The color picture tube isvacuumIn the state of,electron beamExcite thePhosphorAnd glow.The process of converting electric energy into light through devices is controlled by electric signalscathodeThe emitted electrons focus the electrons into an electron beam which deflects and shoots onto the phosphor, transforming the video signal into an optical signal.The electron beam of the color picture tube is restricted by the color selection mechanism. The electron beam is used to excite their corresponding phosphors, so that the phosphors are excited to emit light and reproduce the color image.
The color picture tube is a traditional color display device. Its working principle isBlack and white picture tubeBasically the same, for electrostatic focusing and magnetic deflectioncrt。In recent years, some new color TV display devices have emerged, such asLCD display device、Plasma display deviceBecause of its small size and light weight, it has great advantages over the traditional cathode ray tube, and has obtained rapid development in many image display fields and color TV display fields.Nevertheless, the traditional color picture tube is still the mainstream of TV display devices due to its mature technology and simple display circuit.According to the principle of three primary colors, three kinds of phosphors need to be coated on the fluorescent screen of the color picture tube, and three electron guns need to be set. Therefore, there are many differences between the color picture tube and the black and white picture tube in terms of structure and working process.Structurally, there are three types of color picture tubes: shadow mask type three gun three beam arm, sphene strip type single gun three beam tube, and self converging color picture tubes. At present, self converging color picture tubes are widely used.The fluorescent screen of traditional picture tube is mostly spherical screen. In recent years, ultra flat and pure flat color picture tubes have the same working principle as self converging tubes.
The three gun three beam shadow mask tube is characterized by three independent electron guns, each of which has its own filament, cathode control grid and accelerator, while the collector and anode high voltage are for public use.
The characteristics of the single gun three beam grid picture tube are: large electron beam diameter, high electron transmittance, and simple dynamic focusing correction.
The characteristics of self drawing poly picture tube are: self drawing poly, strip phosphor and short tube neck.
Figure 1.Color kinescope
Structure characteristics and working principle
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electron gun
The electron gun of the color picture tube generates three electron beams that can independently control the beam currentTricolorThe electrical signals R (t), G (t) and B (t) respectively control the beam current of the three electron beams.The electron beam is focused on the fluorescent screen under the effect of the electron gun focusing electric field, and bombards a large number of red, green and blue phosphors coated on the inner side of the fluorescent screen at a high speed under the effect of the high-voltage electric field in the tube.The deflection magnetic field generated by the deflection coil at the pipe cone enables three electron beams to scan full screen at the same time.However, it is necessary to ensure that the three electron beams bombard the corresponding phosphor respectively in the color picture tube.For example, an electron beam is only allowed to bombard the red phosphor during full screen scanning, and is not allowed to hit other phosphors, otherwise it will cause impure color.For this reason, a metal grid is arranged at the inner side of the fluorescent screen, which is a metal plate with many holes on it. The number of holes corresponds to the number of phosphor groups.The structure of color picture tube is shown in Figure 2.
Figure 2. Structure of color picture tube
The electron gun is installed on the pipe neck.The electron gun is composed of some metal cylinders.It has three independent cathodes, each with a filament inside.The end face of the cathode is coated with special oxide.The filament transfers heat to the cathode, and the electrons in the oxide get enough energy to escape from the metal surface and become free electron clouds in space.When a high voltage is applied between the cathode and the high voltage electrode, electrons will move towards the screen and accelerate.The cathode is followed by the control electrode (also calledgrid）, accelerating electrode, focusing electrode and high-voltage electrode.Except for the high voltage electrode, other electrodes are led out from the tube tail.Therefore, there are three RGB cathodes, filaments, grids, accelerators and focusing electrodes in the electrodes at the tube tail.
cathode
The three cathodes are arranged in a straight line, and the rest of the electrodes are common except for the three independent cathodes.This kind of pipe is called single gun three bundle pipe.The grid potential is lower than the cathode potential.The electric field formed between them repels the cathode electrons.Therefore, the more negative the negative voltage between the gate and the cathode, the smaller the electron beam current.In use, the grid is generally grounded, and the cathode is applied with a positive potential, thus forming a negative voltage between the grid and the cathode.Applying R, G, B three primary color electrical signals to the three cathodes can control the current of the three electron beams.This is called cathodic excitation.The cathode excitation requires that the three primary color electrical signals are negative.The signal amplitude is usually tens of volts.
Acceleration pole
The accelerating electrode applies positive potential, which is much higher than the cathode potential (generally 400V~800V).The electric field formed between it and the cathode is a curved electric field symmetrical to the tube axis.This electric field has two effects on the electrons of the cathode: one is to accelerate, and the other is to focus.The function of acceleration is to make the electrons on the cathode surface move towards the screen and make the electrons obtain their initial velocity.Focusing makes each electron in the electron cloud have a force pointing to the tube axis.Therefore, under the action of the accelerating pole, the electrons will converge to form the first focus.After this focus, the electron beam will diverge again.Therefore, the acceleration pole only serves as a pre focus.However, the bending electric field formed between the focusing electrode and the accelerating electrode behind the accelerating electrode, as well as between the focusing electrode and the high voltage electrode, will form a second focus on the divergent electrons. Since the movement speed of the electrons is accelerated at this time, the focus of the second focus will be formed near the screen.In order to make the focus point accurately fall on the screen, it is generally required that the focusing electrode voltage is adjustable.The focusing electrode voltage is generally 4000V~8000V.Since the electric field of the acceleration pole and the focusing pole has a focusing force on the electron, the electron will not fall on the acceleration pole and the focusing pole, that is, the acceleration pole and the focusing pole only need to provide voltage, not current.The focusing electrode voltage is generally obtained by dividing the high voltage of the high voltage electrode through a resistance divider.
