay in the supply of voltage to the anode or modulator of the kinescope. It is interesting that in some of them, despite the fact that they are intended for installation in all-semiconductor televisions, a radio tube is used as a delay element. Later televisions began to be produced industrial production, in which such a delay is provided initially.
Scan
To create an image on the screen, an electron beam must constantly pass across the screen with high frequency- at least 25 times per second. This process is called sweep. There are several ways to scan an image.
Raster scanning
The electron beam passes the entire screen in rows. There are two options:
- 1-2-3-4-5-… (interlaced scanning);
- 1-3-5-7-…, then 2-4-6-8-… (interlaced).
Vector scan
The electron beam passes along the image lines. Vector scanning was used in the Vectrex game console.
Scan on the radar screen
In the case of using the all-round viewing screen, the so-called. typetron, the electron beam passes along the radii of the screen (the screen has the shape of a circle). Service information in most cases (numbers, letters, topographical signs) is additionally deployed through a sign matrix (located in an electron beam gun).
Color picture tubes
Color kinescope device. 1 - Electron guns. 2 - Electron rays. 3 - Focusing coil. 4 - Deflection coils. 5 - Anode. 6 - A mask, thanks to which the red beam hits the red phosphor, etc. 7 - Red, green and blue phosphor grains. 8 - Mask and phosphor grains (enlarged).
A color kinescope differs from a black and white one in that it has three guns - “red”, “green” and “blue” ( 1 ). Accordingly, on the screen 7 three types of phosphor are applied in some order - red, green and blue ( 8 ).
Depending on the type of mask used, the guns in the neck of the kinescope are located delta-shaped (in the corners of an equilateral triangle) or planar (on the same line). Some electrodes of the same name from different electron guns are connected by conductors inside the kinescope. These are accelerating electrodes, focusing electrodes, heaters (connected in parallel) and, often, modulators. This measure is necessary to save the number of outputs of the kinescope, due to the limited dimensions of its neck.
Only the beam from the red gun hits the red phosphor, only the beam from the green gun hits the green one, etc. This is achieved by installing a metal grill, called mask (6 ). In modern picture tubes, the mask is made of invar, a type of steel with a small coefficient of thermal expansion.
Types of masks
There are two types of masks:
There is no clear leader among these masks: the shadow one provides high quality lines, the aperture gives more saturated colors and high efficiency. Slit combines the advantages of shadow and aperture, but is prone to moire.
The smaller the phosphor elements, the higher the image quality the tube can produce. An indicator of image quality is mask step.
- For a shadow grating, the mask pitch is the distance between the two nearest mask holes (accordingly, the distance between the two closest phosphor elements of the same color).
- For aperture and slot gratings, the mask pitch is defined as the horizontal distance between the mask slits (respectively, the horizontal distance between vertical phosphor strips of the same color).
In modern CRT monitors, the mask pitch is 0.25 mm. Television picture tubes, which view images from a greater distance, use steps of about 0.8 mm.
Convergence of rays
Since the radius of curvature of the screen is much greater than the distance from it to the electron-optical system up to infinity in flat picture tubes, and without the use of special measures, the point of intersection of the rays of a color picture tube is at a constant distance from the electron guns, it is necessary to ensure that this point is located exactly at surface of the shadow mask, otherwise a misalignment of the three color components of the image will occur, increasing from the center of the screen to the edges. To prevent this from happening, the electron beams must be properly biased. In picture tubes with a delta-shaped arrangement of guns, this is done by a special electromagnetic system, controlled separately by a device, which in old televisions was placed in a separate block - the mixing block - for periodic adjustments. In picture tubes with a planar arrangement of guns, adjustment is made using special magnets located on the neck of the picture tube. Over time, especially for picture tubes with a delta-shaped arrangement of electron guns, the convergence is disrupted and requires additional adjustment. Most computer repair companies offer a monitor reconvergence service.
Demagnetization
Necessary in color picture tubes to remove residual or random magnetization of the shadow mask and electrostatic screen that affects image quality.
Demagnetization occurs due to the appearance in the so-called demagnetization loop - a ring-shaped flexible coil large diameter, located on the surface of the kinescope - a pulse of a rapidly varying damped magnetic field. To ensure that this current gradually decreases after turning on the TV, thermistors are used. Many monitors, in addition to thermistors, contain a relay, which, upon completion of the kinescope demagnetization process, turns off the power to this circuit so that the thermistor cools down. After this, you can use a special key, or, more often, a special command in the monitor menu, to trigger this relay and carry out repeated demagnetization at any time, without turning off and on the monitor’s power.
