§ 137. Electronic ray tube. Oscilloscope
Oscilloscopes are used to observe, record, measure and control various changing processes in automation devices, telemechanics and other fields of technology (Fig. 198). The main part of the oscilloscope is a cathode ray tube - an electric vacuum device, in the most in simple form designed to convert electrical signals into light.
Let's consider how an electron and an electron beam are deflected in the electric field of a cathode ray tube of an oscilloscope.
If an electron is placed between two parallel plates (Fig. 199, a) having opposite electric charges, then under the influence electric field, arising between the plates, the electron will be deflected, since it is negatively charged. He pushes off the plate A, having a negative charge, and is attracted to the plate B having a positive electrical charge. The electron's movement will be directed along the field lines.
When a person moving at a speed enters the field between the plates V electron (Fig. 199, b), then it is acted upon not only by field forces F, but also strength F 1, directed along its movement. As a result of the action of these forces, the electron will deviate from its straight path and will move along the line OK. - diagonally.
If a narrow beam of moving electrons is passed between the plates - an electron beam (Fig. 199, c), it will deflect under the influence of an electric field. The angle of deflection of the electron beam depends on the speed of movement of the electrons that make up the beam and the magnitude of the voltage creating electric field between the plates.
Each cathode ray tube (Fig. 200) is a cylinder from which air has been pumped out. The conical part of the inner surface of the cylinder is covered with graphite and is called aquadag. Inside the cylinder 3
fits electronic spotlight 8
- electron gun, deflection plates 4
And 6
, and screen 5
. The electron tube illuminator consists of a heated cathode, which emits electrons, and a system of electrodes that form the electron beam. This beam, emitted by the cathode of the tube, moves with high speed towards the screen and is essentially an electric current directed towards the reverse movement electrons.
The cathode is a nickel cylinder, the end of which is coated with a layer of oxide. The cylinder is placed on a thin-walled ceramic tube, and a tungsten filament made in the form of a spiral is placed inside it to heat the cathode.
The cathode is located inside the control electrode 7
shaped like a cup. A small hole is made in the bottom of the cup through which electrons emitted from the cathode pass; this hole is called diaphragm. A small negative voltage (of the order of several tens of volts) is applied to the control electrode relative to the cathode. It creates an electric field that acts on the electrons emitted from the cathode so that they are collected into a narrow beam directed towards the tube screen. The point of intersection of electron flight trajectories is called first focus of the tube. By increasing the negative voltage on the control electrode, some electrons can be deflected so much that they will not pass through the hole and thus the number of electrons hitting the screen will decrease. By changing the voltage of the control electrode, you can regulate the number of electrons in it. This allows you to change the brightness of the luminous spot on the screen of the cathode ray tube, which is covered special composition, which has the ability to glow under the influence of an electron beam striking it.
The electron gun also includes two anodes that create an accelerating field: the first is focusing 1
and the second is the manager 2
. Each of the anodes is a cylinder with a diaphragm, which serves to limit cross section electron beam.
The anodes are located along the axis of the tube at a certain distance from one another. A positive voltage of the order of several hundred volts is applied to the first anode, and the second anode, connected to the aquadag of the tube, has a positive potential several times greater than the potential of the first anode.
Electrons escaping from the hole of the control electrode, entering the electric field of the first anode, acquire high speed. Flying inside the first anode, the electron beam is compressed under the influence of the electric field forces and forms a thin electron beam. Next, the electrons fly through the second anode, acquire an even higher speed (several thousand kilometers per second), and fly through the diaphragm to the screen. On the latter, under the influence of electron impacts, a luminous spot with a diameter of less than one millimeter is formed. This spot is located second focus cathode ray tube.
To deflect the electron beam in two planes, the cathode ray tube is equipped with two pairs of plates 6
And 4
, located in different planes perpendicular to one another.
First pair of plates 6
, which is located closer in the electron gun, serves to deflect the beam in the vertical direction; these plates are called vertically deflecting. Second pair of plates 4
, located closer to the tube screen, serves to deflect the beam in the horizontal direction; these plates are called horizontally deflecting.
Let's consider the principle of operation of deflecting plates (Fig. 201).
