Work tasks
- general acquaintance with the device and the principle of operation of electronic oscilloscopes,
- determination of the sensitivity of the oscilloscope,
- making some measurements in an alternating current circuit using an oscilloscope.
General information about the design and operation of an electronic oscilloscope
Using the cathode of the cathode ray tube of the oscilloscope, an electron flow is created, which is formed in the tube into a narrow beam directed towards the screen. An electron beam focused on the screen of the tube causes a luminous spot at the point of impact, the brightness of which depends on the energy of the beam (the screen is covered with a special luminescent compound that glows under the influence of the electron beam). The electron beam is practically inertialess, so the light spot can be moved almost instantly in any direction on the screen if the electron beam is exposed to an electric field. The field is created using two pairs of plane-parallel plates called deflection plates. The small inertia of the beam makes it possible to observe fast-changing processes with a frequency of 10 9 Hz or more.
Considering the existing oscilloscopes, which are diverse in design and purpose, you can see that their functional diagram is approximately the same. The main and mandatory nodes should be:
Cathode-ray tube for visual observation of the process under study;
Power supplies to obtain the necessary voltages applied to the electrodes of the tube;
A device for adjusting the brightness, focusing and shifting of the beam;
Sweep generator for moving the electron beam (and, accordingly, the luminous spot) across the tube screen at a certain speed;
Amplifiers (and attenuators) used to amplify or attenuate the voltage of the signal under study, if it is not enough to noticeably deflect the beam on the tube screen or, on the contrary, is too high.
Cathode Ray Tube Device
First of all, consider the design of a cathode ray tube (Fig. 36.1). Usually it is a glass flask 3, evacuated to a high vacuum. A heated cathode 4 is located in its narrow part, from which electrons fly out due to thermionic emission. A system of cylindrical electrodes 5, 6, 7 focuses electrons into a narrow beam 12 and controls its intensity. This is followed by two pairs of deflecting plates 8 and 9 (horizontal and vertical) and, finally, a screen 10 - the bottom of the flask 3, coated with a luminescent composition, due to which the trace of the electron beam becomes visible.
The cathode includes a tungsten filament - heater 2, located in a narrow tube, the end of which (to reduce the electron work function) is covered with a layer of barium or strontium oxide and is actually a source of electron flow.
The process of forming electrons into a narrow beam using electrostatic fields is in many ways similar to the action of optical lenses on a light beam. Therefore, the system of electrodes 5,6,7 is called an electron-optical device.
Electrode 5 (modulator) in the form of a closed cylinder with a narrow hole is under a small negative potential relative to the cathode and performs functions similar to the control grid of an electron lamp. By changing the value of the negative voltage on the modulating or control electrode, you can change the number of electrons passing through its hole. Therefore, using a modulating electrode, it is possible to control the brightness of the beam on the screen. The potentiometer that controls the magnitude of the negative voltage on the modulator is displayed on the front panel of the oscilloscope with the inscription “brightness”.
A system of two coaxial cylinders 6 and 7, called the first and second anodes, serves to accelerate and focus the beam. The electrostatic field in the gap between the first and second anodes is directed in such a way that it deflects the diverging electron trajectories back to the axis of the cylinder, just as an optical system of two lenses acts on a diverging light beam. In this case, the cathode 4 and the modulator 5 constitute the first electronic lens, and another electronic lens corresponds to the first and second anodes.
As a result, the electron beam is focused at a point that should lie in the plane of the screen, which is possible with an appropriate choice of potential difference between the first and second anodes. The potentiometer knob that regulates this voltage is displayed on the front panel of the oscilloscope with the inscription “focus”.
When an electron beam hits the screen, a sharply outlined luminous spot (corresponding to the beam cross section) is formed on it, the brightness of which depends on the number and speed of electrons in the beam. Most of the beam energy is converted into heat when the screen is bombarded. In order to avoid burning through the luminescent coating, high brightness is not allowed with a stationary electron beam. The deflection of the beam is carried out using two pairs of plane-parallel plates 8 and 9, located at right angles to each other.
