Design and principle of operation of a cathode ray tube p. What is a cathode ray tube

A cathode ray tube, invented back in 1897, is an electron-vacuum device that has much in common with a conventional vacuum tube. Externally, the tube is a glass flask with an elongated neck and a flat end part—a screen.

Inside the bulb and neck, as well as inside the cylinder of an electronic lamp, there are electrodes, the leads of which, like those of the lamp, are soldered to the legs of the base.

The main purpose of a cathode ray tube is to produce a visible image using electrical signals. By applying appropriate voltages to the electrodes of the tube, you can draw on its screen graphs of alternating voltages and currents, the characteristics of various radio devices, and also obtain moving images similar to those we see on a movie screen.

Rice. 1. Wonderful pencil.

All this makes the cathode ray tube an indispensable part of televisions, radars, and many measuring and computing instruments.

What kind of “fast pencil” manages to sketch current pulses on the screen of a cathode ray tube that last millionths of a second? How do you choose tones? complex pattern? How can you instantly “erase” one image from the screen and create another with the same speed? (Fig. 1).

Fluorescent screen to electron beam. The operation of a cathode ray tube is based on the ability of certain substances (willite, zinc sulfide, zinc aluminate:) to glow (luminesce) under the influence of electron bombardment.

If the anode of a conventional electron tube is coated from the inside with such a luminescent substance, it will glow brightly due to bombardment by electrons forming the anode current. By the way, such a luminescent anode is used in one of the special electron tubes - the 6E5C optical tuning indicator. The inside of the thickened end of the flask is coated with a luminescent composition, thus forming a luminescent screen of a cathode ray tube. Using a special device—an “electron gun”—a narrow beam of electrodes—an “electron beam”—is directed from the neck of the tube onto the screen.

Rice. 2. The screen glows under the action of a beam of electrons.

In the place where the electrons hit the luminescent layer, a luminous point is formed on the screen, which is clearly visible (from the end) from the outside of the tube through the glass. How large quantity electrons forms a beam and the faster these electrons move, the brighter the luminous point on the luminescent screen.

If the electron beam is moved in space, then the luminous point will also move across the screen, and if the beam moves quickly enough, then our eye will see solid luminous lines on the screen instead of a moving point (Fig. 2).

If you quickly trace the entire screen line by line with an electron beam and at the same time change the beam current (i.e., the brightness of the luminous point) accordingly, then you can get a complex and fairly clear picture on the screen.

Thus, the image on the luminescent screen of the tube is obtained using a sharply directed beam of electrons and therefore, just like in an electron tube, the main processes in the tube are associated with the production and ordered movement of free electrons in a vacuum.

Cathode ray tube and triode

A cathode ray tube is in many ways similar to an amplification tube - a triode. Just like a lamp, the tube contains a cathode that emits the electrons needed to produce the electron beam. From the cathode of the tube, electrons move to the screen, which, like the anode of the triode, has a high positive potential relative to the cathode.

Rice. 3. Emergence of secondary electrons

However, applying positive voltage directly to the screen is difficult, since the luminescent substance is a semiconductor. Therefore, positive voltages on the screen have to be created indirectly. The inside of the flask is covered with a layer of graphite, to which a positive voltage is applied. The electrons forming the beam, hitting the luminescent substance with force, “knock out” the so-called “secondary” electrons from it, which move in an orderly manner towards the graphite coating under the influence of a positive voltage on it (Fig. 3).

At the first moment, the number of secondary electrons leaving the screen is much greater than the number of beam electrons entering it. This leads to the formation of a shortage of electrons in the atoms of the luminescent substance, i.e. the screen acquires a positive potential. Equilibrium between the number of electrons hitting the screen and the number of secondary electrons knocked out of it will be established only when the voltage on the tube screen is close to the voltage on the graphite coating. Thus, the current in the cathode ray tube is closed along the path cathode - screen - graphite coating, and therefore, it is the graphite coating that plays the role of the anode, although the electrodes flying out of the cathode do not directly hit it.