Deflection plates IN 2 and G 2 connected to potentiometer sliders P in and P d. A constant voltage is applied to the ends of the potentiometers. Deflection plates IN 1 and G 1, like the midpoints of the potentiometers, are grounded and their potentials are zero.
When the potentiometer sliders are in the middle position, the potential on all plates is zero, and the electron beam creates a luminous spot in the center of the screen - a point ABOUT. When moving the potentiometer slider P g left onto the plate G 2, a negative voltage is applied and therefore the electron beam, repelling from this plate, will deviate and the luminous point on the screen will shift in the direction of the point A.
When moving the potentiometer slider P g right plate potential G 2 the electron beam will increase and, consequently, the luminous point on the screen will shift horizontally to the point B. Thus, with a continuous change in the potential on the plate G 2 electron beam will be drawn on the screen horizontal line AB.
Similarly when changing with a potentiometer P in the voltage on the vertical deflection plates, the beam will deflect vertically and draw a vertical line on the screen VG. By simultaneously changing the voltage on both pairs of deflection plates, the electron beam can be moved in any direction.
The screen of a cathode ray tube is coated with a special compound - a phosphor that can glow under the influence of impacts from rapidly flying electrons. Thus, when a focused beam hits one or another point on the screen, it begins to glow.
To cover the screens of cathode ray tubes, phosphors are used in the form of zinc oxide, beryllium zinc, a mixture of zinc sulfate with cadmium sulfate, etc. These materials have the property of continuing to glow for some time after the electron impacts have stopped. This means that they have afterglow.
It is known that the human eye, having received a visual impression, can hold it for about 1/16 of a second. In a cathode ray tube, the beam can move across the screen so quickly that a number of successive luminous points on the screen are perceived by the eye as a continuous luminous line.
The voltage to be studied (considered) using an oscilloscope is applied to the vertical deflection plates of the tube. A sawtooth voltage is applied to the horizontal deflection plates, the graph of which is shown in Fig. 202, a.
This voltage is supplied by an electronic sawtooth pulse generator, which is mounted inside the oscilloscope. Under the influence of a sawtooth voltage, the electron beam moves horizontally across the screen. During t 1 - t 8 the beam moves across the screen from left to right, and over time t 9 - t 10 quickly returns to its original position, then moves again from left to right, etc.
Let's find out how you can see on the screen of a cathode ray tube of an oscilloscope the shape of the curve of instantaneous voltage values supplied to the vertical deflection plates. Let us assume that a sawtooth voltage with an amplitude of 60 is applied to the horizontal deflection tubes V and with a change period of 1/50 sec.
In Fig. Figure 202, b shows one period of sinusoidal voltage, the shape of the curve of which we want to see, and the circle (Fig. 202, c) shows the resulting movement of the electron beam on the screen of the oscilloscope tube.
Voltages at the same instants have the same designations on the top two graphs.
At a moment in time t 1 sawtooth voltage ( U d), deflecting the electron beam horizontally, is equal to 60 V, and the stress on the vertical plates U equals zero and a dot lights up on the screen O 1 . At a moment in time t 2 voltage U g = - 50 V, and the voltage U in = 45 V. In a time equal to t 2 - t 1, the electron beam will move to position O 2 on line O 1 - O 2. At a moment in time t 3 voltage U g = 35 V, and the voltage U in = 84.6 V. During t 3 - t 2 beam will move to the point O 3 on line O 2 - O 3, etc.
The process of action of the electric fields created by both pairs of deflection plates on the electron beam will continue, and the beam will be deflected further along the line O 3 - O 4 - o 6, etc.
During t 10 - t 9, the electron beam will quickly deviate to the left (the beam will reverse), and then the process will be repeated: The voltage being tested changes periodically, so the electron beam will move repeatedly along the same path, resulting in a fairly bright line, similar in shape to the shape of the voltage curve applied to the vertical deflection plates of the tube.
Since the period (and frequency) of the voltages of the sawtooth sweep pulses and the voltage under study are equal, the sinusoid on the screen will be motionless. If the frequency of these voltages is different and not a multiple of each other, then the image will move along the tube screen.