If there is a potential difference on the plates of one pair, a uniform electric field between them deflects the trajectory of the electron beam, depending on the magnitude and sign of this field. Calculations show that the amount of beam deflection on the tube screen D(in millimeters) is related to the stress on the plates U D and voltage at the second anode Ua 2(in volts) as follows:
(36.1),
A cathode ray tube (CRT) uses a beam of electrons from a heated cathode to reproduce an image on a fluorescent screen. The cathode is made of oxide, with indirect heating, in the form of a cylinder with a heater. The oxide layer is deposited on the bottom of the cathode. Around the cathode is a control electrode, called a modulator, of a cylindrical shape with a hole in the bottom. This electrode serves to control the density of the electron beam and to pre-focus it. A negative voltage of several tens of volts is applied to the modulator. The higher this voltage, the more electrons return to the cathode. Other electrodes, also cylindrical, are anodes. There are at least two of them in a CRT. On the second anode, the voltage is from 500 V to several kilovolts (about 20 kV), and on the first anode, the voltage is several times less. Inside the anodes there are partitions with holes (diaphragms). Under the action of the accelerating field of the anodes, the electrons acquire a significant speed. The final focusing of the electron beam is carried out using a non-uniform electric field in the space between the anodes, as well as due to diaphragms. A system consisting of a cathode, modulator and anodes is called an electron searchlight (electron gun) and serves to create an electron beam, i.e. a thin stream of electrons flying at high speed from the second anode to the luminescent screen. An electronic searchlight is placed in the narrow neck of the CRT bulb. This beam is deflected by an electric or magnetic field, and the intensity of the beam can be changed by means of a control electrode, thereby changing the brightness of the spot. The luminescent screen is formed by applying a thin layer of phosphor on the inner surface of the end wall of the conical part of the CRT. The kinetic energy of the electrons bombarding the screen is converted into visible light.
CRT With electrostatic control.
Electric fields are commonly used in small screen CRTs. In electric field deflection systems, the field vector is oriented perpendicular to the initial beam path. The deflection is carried out by applying a potential difference to a pair of deflecting plates (Figure below). Typically, deflection plates make the deflection in the horizontal direction proportional to time. This is achieved by applying a voltage to the deflecting plates, which increases uniformly as the beam travels across the screen. Then this voltage quickly drops to its original level and again begins to increase evenly. The signal to be investigated is applied to the plates deflecting in the vertical direction. If the duration of a single horizontal sweep is equal to the period or corresponds to the frequency of the signal, the screen will continuously display one period of the wave process.
1 - CRT screen, 2 - cathode, 3 - modulator, 4 - first anode, 5 - second anode, P - deflecting plates.
CRT with electromagnetic control
In cases where a large deflection is required, the use of an electric field to deflect the beam becomes inefficient.
Electromagnetic tubes have an electron gun, the same as electrostatic tubes. The difference is that the voltage at the first anode does not change, and the anodes are only there to speed up the electron flow. Magnetic fields are required to deflect the beam in television CRTs with large screens.
Focusing of the electron beam is carried out using a focusing coil. The focusing coil has an ordinary winding and is put on directly on the tube flask. The focusing coil creates a magnetic field. If the electrons move along the axis, then the angle between the velocity vector and the magnetic field lines will be equal to 0, therefore, the Lorentz force is equal to zero. If an electron flies into the magnetic at an angle, then due to the Lorentz force, the trajectory of the electron will deviate towards the center of the coil. As a result, all electron trajectories will intersect at one point. By changing the current through the focusing coil, you can change the location of this point. Achieve that this point was in the plane of the screen. The beam is deflected using magnetic fields generated by two pairs of deflecting coils. One pair is vertical deflection coils, and the other is coils in such a way that their magnetic lines of force on the center line will be mutually perpendicular. Coils have a complex shape and are located on the neck of the tube.
When using magnetic fields to deflect the beam at large angles, the CRT turns out to be short, and also allows you to make screens of large sizes.
kinescopes.
Kinescopes are combined CRTs, that is, they have electrostatic focusing and electromagnetic beam deflection to increase sensitivity. The main difference between kinescopes and CRTs is the following: the electron gun of kinescopes has an additional electrode, which is called an accelerating electrode. It is located between the modulator and the first anode, a positive voltage of several hundred volts relative to the cathode is applied to it, and it serves to additionally accelerate the electron flow.