Near the cathode of the tube there is a control electrode (modulator), which plays the same role as the control grid of the triode. By changing the voltage on the control electrode, you can change the amount of beam current, which in turn will lead to a change in the brightness of the point glowing on the screen.

However, along with the similarities between an amplifying electron tube and a cathode ray tube, there are features in the operation of the latter that fundamentally distinguish it from a triode.

First, electrons move from the cathode to the tube screen in a narrow beam, while they move in a “broad front” to the anode of the lamp.

Secondly, in order to create an image on it by moving a luminous point across the screen, it is necessary to change the direction of movement of the electrons flying towards the screen and, thus, move the electron beam in space.

From all this it follows that the most important processes What distinguishes a tube from a triode is the formation of a thin electron beam and the deflection of this beam in different directions.

Formation and focusing of the electron beam

The formation of an electron beam begins already near the cathode of the cathode ray tube, which consists of a small nickel cylinder with a cap coated with an emitting material (well emitting electrons when heated) material. An insulated wire—a heater—is placed inside the cylinder. Thanks to this cathode design, electrons are emitted from a much smaller surface area than in a conventional vacuum tube. This immediately creates a certain directionality of the beam of electrons flying from the cathode.

The cathode of the cathode ray tube is placed in a heat shield - a metal cylinder, the end part of which, directed towards the bulb, is open. Due to this, electrons do not move from the cathode in all directions, as is the case in a lamp, but only in the direction of the luminescent screen. However, despite the special design of the cathode and the heat shield, the flow of moving electrons remains excessively wide.

A sharp narrowing of the electron flow is carried out by the control electrode, which, although it plays the role of a control grid, structurally has nothing in common with the grid. The control electrode is made in the form of a cylinder covering the cathode, in the end part of which a round hole with a diameter of several tenths of a millimeter is made.

A significant (several tens of volts) negative bias is applied to the control electrode, due to which it repels electrons, which, as is known, have a negative charge. Under the influence of a negative voltage, the trajectories (paths of movement) of electrons passing through a narrow hole in the control electrode are “compressed” towards the center of this hole and thus a rather thin electron beam is formed.

However, for the tube to operate normally, it is necessary not only to create an electron beam, but also to focus it, i.e., to ensure that the trajectories of all the electrons of the beam converge on the screen at one point. If the beam is not focused, then a rather large luminous spot will appear on the screen instead of a luminous point and, as a result, the image will be blurry or, as amateur photographers say, “unsharp.”

Rice. 4. Electron gun and its optical analogy.

The beam is focused by an electronic optical system, which acts on moving electrons in the same way as conventional optics on light rays. Electronic optical system formed by electrostatic lenses (static focusing) or electromagnetic lenses (magnetic focusing), final result whose actions are the same.

An electrostatic lens is nothing more than (Fig. 4a) an electric field formed with the help of special electrodes, under the influence of which the trajectories of the beam electrons are bent. In a tube with static focusing (Fig. 4, b) there are usually two lenses, for the formation of which they use a control electrode already known to us, as well as two special electrodes: the first and second anodes. Both of these electrodes are metal cylinders, sometimes of different diameters, to which a large positive (relative to the cathode) voltage is applied: the first anode is usually 200-500 V, the second is 800-15,000 V.

A first lens is formed between the control electrode and the first anode. Its optical analogue is a short-focus collecting lens, consisting of two elements: a biconvex and a biconcave lens. This lens produces an image of the cathode inside the first anode, which in turn is projected onto the tube screen using the second lens.

The second lens is formed by the field between the first and second anodes and is similar to the first lens, except that its focal length much bigger. Thus, the first lens plays the role of a condenser, and the second lens acts as the main projection lens.

Inside the anodes there are thin metal plates with holes in the center - diaphragms, which improve the focusing properties of the lenses.

By changing the voltage on any of the three electrodes that form electrostatic lenses, you can change the properties of the lenses, achieving good focusing of the beam. This is usually done by changing the voltage at the first anode.