Trinescope
A trinescope is a design consisting of three black-and-white picture tubes, light filters and translucent mirrors (or dichroic mirrors that combine the functions of translucent mirrors and filters), used to obtain a color image.
Application
CRTs are used in raster image formation systems: various types of televisions, monitors, and video systems.
Oscilloscope CRTs are most commonly used in display systems functional dependencies: oscilloscopes, wobuloscopes, also as a display device on radar stations, in devices special purpose; during the Soviet years they were also used as visual aids when studying the design of electron beam devices in general.
Character-printing CRTs are used in various special-purpose equipment.
Designation and marking
The designation of domestic CRTs consists of four elements:
- The first element: a number indicating the diagonal of the rectangular or the diameter of the round screen in centimeters;
- The second element: two letters indicating that the CRT belongs to a certain design type. LC - kinescope, LM - tube with electromagnetic beam deflection, LO - tube with electrostatic beam deflection, LN - tubes with memory (indicator and oscillographic);
- Third element: a number indicating the model number of a given tube with a given diagonal, while for oscilloscope tubes in the microwave range, the numbering begins with number 101;
- Fourth element: a letter indicating the color of the screen glow. C - colored, B - white glow, I - green glow, B - yellow-green glow, C - orange glow, P - red glow, A - blue glow. X - denotes a specimen that has worse lighting parameters compared to the prototype.
IN special cases a fifth element may be added to the designation, carrying additional information.
Example: 50LK2B - black and white kinescope with a screen diagonal of 50 cm, second model, 3LO1I - oscilloscope tube with a green screen diameter of 3 cm, first model.
Health effects
Electromagnetic radiation
This radiation is created not by the kinescope itself, but by the deflection system. Tubes with electrostatic deflection, in particular oscilloscopes, do not emit it.
In monitor picture tubes, to suppress this radiation, the deflection system is often covered with ferrite cups. Television picture tubes do not require such shielding, since the viewer usually sits at a much greater distance from the TV than from the monitor.
Ionizing radiation
Present in picture tubes ionizing radiation two types.
The first of these is the electron beam itself, which is essentially a stream of low-energy beta particles (25 keV). This radiation does not escape outside and does not pose a danger to the user.
The second is bremsstrahlung X-ray radiation, which occurs when the screen is bombarded with electrons. To reduce the output of this radiation to completely safe levels, the glass is doped with lead (see below). However, in the event of a malfunction of the TV or monitor, leading to a significant increase in the anode voltage, the level of this radiation can increase to noticeable levels. To prevent such situations, line scanning units are equipped with protection units.
In domestic and foreign color TVs produced before the mid-1970s, additional sources of X-ray radiation may be found - stabilizing triodes connected in parallel to the kinescope, and used to stabilize the anode voltage, and therefore the size of the image. The Raduga-5 and Rubin-401-1 TVs use 6S20S triodes, and the early ULPTsT models use GP-5. Since the glass of the container of such a triode is much thinner than that of a kinescope and is not doped with lead, it is a much more intense source of X-ray radiation than the kinescope itself, so it is placed in a special steel screen. In later models of ULPTST TVs, other methods of stabilizing high voltage are used, and this source of X-ray radiation is excluded.
Flicker
Mitsubishi Diamond Pro 750SB monitor (1024x768, 100 Hz), shot at 1/1000 s shutter speed. Brightness is artificially high; shows the actual brightness of the image at different points on the screen.
The beam of a CRT monitor, forming an image on the screen, causes phosphor particles to glow. Before the next frame is formed, these particles have time to go out, so you can observe “screen flickering.” The higher the frame rate, the less noticeable the flickering. Low frequency leads to eye fatigue and harms health.
For most televisions based on a cathode ray tube, 25 frames change every second, which, taking into account interlaced scanning, is 50 fields (half frames) per second (Hz). IN modern models TVs artificially increase this frequency to 100 hertz. When working behind a monitor screen, flickering is felt more strongly, since the distance from the eyes to the kinescope is much smaller than when watching TV. The minimum recommended monitor refresh rate is 85 hertz. Early models of monitors do not allow working with a scanning frequency of more than 70-75 Hz. The flickering of a CRT can clearly be observed with peripheral vision.