Schematic device of a kinescope for black-and-white television: 1- thread of the cathode heater; 2- cathode; 3- control electrode; 4- accelerating electrode; 5- first anode; 6- second anode; 7 - conductive coating (aquadag); 8 and 9 - coils for vertical and horizontal deflection of the beam; 10 - electron beam; 11 - screen; 12 - output of the second anode.
The second difference is that the kinescope screen, unlike the CRT, is three-layered:
1 layer - outer layer - glass. The glass of the kinescope screen is subject to increased requirements for the parallelism of the walls and the absence of foreign inclusions.
Layer 2 is a phosphor.
Layer 3 is a thin aluminum film. This film has two functions:
Increases the brightness of the screen, acting like a mirror.
The main function is to protect the phosphor from heavy ions that fly out of the cathode along with electrons.
Colored kinescopes.
The principle of operation is based on the fact that any color and shade can be obtained by mixing three colors - red, blue and green. Therefore, color kinescopes have three electron guns and one common deflection system. The screen of a color kinescope consists of separate sections, each of which contains three phosphor cells that glow in red, blue and green. Moreover, the sizes of these cells are so small and they are located so close to each other that their glow is perceived by the eye as a total. This is the general principle of building color kinescopes.
Mosaic (triads) of a color kinescope screen with a shadow mask: R - red, G - green, B - blue phosphor "dots".
Electrical conductivity of semiconductors
Intrinsic conductivity of semiconductors.
An intrinsic semiconductor is a perfectly chemically pure semiconductor with a homogeneous crystal lattice in the valence orbit of which there are four electrons. Silicon is most commonly used in semiconductor devices. Si and germanium Ge.
The electron shell of a silicon atom is shown below. Only four electrons of the outer shell, called valence electrons, can participate in the formation of chemical bonds and in the conduction process. Ten internal electrons are not involved in such processes.
The crystal structure of a semiconductor in a plane can be represented as follows.
If an electron has received an energy greater than the band gap, it breaks the covalent bond and becomes free. In its place, a vacancy is formed, which has a positive charge equal in magnitude to the electron charge and is called hole. In a chemically pure semiconductor, the electron concentration n is equal to the hole concentration p.
The process of formation of a pair of electron and hole charges is called charge generation.
A free electron can take the place of a hole, restoring a covalent bond and, in doing so, radiating an excess of energy. This process is called charge recombination. In the process of recombination and generation of charges, the hole, as it were, moves in the opposite direction from the direction of electron movement; therefore, the hole is considered to be a mobile positive charge carrier. Holes and free electrons resulting from the generation of charge carriers are called intrinsic charge carriers, and the conductivity of a semiconductor due to its own charge carriers is called intrinsic conductivity of the conductor.
Impurity conductivity of conductors.
Since the conductivity of chemically pure semiconductors depends significantly on external conditions, doped semiconductors are used in semiconductor devices.
If a pentavalent impurity is introduced into a semiconductor, then 4 valence electrons restore covalent bonds with semiconductor atoms, and the fifth electron remains free. Due to this, the concentration of free electrons will exceed the concentration of holes. admixture, due to which n> p, is called donor impurity. A semiconductor that n> p, is called a semiconductor with an electronic type of conductivity, or a semiconductor n-type.
in semiconductor n-type electrons are called majority charge carriers and holes are called minority charge carriers.
When a trivalent impurity is introduced, three of its valence electrons restore a covalent bond with semiconductor atoms, and the fourth covalent bond is not restored, i.e., a hole occurs. As a result, the hole concentration will be greater than the electron concentration.
The impurity at which p> n, is called acceptor impurity.
A semiconductor that p> n, is called a semiconductor with a hole type of conductivity, or a semiconductor p-type. in semiconductor p-type holes are called majority charge carriers and electrons are called minority charge carriers.
Formation of an electron-hole transition.
Due to uneven concentration at the interface R And n semiconductor, a diffusion current arises, due to which electrons from n- areas move into p-region, and uncompensated charges of positive ions of the donor impurity remain in their place. Electrons arriving in the p-region recombine with holes, and uncompensated charges of negative ions of the acceptor impurity arise. Width R-n transition - tenths of a micron. At the interface, an internal electric field of the p-n junction arises, which will be retarding for the main charge carriers and will reject them from the interface.