When two sinusoidal voltages of equal amplitudes and frequencies, but shifted in phase by 90°, are connected to both pairs of deflection plates, a circle will be visible on the tube screen. Thus, using an oscilloscope, you can observe and examine various processes occurring in electrical circuits. In addition to the sawtooth pulse generator, the oscilloscope has amplifiers to amplify the voltage applied to the vertical beam deflection plates and the sawtooth voltage applied to the horizontal deflection plates.
The student should know : block diagram of an oscilloscope; purpose of the main blocks of the oscilloscope; device and principle of operation of a cathode ray tube; operating principle of a sweep generator (sawtooth voltage), addition of mutually perpendicular oscillations.
The student must be able to : determine experimentally the price of dividing horizontally and vertically, measure the magnitude of direct voltage, period, frequency and amplitude of alternating voltage.
Brief theory Oscilloscope structure
An electronic oscilloscope is a universal device that allows you to monitor fast electrical processes (lasting up to 10-12 s). Using an oscilloscope, you can measure voltage, current, time intervals, and determine the phase and frequency of alternating current.
Because Since potential differences arise in the functioning nerves and muscles of living organisms, the electronic oscilloscope or its modifications are widely used in biological and medical studies of the functioning of various organs, the heart, nervous system, eyes, stomach, etc.
The device can be used to observe and measure non-electrical quantities if special primary transducers are used.
There are no moving mechanical parts in the oscilloscope (see Fig. 1), but the electron beam is deflected in an electric or magnetic field. A narrow beam of electrons hitting a screen coated with a special composition causes it to glow at that point. When a beam of electrons moves, you can follow it by the movement of a luminous dot on the screen.
The electron beam “monitors” the change in the electric field being studied, keeping pace with it, because the electron beam is practically inertia-free.
Rice. 1. Fig. 2.
Structure of cathode ray tube Cathode and modulator
In that great dignity electronic oscilloscope compared to other recording instruments.
A modern electronic oscilloscope has the following main components: a cathode ray tube (CRT), a scan generator, amplifiers, and a power supply.
Design and operation of a cathode ray tube
Let us consider the device of a cathode ray tube with electrostatic focusing and electrostatic control of the electron beam.
CRT, schematically shown in Fig. 1, is a specially shaped glass flask in which a high vacuum is created (about 10 -7 mm Hg). Inside the flask there are electrodes that perform the function of an electron gun to produce a narrow beam of electrons; beam-deflecting plates and a screen covered with a layer of phosphor.
The electron gun consists of a cathode 1, a control (modulating) electrode 2, an additional shielding electrode 3 and the first and second anodes 4, 5.
Heating cathode 1 is made in the form of a small nickel cylinder, inside of which there is a filament; it has an oxide layer on the front end with a low electron work function for obtaining electrons (Fig. 2).
The cathode is located inside the control electrode or modulator, which is a metal cup with a hole in the end through which electrons can pass. The control electrode has a negative potential relative to the cathode and, by changing the value of this potential, you can regulate the intensity of the flow of electrons passing through its hole and thereby change the brightness of the screen. At the same time, the electric field between the cathode and the modulator focuses the electron beam (Fig. 2).
The shielding electrode 3 has a potential slightly higher than the cathode potential and serves to facilitate the exit of electrons, eliminating the interaction of the electric fields of the control electrode 2 and the first anode 4.
Additional focusing and acceleration of electrons occurs by the electric field between the first and second anodes, forming an electron lens. These anodes are made in the form of cylinders with diaphragms inside. The first anode 4 is supplied with a positive potential relative to the cathode of the order of hundreds of volts, and the second 5 of the order of a thousand volts. The electric field strength lines between these anodes are shown in Fig. 3.
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 forced 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 AC voltage, and to the vertical plates - a linear voltage of the same frequency.
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 of the magnetic field of a 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). By passing alternating currents of the appropriate shape through such magnets, they 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 to withstand than flat sections. 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.
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 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 you apply to these plates alternating current, then 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.
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 electrons must fly then to arise electricity 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. feeding on the grid comparatively low voltage, you can control the current flowing through the lamp. 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)