Fuzzy image
The image on a cathode ray tube is blurry compared to other types of screens. Blurred images are believed to be one of the factors contributing to user eye fatigue. On the other hand, when using high-quality monitors, blur does not have a strong impact on human health, and the blur effect itself allows you to avoid the use of smoothing of screen fonts on the monitor, which is reflected in the quality of image perception; there is no font distortion inherent in LCD monitors.
High voltage
A CRT uses high voltage to operate. Residual voltage of hundreds of volts, if no measures are taken, can linger on CRTs and wiring circuits for weeks. Therefore, discharge resistors are added to the circuits, which make the TV completely safe within a few minutes after turning it off.
Contrary to popular belief, the anode voltage of a CRT cannot kill a person due to the low power of the voltage converter - there will only be a noticeable blow. However, it can also be fatal if a person has heart defects. It can also cause injury, including death, indirectly when a person withdraws their hand and touches other circuits in the television and monitor that contain extremely life-threatening voltages - which are present in all models of televisions and monitors that use CRTs, as well as including purely mechanical injuries associated with a sudden uncontrolled fall caused by an electrical spasm.
Toxic substances
Any electronics (including CRTs) contain substances harmful to health and environment. Among them: barium compounds in cathodes, phosphors.
Used CRTs are considered hazardous waste in most countries and must be recycled or disposed of in separate landfills.
CRT explosion
Since there is a vacuum inside the CRT, due to air pressure, the screen of a 17-inch monitor alone places a load of about 800 kg - the weight of a minicar. Due to the design, the pressure on the screen and cone of the CRT is positive, and on side part screen is negative, which creates a danger of explosion. When working with early models of picture tubes, safety regulations required the use of protective gloves, a mask and goggles. A glass protective screen was installed in front of the kinescope screen on the TV, and a metal protective mask was installed at the edges.
Since the second half of the 1960s, the dangerous part of the picture tube has been covered with a special metal explosion-proof bandage, made in the form of an all-metal stamped structure or wound in several layers of tape. Such a bandage eliminates the possibility of spontaneous explosion. Some models of picture tubes additionally used a protective film to cover the screen.
Despite the use protective systems, it is not excluded that people will be injured by shrapnel when the picture tube is deliberately broken. In this regard, when destroying the latter, for safety, the extension is first broken - a technological glass tube at the end of the neck under a plastic base, through which air is pumped out during production.
Small-sized CRTs and picture tubes with a screen diameter or diagonal of up to 15 cm do not pose a danger and are not equipped with explosion-proof devices.
Other types of electron beam devices
In addition to the kinescope, cathode ray devices include:
- Quantoscope (laser kinescope), a type of kinescope, the screen of which is a matrix of semiconductor lasers pumped by an electron beam. Quantoscopes are used in image projectors.
- Sign-printing cathode ray tube.
- Indicator cathode ray tubes are used in radar indicators.
- Storage cathode ray tube.
- Graphecon
- The transmitting television tube converts light images into electrical signals.
- A monoscope is a transmitting cathode ray tube that converts a single image made directly on the photocathode into an electrical signal. Used to transmit images of a television test table (for example, TIT-0249).
- Kadroscope is a cathode ray tube with a visible image, designed for adjusting scanning units and focusing the beam in equipment using cathode ray tubes without a visible image (graphecons, monoscopes, potentialoscopes). The framescope has a pinout and reference dimensions similar to the cathode ray tube used in the equipment. Moreover, the main CRT and framescope are selected according to parameters with very high accuracy and are supplied only as a set. When setting up, a framescope is connected instead of the main tube.
see also
Notes
Literature
- D. Brilliantov, F. Ignatov, V. Vodychko. Single-beam color kinescope - chromoscope 25LK1TS. Radio No. 9, 1976. P. 32, 33.
Links
- S. V. Novakovsky. 90 years of electronic television // Electrosvyaz No. 6, 1997
- P. Sokolov. Monitors // iXBT, 1999
- Mary Bellis. The History of the Cathode Ray Tube // About:Inventors
- Evgeny Kozlovsky. An old friend is better "Computerra" No. 692, June 27, 2007
- Mukhin I. A. How to choose a CRT monitor Computer business market No. 49(286), November-December 2004. P. 366-371
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