For minority charge carriers, the field will be accelerating and will transfer them to the region where they will be the main ones. The maximum electric field strength is at the interface.
The distribution of the potential across the width of the semiconductor is called the potential diagram. Potential difference on R-n transition is called contact difference potentials or potential barrier. In order for the main charge carrier to overcome R-n transition, its energy must be sufficient to overcome the potential barrier.
Direct and reverse inclusion p-ntransition.
We apply an external voltage plus to R- areas. The external electric field is directed towards the internal field R-n transition, which leads to a decrease in the potential barrier. The main charge carriers can easily overcome the potential barrier, and therefore, through R-n the junction will flow a relatively large current caused by the majority charge carriers.
Such inclusion R-n transition is called direct, and the current through R-n the transition caused by majority charge carriers is also called forward current. It is believed that with direct connection R-n transition is open. If you connect an external voltage with a minus to p-region, and plus on n-region, then an external electric field arises, the intensity lines of which coincide with the internal field R-n transition. As a result, this will increase the potential barrier and the width R-n transition. Major charge carriers will not be able to overcome R-n transition, and it is considered that R-n the transition is closed. Both fields - both internal and external - are accelerating for minority charge carriers, so minority charge carriers will pass through R-n junction, producing a very small current called reverse current. Such inclusion R-n transition is also called reverse.
Properties p-ntransition.Current-voltage characteristic p-ntransition
Back to main features R-n transitions include:
- the property of one-way conduction;
Temperature Properties R-n transition;
Frequency properties R-n transition;
Breakdown R-n transition.
Property of one-way conduction R-n consider the transition on the current-voltage characteristic.
The current-voltage characteristic (CVC) is a graphically expressed dependence of the value of the current flowing through R-n transition of current from the magnitude of the applied voltage I= f(U) - Fig.29.
Since the magnitude of the reverse current is many times less than the direct current, the reverse current can be neglected and assumed that R-n The junction conducts current in one direction only. temperature property R-n transition shows how work changes R-n transition with temperature change. On R-n the transition is largely affected by heating, to a very small extent - cooling. With an increase in temperature, the thermal generation of charge carriers increases, which leads to an increase in both forward and reverse current. Frequency properties R-n transitions show how it works R-n transition when a high-frequency alternating voltage is applied to it. Frequency properties R-n junctions are defined by two kinds of junction capacitance.
The first type of capacitance is the capacitance due to the immobile charges of the ions of the donor and acceptor impurities. It is called the charging or barrier capacitance. The second type of capacitance is the diffusion capacitance due to the diffusion of mobile charge carriers through R-n direct transition.
If on R-n junction to supply alternating voltage, then the capacitance R-n transition will decrease with increasing frequency, and at some high frequencies, the capacitance may become equal to the internal resistance R-n transition with direct connection. In this case, when switched back on, a sufficiently large reverse current will flow through this capacitance, and R-n the transition will lose the property of one-way conduction.
Conclusion: the smaller the capacitance value R-n transition, the higher frequencies it can operate.
The barrier capacitance has the main effect on the frequency properties, since the diffusion capacitance occurs with direct connection, when the internal resistance R-n little transition.
Breakdown p-ntransition.
With an increase in the reverse voltage, the energy of the electric field becomes sufficient to generate charge carriers. This leads to a strong increase in reverse current. The phenomenon of a strong increase in reverse current at a certain reverse voltage is called electrical breakdown. R-n transition.
Electrical breakdown is a reversible breakdown, that is, with a decrease in reverse voltage R-n the transition restores the property of one-way conduction. If the reverse voltage is not reduced, then the semiconductor will become very hot due to the thermal effect of the current and R-n transition is on fire. This phenomenon is called thermal runaway. R-n transition. Thermal breakdown is irreversible.
Semiconductor diodes
A semiconductor diode is a device consisting of a semiconductor crystal, usually containing one p-n junction and having two terminals. There are many different types of diodes - rectifier, pulse, tunnel, inverted, microwave diodes, as well as zener diodes, varicaps, photodiodes, LEDs, etc.