A few words about the names of the electrodes “first anode” and “second anode”. Previously, we established that the role of the anode in a cathode ray tube is played by the graphite coating near the screen. However, the first and second anodes, mainly intended for focusing the beam, due to the presence of a large positive voltage on them, accelerate electrons, i.e., they do the same as the anode of an intensifying lamp. Therefore, the names of these electrodes can be considered justified, especially since some of the electrons escaping from the cathode fall on them.

Rice. 5. Magnetic focusing tube. 1—control electrode; 2—first anode; 3—focusing coil; 4—graphite coating; 5—luminescent screen; 6—flask.

In cathode ray tubes with magnetic focusing (Fig. 5), there is no second anode. The role of a collecting lens in this tube is played by a magnetic field. This field is formed by a coil covering the neck of the tube through which a direct current is passed. The magnetic field of the coil creates rotational movement electrons. At the same time, electrons move at high speed parallel to the axis of the tube towards the luminescent screen under the influence of a positive voltage on it. As a result, electron trajectories form a curve “resembling a helix.

As they approach the screen, the speed of the electrons' translational motion increases, and the effect of the magnetic field weakens. Therefore, the radius of the curve gradually decreases and near the screen the electron beam is stretched into a thin straight beam. Good focusing is usually achieved by changing the current in the focusing coil, that is, by changing the magnetic field strength.

The entire system for producing an electron beam in tubes is often called an “electron gun” or “electron spotlight.”

Electron beam deflection

The deflection of the electron beam, as well as its focusing, is carried out using electric fields (electrostatic deflection) or using magnetic fields (magnetic deflection).

In tubes with electrostatic (Fig. 6a) deflection, the electron beam, before hitting the screen, passes between four flat metal electrode plates, which are called deflection plates.

Rice. 6. Beam control using. a—electrostatic and b—magnetic fields.

After the deflection system, the electrons fall on the CRT screen. The screen consists of a thin layer of phosphor applied to the inner surface of the end part of the balloon and capable of glowing intensely when bombarded with electrons.

In some cases, a conductive thin layer of aluminum is applied on top of the phosphor layer. Screen properties are determined by its

characteristics and parameters. The main screen parameters include: first And second critical screen potentials, glow brightness, luminous efficiency, afterglow duration.

Screen potential. When the screen is bombarded by a stream of electrons from its surface, secondary electron emission occurs. To remove secondary electrons, the tube walls near the screen are coated with a conductive graphite layer, which is connected to the second anode. If this is not done, then the secondary electrons, returning to the screen, together with the primary ones, will lower its potential. In this case, a braking electric field is created in the space between the screen and the second anode, which will reflect the electrons of the beam. Thus, to eliminate the braking field, it is necessary to remove the electric charge carried by the electron beam from the surface of the non-conducting screen. Almost the only way to compensate for the charge is to use secondary emission. When electrons fall on the screen, they kinetic energy is converted into screen glow energy, heats it and causes secondary emission. The value of the secondary emission coefficient o determines the screen potential. The coefficient of secondary electron emission a = / in // l (/„ is the current of secondary electrons, / l is the beam current, or the current of primary electrons) from the screen surface in a wide range of changes in the energy of primary electrons exceeds unity (Fig. 12.8, O < 1 на участке O A curve at V < С/ кр1 и при 15 > S/cr2).

At And < (У кр1 число уходящих-от экрана вторичных электронов меньше числа первичных, что приводит к накоплению отрицательного заряда на экране, формированию тормозящего поля для электронов луча в пространстве между вторым анодом и экраном и их отражению; свечение экрана отсутствует. Потенциал and l2= Г/крР corresponding to point A in Fig. 12.8, called first critical potential.

At C/a2 = £/cr1 the screen potential is close to zero.

If the beam energy becomes greater than e£/cr1, then o > 1 and the screen begins to charge

Rice. 12.8

relative to the last anode of the spotlight. The process continues until the screen potential becomes approximately equal to the potential of the second anode. This means that the number of electrons leaving the screen is equal to the number of incident ones. In the range of beam energy changes from e£/cr1 to C/cr2 c > 1 and the screen potential is quite close to the potential of the projector anode. At and &2 > N cr2 secondary emission coefficient a< 1. Потенциал экрана вновь снижается, и у экрана начинает формироваться тормозящее для электронов луча поле. Потенциал And kr2 (corresponds to the point IN in Fig. 12.8) is called second critical potential or maximum potential.