Diode marking consists of 4 designations:
K C -156 A
After the deflecting system, the electrons enter the CRT screen. The screen is a thin layer of phosphor deposited on 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 aluminum layer is deposited over the phosphor layer. Screen properties are determined by its
characteristics and settings. The main screen options are: first And second critical screen potentials, glow brightness, light output, 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 walls of the balloon tube near the screen are covered 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 decelerating 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 decelerating field from the surface of a non-conductive screen, it is necessary to remove the electric charge carried by the electron beam. Almost the only way to compensate for the charge is to use secondary emission. When electrons fall on the screen, their kinetic energy is converted into the energy of the screen glow, goes to heat it and causes secondary emission. The value of the secondary emission coefficient o determines the potential of the screen. The coefficient of secondary electron emission a \u003d / in // l (/ „ is the current of secondary electrons, / l is the current of the beam, or the current of primary electrons) from the screen surface in a wide range of changes in the energy of primary electrons exceeds one (Fig. 12.8, O < 1 на участке O A curve at V < С/ кр1 и при 15 > C/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\u003d Г / kr corresponding to point A in fig. 12.8, called the first critical potential.
At C/a2 = £/cr1, the screen potential is close to zero.
If the beam energy becomes greater than e£/cr1, then about > 1 and the screen starts charging half-
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 variation from e£/cr1 to C/cr2 c > 1 and the screen potential is quite close to the projector anode potential. At and &2> N cr2 coefficient of secondary emission 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) are called second critical potential or ultimate potential.
At energies of the electron beam above e11 kr2 The brightness of the screen does not increase. For various screens G/ kr1 = = 300...500 V, and cr2= 5...40 kV.
If it is necessary to obtain high brightness, the screen potential is forcibly maintained equal to the potential of the last spotlight electrode 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 normally to the screen surface, to the power of the electron beam P el incident on the screen:
Light output ts determines the efficiency of the phosphor. Not all of the kinetic energy of primary electrons is converted into the energy of visible radiation, part of it goes to heating the screen, secondary emission of electrons and radiation in the infrared and ultraviolet ranges of the spectrum. Light output is measured in candelas per watt: for various screens, it varies between 0.1 ... 15 cd / W. At low electron velocities, luminescence occurs in the surface layer and part of the light is absorbed by the phosphor. As the energy of the electrons increases, the light output increases. However, at very high speeds, many electrons penetrate the phosphor layer without producing excitation, and the light output decreases.
Glow brightness. This is a parameter that is determined by the intensity of light emitted in the direction of the observer by one square meter of a uniformly luminous surface. Luminance is measured in cd/m 2 . It depends on the properties of the phosphor (characterized by the 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 screen luminescence is still observed. The brightness of the glow obeys the law
Exponent values p y potential £/ 0 for different phosphors vary within 1...2.5, respectively, and
30 ... 300 V. In practice, the linear nature of the dependence of brightness on current density y remains approximately up to 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 property of a CRT to reproduce image details. The resolution is estimated by the number of separately distinguishable luminous dots or lines (lines) corresponding to 1 cm 2 of the surface or 1 cm of the screen height, or to the entire height of the screen working surface, respectively. Consequently, to increase the resolution, it is necessary to reduce the beam diameter, i.e., a well-focused thin beam with a diameter of tenths of a mm is required. The resolution is the higher, the lower the beam current and the higher the accelerating voltage. In this case, the best focusing is realized. Resolution also depends on the quality of the phosphor (large phosphor grains scatter light) and the presence of halos due to total internal reflection in the glass part of the screen.
Afterglow duration. The time during which the brightness of the glow decreases to 1% of the maximum value is called the screen persistence 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 H.Lb s) and very long (more than 16 s) afterglow. Tubes with short and very short afterglow are widely used in oscillography, and with medium afterglow - in television. Radar displays typically use tubes with a long afterglow.
In radar tubes, long-lasting screens with a two-layer coating are often used. The first layer of the phosphor - with a short blue afterglow - is excited by an electron beam, and the second - with a yellow glow and a long afterglow - is excited by the light of the first layer. In such screens, it is possible to obtain an afterglow of up to several minutes.