At electron beam energies higher e11 kr2 The brightness of the screen does not increase. For various screens Г/кр1 = = 300...500 V, and kr2= 5...40 kV.

If it is necessary to obtain high brightness, the screen potential is forcibly maintained equal to the potential of the last electrode of the spotlight using a conductive coating. The conductive coating is electrically connected to this electrode.

Light output. This is a parameter that determines the ratio of light intensity J cv, emitted by the phosphor normal to the screen surface, to the power of the electron beam R el incident on the screen:

Light output μ determines the efficiency of the phosphor. Not all the kinetic energy of primary electrons is converted into visible radiation energy, part goes on screen heating, secondary electron emission and radiation in the infrared and ultraviolet spectral ranges. Light output is measured in candelas per watt: for different screens it varies within 0.1... 15 cd/W. At low electron velocities, glow occurs in the surface layer and part of the light is absorbed by the phosphor. As electron energy increases, light output increases. However, at very high speeds, many electrons penetrate the phosphor layer without producing excitation, and a decrease in light output occurs.

Brightness of the glow. This is a parameter that is determined by the strength of light emitted in the direction of the observer by one square meter uniformly luminous surface. Brightness is measured in cd/m2. It depends on the properties of the phosphor (characterized by coefficient A), the current density of the electron beam y, the potential difference between the cathode and the screen II and minimum screen potential 11 0, at which luminescence of the screen is still observed. The brightness of the glow obeys the law

Exponent values p y potential £/ 0 for different phosphors vary within the limits of 1...2.5 and

30...300 V. In practice, the linear nature of the dependence of brightness on current density y is maintained up to approximately 100 μA/cm 2. At high current densities, the phosphor begins to heat up and burn out. The main way to increase brightness is to increase And.

Resolution. This important parameter is defined as the ability of a CRT to reproduce image detail. Resolution is assessed by the number of separately distinguishable glowing points or lines (lines), respectively, per 1 cm 2 of the surface or 1 cm of the screen height, or the entire height of the working surface of the screen. Consequently, to increase the resolution it is necessary to reduce the diameter of the beam, i.e., a well-focused thin beam with a diameter of tenths of a mm is required. The lower the beam current and the higher the accelerating voltage, the higher the resolution. In this case, the best focusing is achieved. Resolution also depends on the quality of the phosphor (large phosphor grains scatter light) and the presence of halos resulting from complete internal reflection in the glass part of the screen.

Duration of afterglow. The time during which the brightness decreases to 1% of the maximum value is called screen afterglow time. All screens are divided into screens with very short (less than 10 5 s), short (10“ 5 ...10“ 2 s), medium (10 2 ...10 1 s), long (10 Ch.Lb s) and very long (more than 16 s) afterglow. Tubes with short and very short persistence are widely used in oscillography, and with medium persistence - in television. Radar indicators typically use tubes with a long persistence.

In radar tubes, long-lasting screens with a two-layer coating are often used. The first layer of phosphor - with a short afterglow of blue color- is excited by an electron beam, and the second - with yellow glow and long afterglow - excited by the light of the first layer. In such screens it is possible to obtain an afterglow of up to several minutes.

Types of screens. Very great importance has the glow color of the phosphor. In oscillographic technology, when visually observing the screen, CRTs with a green glow are used, which is the least tiring for the eye. Zinc orthosilicate activated with manganese (willemite) has this glow color. For photography, screens with a blue emission color characteristic of calcium tungstate are preferred. In receiving television tubes with black and white images, they try to obtain White color, for which phosphors are used from two components: blue and yellow.

The following phosphors are also widely used for the manufacture of screen coatings: zinc and cadmium sulfides, zinc and magnesium silicates, oxides and oxysulfides of rare earth elements. Phosphors based on rare earth elements have a number of advantages: they are more resistant to various influences than sulfide ones, are quite efficient, have a narrower spectral band of emission, which is especially important in the production of color picture tubes, where high color purity is required, etc. As An example is the relatively widely used phosphor based on yttrium oxide activated by europium U 2 0 3: Ey. This phosphor has a narrow emission band in the red region of the spectrum. Good characteristics There is also a phosphor consisting of yttrium oxysulfide with an admixture of europium Y 2 0 3 8: Eu, which has a maximum emission intensity in the red-orange region of the visible spectrum and better chemical resistance than Y 2 0 3: Eu phosphor.

Aluminum is chemically inert when interacting with screen phosphors, is easily applied to the surface by evaporation in a vacuum and reflects light well. The disadvantages of aluminized screens include the fact that the aluminum film absorbs and scatters electrons with an energy of less than 6 keV, so in these cases the light output drops sharply. For example, the luminous efficiency of an aluminized screen at an electron energy of 10 keV is approximately 60% greater than at 5 keV. Tube screens have a rectangular or round shape.

Do you love television as much as I don't?

TV is generally a disgusting thing. Instead of sitting for hours in front of a blue screen, it is much more useful to conduct healthy image life: slowly, with a cup of coffee - at the computer...

Nevertheless, the things that I will tell in this series of articles may be quite useful in our practical activities.

So, now we will figure out how the video signal is transmitted. We will consider the painfully dear SECAM system, because in our country (namely - Russian Federation) this television system has been officially adopted. However - first things first.

How does TV work?

The TV operates 24 hours a day, 7 days a week. It's clear.
It has a screen - 1 piece and a speaker - from 1 to infinity, depending on the “sophistication” of the unit. It also has an antenna and a control panel. But now we are only interested in the screen. And translating from the language of housewives into the language of wise cats - kinescope(Cathode ray tube - CRT).

I understand perfectly well that in our age of plasma and liquid crystal, a cathode-ray kinescope seems to some to be a relic of antiquity. However, the easiest way to understand how a TV works is to understand the CRT.

Cathode-ray tube

What do you think? What do electrons have to do with it? What do the rays have to do with it?

The fact is that the picture on the screen is drawn using an electron beam. An electron beam is very similar to a light beam. But a light beam consists of photons, and an electron beam consists of electrons, and we cannot see it. A bunch of electrons rush at breakneck speed in a straight line from point A to point B. This is how a “beam” is formed.

Point B is the anode. It's right on the back of the screen. Also, the screen (with reverse side) is smeared with a special substance - phosphor. When an electron collides at breakneck speed with a phosphor, the latter emits visible light. The faster the electron flew before the collision, the brighter the light will be. That is, a phosphor is a converter of the “light” of an electron beam into light visible to the human eye.

Point B is dealt with. What is point "A"? A is " electron gun". The name is scary. But there is nothing scary about it. It is not intended to brutally shoot aliens from Mars. But it still knows how to “shoot” - with an electron beam at the screen.

How does it all work?

In general, a CRT is a large electron tube. How? You don't know what a lamp is? OK…

Electronic tubes- these are the same amplifying elements as the transistors we all love. But lamps appeared much earlier than their silicon “colleagues,” back in the first half of the last century.

Lamp- this is a glass cylinder from which the air has been pumped out.
The simplest lamp has 4 terminals: a cathode, an anode and two filament terminals. The filament is needed to heat the cathode. And the cathode needs to be heated in order for electrons to fly from it. And the electrons must fly in order for an electric current to arise through the lamp. To do this, a voltage of 6.3 or 12.6 V is usually applied to the filament (depending on the type of lamp)

In addition, in order for electrons to fly, a high voltage is needed between the cathode and the anode. It depends on the distance between the electrodes and the power of the lamp. In conventional radio tubes this voltage is several hundred volts; the distance from the cathode to the anode in such tubes does not exceed a few millimeters.
In a kinescope, the distance from the cathode located in the electron gun to the screen can exceed several tens of centimeters. Accordingly, much more tension is needed there - 15…30 kV.