Screen types. The color of the glow of the phosphor is very important. In oscillographic technology, when visually observing the screen, a CRT with a green glow is used, which is the least tiring for the eye. Zinc orthosilicate activated with manganese (willemite) has this luminescence color. For photography, screens with a blue glow characteristic of calcium tungstate are preferred. In receiving television tubes with a black and white image, they try to get a white color, for which phosphors from two components are used: 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, they are quite effective, they have a narrower spectral emission band, which is especially important in the production of color picture tubes, where high color purity is required, etc. An example is the relatively widely used phosphor based on yttrium oxide activated with europium Y 2 0 3: Eu. This phosphor has a narrow emission band in the red region of the spectrum. A phosphor consisting of yttrium oxysulfide with an admixture of europium Y 2 0 3 8: Eu also has good characteristics, which has a maximum radiation intensity in the red-orange region of the visible region of the spectrum and better chemical resistance than the 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 energies less than 6 keV, therefore, in these cases, the light output drops sharply. For example, the light output of an aluminized screen at an electron energy of 10 keV is about 60% greater than at 5 keV. Tube screens are rectangular or round.
cathode ray tubes(CRT) - electrovacuum devices designed to convert an electrical signal into a light image using a thin electron beam directed to a special screen covered with phosphor- a composition capable of glowing when bombarded with electrons.
On fig. 15 shows the device of a cathode ray tube with an 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 set on the modulator relative to the cathode. Further along the axis of the tube (and along the beam) is a focusing electrode, also called the first anode, its positive potential contributes to the extraction of electrons from the near-cathode space through the modulator hole and the formation of 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 the most positive and amounts to units - tens of kilovolts. The combination of the cathode, modulator and accelerating electrode forms an electron gun (electron searchlight). The inhomogeneous electric field in the space between the electrodes acts on the electron beam as a collective electrostatic lens. Electrons under the action of this lens converge to a point on the inner side of the screen. The screen is covered from the inside with a layer of phosphor - a substance that converts the energy of the electron flow into light. Outside, the place where the electron flow falls on 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 deflection angle of the beam depends on the voltage applied to the plates. Under the action of alternating deflecting stresses on the plates, the beam goes around different points on the screen. The brightness of the dot glow depends on the strength of the beam current. To control the 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 changing horizontal scan voltage X with the signal under study, which simultaneously enters the vertical deflection plates Y. In this way, images are formed on the screen CRT. The electron beam has a low inertia.
In addition to electrostatic, it is also used magnetic focus electron beam. For it, a DC coil is used, into which a CRT is inserted. The quality of magnetic focusing is higher (smaller spot size, less distortion), but magnetic focusing is cumbersome and continuously consumes power.
Widely used (in kinescopes) is the magnetic deflection of the beam, carried out by two pairs of coils with currents. In a magnetic field, the electron is deflected along the radius of the circle, and the deflection angle can be much larger than in a CRT with electrostatic deflection. However, the speed of the magnetic deflection system is low due to the inertia of the current-carrying coils. Therefore, in oscilloscope tubes, only electrostatic beam deflection is used as less inertial.
The screen is the most important part of the 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 impurities of activators (copper, manganese, bismuth, etc.). The main parameters of the phosphor: glow color, brightness, spot light intensity, light output, afterglow. The color of the glow is determined by the composition of the phosphor. The brightness of the glow of the phosphor in Cd / m 2
B ~ (dn/dt)(U-U 0) m ,
where dn/dt is the electron flow per second, that is, the beam current, A;
U 0 - glow potential of the phosphor, V;
U is the accelerating voltage of the second anode, V;
The light intensity of the spot is proportional to the brightness. Luminous efficacy is the ratio of the luminous intensity of the spot to the power of the beam in cd/W.
afterglow- this is the time during which the brightness of the spot after turning off the beam drops to 1% of the original value. Phosphors are distinguished with a very short (less than 10 μs) afterglow, with a 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. afterglow. The choice of the afterglow value is determined by the scope of the CRT. For kinescopes, phosphors with a small afterglow are used, since the image on the kinescope screen is constantly changing. For oscilloscope tubes, phosphors are used with a medium to very long afterglow, depending on the frequency range of the signals to be displayed.