Such brutal voltages are created by a special step-up transformer. It is also called a horizontal transformer because it operates at a horizontal frequency. But more on that later.

When an electron hits a screen, in addition to visible light, other radiations are also “knocked out”. In particular - radioactive. This is why it is not recommended to watch TV closer than 1...2 meters from the screen.

So, we received the beam. And it shines so beautifully right in the center of the screen. But we need it to “draw” lines on the screen. That is, you need to make it deviate from the center. And electromagnets will help you with this. The fact is that the electron beam, unlike the light beam, is very sensitive to magnetic field. That's why it is used in CRTs.

It is necessary to install two pairs of deflection coils. One pair will deflect horizontally, the other will deflect vertically. By skillfully controlling them, you can drive the beam anywhere on the screen.

And anywhere?

This is where we begin our story about dot lines and hooks...

The Tale of Stitches, Dots and Hooks

The picture on the TV screen is formed as a result of the fact that the beam draws at breakneck speed from left to right, top to bottom, across the screen. This method of sequential drawing of an image is called " scan".

Since the scanning occurs very quickly, for the eye all the points merge into lines and the lines into a single frame.

In PAL and SECAM systems, in one second the beam manages to run across the entire screen 50 times.
In the American NTSC system - even more - as many as 60 times! Generally speaking, the PAL and SECAM systems differ only in color reproduction. Everything else is the same for them.

The picture is formed due to the fact that during the “run” the beam changes its brightness in accordance with the received video signal. How is brightness controlled?

And it's very simple! The fact is that in addition to the electrodes considered - anode And cathode, in lamps there is also a third electrode - net. Net- this is the control electrode. By applying a relatively low voltage to the grid, the current flowing through the lamp can be controlled. In other words, you can control the intensity of the flow of electrons “flying” from the cathode to the anode.

In a CRT, a grid is used to change the brightness of the beam.

By applying a negative voltage to the grid (relative to the cathode), you can weaken the intensity of the electron flow in the beam, or even close the “road” for electrons. This may be necessary, for example, when moving a beam from the end of one line to the beginning of another.

Now let's talk in more detail about the principles of scanning.
To begin with, it’s worth remembering a few simple numbers and terms:

Raster- this is one “line” that the beam draws on the screen.
Field- these are all the lines that the beam drew in one vertical pass.
Frame- this is an elementary unit of video sequence. Each frame consists of two fields - even and odd.

This is worth explaining: the image on the TV screen rotates at a frequency of 50 fields per second. However, the television standard is 25 frames per second. Therefore, during transmission, one frame is divided into two fields - even and odd. The even field contains only the even lines of the frame (2,4,6,8...), the odd field contains only the odd ones. The image on the screen is also "drawn" across the line. This kind of development is called "interlace scanning".

It still happens" progressive scan" - when the entire frame is unfolded in one vertical stroke of the beam. It is used in computer monitors.

So, now the dry numbers. All numbers given are valid for PAL and SECAM systems.

Number of fields per second - 50
Number of lines per frame - 625
Number of effective lines per frame - 576
Number of effective dots per line - 720

And these numbers are derived from the above:

Number of lines in the field - 312.5
Line frequency - 15625 Hz
Duration of one line - 64 µS (including beam return)

Cathode-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 the technical parameters that 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.

Cathode ray tubes (CRT) - electrovacuum devices designed to convert an electrical signal into a light image using a thin electron beam directed onto a special screen covered phosphor- a composition capable of glowing when bombarded with electrons.

In Fig. Figure 15 shows the device of a cathode ray tube with electrostatic focusing and electrostatic beam deflection. The tube contains an oxide heated cathode with an emitting surface facing the hole in the modulator. A small negative potential is established on the modulator relative to the cathode. Further along the axis of the tube (and along the beam) there is a focusing electrode, also called the first anode; its positive potential helps to draw electrons from the near-cathode space through the modulator hole and form a narrow beam from them. Further focusing and acceleration of electrons is carried out by the field of the second anode (accelerating electrode). Its potential in the tube is most positive and ranges from units to tens of kilovolts. The combination of the cathode, modulator and accelerating electrode forms an electron gun (electronic spotlight). The inhomogeneous electric field in the space between the electrodes acts on the electron beam as a collecting electrostatic lens. Electrons under the influence of this lens converge to a point on inside screen. The inside of the screen is covered with a layer of phosphor - a substance that converts the energy of the electron flow into light. Outside, the place where the flow of electrons falls onto the screen glows.

To control the position of the luminous spot on the screen and thereby obtain an image, the electron beam is deflected along two coordinates using two pairs of flat electrodes - deflection plates X and Y. The angle of deflection of the beam depends on the voltage applied to the plates. Under the influence of variable deflecting voltages on the plates, the beam runs around different points on the screen. The brightness of the dot depends on the current strength of the beam. To control brightness, an alternating voltage is applied to the input of the modulator Z. To obtain a stable image of a periodic signal, it is periodically scanned on the screen, synchronizing the linearly varying horizontal scan voltage X with the signal under study, which is simultaneously supplied to the vertical deflection plates Y. In this way, images are formed on the screen CRT. The electron beam has low inertia.

In addition to electrostatic, it is also used magnetic focusing electron beam. It uses a direct current coil into which a CRT is inserted. The quality of magnetic focusing is higher (smaller spot size, less distortion), but magnetic focusing is bulky and continuously consumes energy.

Magnetic beam deflection, carried out by two pairs of coils with currents, is widely used (in picture tubes). In a magnetic field, an electron is deflected along the radius of a circle, and the deflection angle can be significantly larger than in a CRT with electrostatic deflection. However, the performance of the magnetic deflection system is low due to the inertia of the current-carrying coils. Therefore, in oscillographic tubes, exclusively electrostatic beam deflection is used as it has less inertia.

The screen is the most important part of a CRT. As electroluminophores Various inorganic compounds and their mixtures are used, for example, zinc and zinc-cadmium sulfides, zinc silicate, calcium and cadmium tungstates, etc. with admixtures of activators (copper, manganese, bismuth, etc.). The main parameters of the phosphor: glow color, brightness, spot luminous intensity, luminous efficiency, afterglow. The color of the glow is determined by the composition of the phosphor. Luminescent brightness in cd/m2

B ~ (dn/dt)(U-U 0) m,

where dn/dt is the flow of electrons per second, that is, the beam current, A;

U 0 - phosphor glow potential, V;

U – accelerating voltage of the second anode, V;

The light intensity of the spot is proportional to the brightness. Luminous efficiency is the ratio of the luminous intensity of the spot to the beam power in cd/W.

Afterglow– this is the time during which the brightness of the spot after turning off the beam decreases to 1% of the original value. There are phosphors with very short (less than 10 μs) afterglow, short (from 10 μs to 10 ms), medium (from 10 to 100 ms), long (from 0.1 to 16 s) and very long (more than 16 s) afterglow. The choice of the afterglow value is determined by the field of application of the CRT. For kinescopes, phosphors with low afterglow are used, since the image on the kinescope screen is constantly changing. For oscilloscope tubes, phosphors with a medium to very long persistence are used, depending on the frequency range of the signals to be displayed.

Important question, requiring more detailed consideration, is related to the potential of the CRT screen. When an electron hits the screen, it charges the screen with a negative potential. Each electron recharges the screen, and its potential becomes increasingly negative, so that a braking field very quickly arises, and the movement of electrons towards the screen stops. In real CRTs this does not happen, because each electron that hits the screen knocks out secondary electrons from it, that is, secondary electron emission occurs. Secondary electrons carry away a negative charge from the screen, and to remove them from the space in front of the screen, the inner walls of the CRT are covered with a carbon-based conductive layer, electrically connected to a second anode. In order for this mechanism to work, secondary emission factor, that is, the ratio of the number of secondary electrons to the number of primary ones must exceed one. However, for phosphors, the secondary emission coefficient Kve depends on the voltage at the second anode U a. An example of such a dependence is shown in Fig. 16, from which it follows that the screen potential should not exceed the value