An important issue that requires 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 more and more negative, so that a decelerating 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 secondary electrons out of it, that is, secondary electron emission takes place. 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 conductive layer based on carbon, electrically connected to the second anode. 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 potential of the screen 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 screen is covered from the inside with a thin film of metal permeable to electrons (usually aluminum - 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 get a higher brightness of the screen. The brightness of the glow also increases due to the reflection of light emitted inside the tube from the aluminum film. The latter is transparent only for 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 electrovacuum devices, but also by the destruction of the phosphor on the screen. First, 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 heat the phosphor, while its destruction occurs, which is expressed in the fact that the light output 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 intense in places where the beam falls for a longer time. Another factor that reduces the service life of a cathode ray tube is the bombardment of the screen by negative ions formed from the atoms of the cathode oxide coating. Accelerated by the accelerating field, these ions move towards the screen, passing through the deflecting system. In electrostatic deflection tubes, ions are deflected just as efficiently as electrons, so they hit different parts of the screen more or less evenly. In tubes with magnetic deflection, ions are deflected more weakly due to their many times greater mass than electrons, and fall mainly into the central part of the screen, eventually 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.
There are two types of cathode ray tubes most widely used: oscilloscope And kinescopes. Oscilloscope tubes are designed to display a variety of processes represented by electrical signals. They have electrostatic beam deflection as it allows the oscilloscope to display higher frequency signals. Beam focusing is also electrostatic. Typically, the oscilloscope is used in a 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 the sweep frequency, a stationary graph of the signal over time appears on the screen ( waveform). Modern oscilloscope tubes are more complex in design than the one shown in Fig. 15, they have more electrodes, also apply two-beam oscillographic CRTs that have a double set of all electrodes with one common screen and allow you to display two different signals synchronously.
Kinescopes are CRTs with brightness mark, that is, with beam brightness control 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. From oscillographic CRTs, kinescopes differ in their large screen sizes, the nature of the image ( halftone on the entire surface of the screen), the use of magnetic deflection of the beam in two coordinates, the relatively small size of the luminous spot, stringent requirements for the stability of the spot size and the linearity of scans. The most perfect are color kinescopes for computer monitors, they have high resolution (up to 2000 lines), minimal geometric distortion of the raster, and correct color reproduction. At different times, kinescopes were produced with a diagonal screen size of 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 screen of a color kinescope is covered from the inside with many dots or narrow strips of phosphors of various compositions that convert an electric beam into one of three primary colors: red, green, blue. There are three electron guns in a color kinescope, one for each primary color. When scanning 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 kinescopes for direct observation, there are projection kinescopes, which, with their small size, have a high image brightness on the screen. This bright image is then optically projected onto a flat white screen, resulting in a large image.
How does a cathode ray tube work?
Cathode-ray tubes are vacuum devices in which an electron beam of small cross section is formed, and the electron beam can deviate in the desired direction and, hitting the luminescent screen, cause it to glow (Fig. 5.24). A cathode ray tube is an electron-optical converter that converts an electrical signal into its corresponding image in the form of a pulsed waveform, which is reproduced on the screen of the tube. The electron beam is formed in an electron projector (or electron gun) consisting of a cathode and focusing electrodes. The first focusing electrode, also called modulator, performs the functions of a grid with a negative bias that guides the electrons to the axis of the tube. Changing the bias voltage of the grid affects the number of electrons and, consequently, the brightness of the image obtained on the screen. Behind the modulator (toward the screen) are the following electrodes, whose task is to focus and accelerate the electrons. They operate on the principle of electronic lenses. Focusing accelerating electrodes are called anodes and a positive voltage is applied to them. Depending on the type of tube, the anode voltages range from several hundred volts to several tens of kilovolts.
Rice. 5.24. Schematic representation of a cathode ray tube:
1 - cathode; 2 - anode I: 3 - anode II; 4 - horizontal deflecting plates; 5 - electron beam; 6 - screen; 7 - vertical deflecting plates; 8 - modulator
In some tubes, the beam is focused using a magnetic field by using coils located outside the lamp, instead of electrodes located inside the tube and creating a focusing electric field. Beam deflection is also carried out by two methods: using an electric or magnetic field. In the first case, deflecting plates are placed in the tube, in the second, deflecting coils are mounted outside the tube. For deflection in both horizontal and vertical directions, plates (or coils) of vertical or horizontal deflection of the beam are used.
The screen of the tube is covered from the inside with a material - a phosphor, which glows under the influence of electron bombardment. Phosphors are distinguished by a different glow color and a different glow time after the termination of excitation, which is called afterglow time. Usually it ranges from fractions of a second to several hours, depending on the purpose of the tube.