U a max , otherwise the brightness of the image will not increase, but decrease. Depending on the phosphor material, the voltage U a max = 5...35 kV. To increase the limiting potential, the inside of the screen is covered with a thin film of metal (usually aluminum, permeable to electrons). aluminized screen) electrically connected to the second anode. In this case, the screen potential is determined not by the secondary emission coefficient of the phosphor, but by the voltage at the second anode. This allows you to use a higher voltage of the second anode and obtain more high brightness screen glow. The brightness of the glow also increases due to the reflection of light emitted into the tube from the aluminum film. The latter is transparent only to sufficiently fast electrons, so the voltage of the second anode must exceed 7...10 kV.

The service life of cathode ray tubes is limited not only by the loss of emission from the cathode, as with other vacuum devices, but also by the destruction of the phosphor on the screen. Firstly, the power of the electron beam is used extremely inefficiently. No more than two percent of it turns into light, while more than 98% only heats up the phosphor, and its destruction occurs, which is expressed in the fact that the luminous efficiency of the screen gradually decreases. Burnout occurs faster with an increase in the power of the electron flow, with a decrease in the accelerating voltage, and also more intensely in places where the beam falls longer time. Another factor that reduces the life of a cathode ray tube is the bombardment of the screen by negative ions generated from the atoms of the cathode oxide coating. Accelerated by the accelerating field, these ions move towards the screen, passing through the deflection system. In electrostatic deflection tubes, ions are deflected just as efficiently as electrons, so they hit different areas of the screen more or less evenly. In tubes with magnetic deflection, ions are deflected weaker due to their many times greater mass than electrons, and fall mainly into the central part of the screen, over time forming a gradually darkening so-called “ion spot” on the screen. Tubes with an aluminized screen are much less sensitive to ion bombardment, since the aluminum film blocks the path of ions to the phosphor.

The two most widely used types of cathode ray tubes are: oscillographic And kinescopes. Oscilloscope tubes are designed to display a variety of processes represented by electrical signals. They have electrostatic beam deflection because it allows the oscilloscope to display higher frequency signals. The beam focusing is also electrostatic. Typically, an oscilloscope is used in periodic sweep mode: a sawtooth voltage with a constant frequency ( sweep voltage), an amplified voltage of the signal under study is applied to the vertical deflection plates. If the signal is periodic and its frequency is an integer number of times higher than the sweep frequency, a stationary graph of the signal over time appears on the screen ( oscillogram). Modern oscilloscope tubes are more complex in design than the one shown in Fig. 15, they have a larger number of electrodes, they are also used double beam oscillographic CRTs, which have a double set of all electrodes with one common screen and allow you to display two different signals synchronously.

CRTs are CRTs with brightness mark, that is, with control of the brightness of the beam by changing the modulator potential; they are used in household and industrial televisions, as well as monitors computers to convert an electrical signal into a two-dimensional image on a screen. CRTs differ from oscillographic CRTs large sizes screen, the nature of the image ( halftone on the entire surface of the screen), the use of magnetic deflection of the beam along two coordinates, a relatively small size of the luminous spot, strict requirements for the stability of the spot size and the linearity of the scans. The most advanced are color picture tubes for computer monitors; they have a high resolution(up to 2000 lines), minimal geometric raster distortion, correct color rendition. IN different time kinescopes were produced with a diagonal screen size from 6 to 90 cm. The length of the kinescope along its axis is usually slightly less than the diagonal size, the maximum beam deflection angle is 110...116 0. The inside of a color picture tube screen is covered with many dots or narrow stripes of phosphors different compositions, converting an electric beam into one of three primary colors: red, green, blue. A color picture tube has three electron guns, one for each primary color. When scanned across the screen, the rays move in parallel and illuminate adjacent areas of the phosphor. The beam currents are different and depend on the color of the resulting image element. In addition to picture tubes for direct observation, there are projection picture tubes that, despite their small size, have a high brightness of the image on the screen. This bright image is then projected optically onto a flat White screen, obtaining a large image.