High voltage pole
On the one hand, a bending electric field is formed between the high voltage electrode and the focusing electrode to focus the electrons; on the other hand, its high voltage accelerates the electrons strongly to bombard the phosphor at a high speed.The voltage of high voltage pole is generally up to 25kV~30kV.The high voltage electrode is connected with the inner wall of the pipe at the intersection of the pipe neck and the cone through a metal spring.The inner and outer walls of the pipe cone are coated with graphite conductive layer, and the inner graphite conductive layer is connected with the grid of the screen part, so the inner graphite conductive layer and grid of the pipe are part of the high voltage electrode.
Because the voltage of the high voltage electrode is too high, it cannot be led out from the tube tail (otherwise, it will cause breakdown discharge between the high voltage electrode and other electrodes). Instead, a metal terminal, called the high voltage nozzle, is fused into the cone.The high-pressure nozzle is connected with the inner graphite conductive layer (so it is connected with the high-pressure electrode), but not with the outer graphite conductive layer.The outer graphite conductive layer shall be grounded when used.In this way, the capacitance formed by the inner and outer graphite conductive layers is used as the filter capacitance of the high-voltage electrode.
Figure 3. Schematic diagram of picture tube current circuit
The electrons emitted from the cathode bombard the phosphor at a high speed, which will blow out the electrons in the phosphor, which is called secondary electrons.The secondary electrons will be absorbed by the grid, that is, by the high-voltage electrode, forming a current loop as shown in Figure 3.
Therefore, the electron beam current is actually the high voltage load current and the main energy consumption of the picture tube.For example, if the high voltage is 30kV and the average beam current (three beams) is 1mA, then the pipe consumption=30kV × 1mA=30W.This pipe consumption requirementhigh-voltage power supplyIt can provide, and the smaller the internal resistance of the high-voltage power supply is required, the better.If the internal resistance of the high-voltage power supply is large, the output voltage of the high-voltage power supply will change significantly due to the obvious change of beam current when the TV image changes from dark field to bright field.However, when the high pressure is low, the movement speed of the electron beam will slow down, and the scanning amplitude will increase. On the contrary, when the high pressure is high, the scanning amplitude will decrease, resulting in the phenomenon of "expansion and contraction" of the image when converting between the bright field and the dark field.This "expansion and contraction" phenomenon is also called "breathing" effect.Therefore, the technical standard of TV receiver requires that the fluctuation of high voltage and accelerating electrode voltage shall not exceed 5% when the beam current changes from 100 µ A to the rated maximum current of the picture tube.
classification
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In the large screen color picture tube, it can be divided into ultra flat picture tube, pure flat picture tube, ultra-thin picture tube and ultra clean picture tube.Three gun three beam color kinescope and single gun three beam kinescope are the main ones.
Three gun three beam color kinescope
Three gun three beam color picture tubeShadow maskTube is an earlier developed picture tube, which is composed ofFluorescent screen, shadow mask, electron gun and glass shell.It has three independent electron guns, arranged in the shape of a finished product, emitting three electron beams of red, green and blue respectively.
The shadow mask plate is placed about 10 mm from the inner surface of the fluorescent screen, and there are hundreds of thousands of small round holes regularly arranged on it. Each hole corresponds to a group of three color dots on the fluorescent screen.The red, green and blue electron beams intersect and pass through the shadow mask hole from different angles at the same time. The three gun three beam color picture tube is rarely used now, mainly because its convergence circuit is complex and adjustment is troublesome.
Single gun three beam color kinescope
Single gun three beam color picture tube A single gun three beam color picture tube emits an electron beam from an electron gun. It has three independent cathodes arranged in a straight line. The other poles are shared.
The red, green and blue phosphors on the single gun three beam fluorescent screen are coated on the screen in a longitudinal strip shape. There is a metal plate at the inner side of the screen, called the color separation plate, which is used to make three electron beams bombard only their respective phosphors.
The advantage of single gun three beam color picture tube is that the convergence adjustment is simpler than that of three gun three beam tube, and the transmissivity is higher than that of three gun three beam tube, which makes its screen brightness higher.The disadvantage of this pipe is that the convergence adjustment is still troublesome, and the production is difficult, so it is not easy to mass produce.
Modulation characteristic
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R. The G and B primary color signals act on the three cathodes of the color picture tube to control the beam current of the three electron beams.The relationship between the control voltage between the grid and cathode and the electron beam current is called the modulation characteristic of the picture tube.
Fig. 4 Modulation characteristic curve of kinescope
It is expected that the relationship between beam current and grid cathode voltage is linear.But in fact, the beam current and the voltage between the gate and the cathode are in the γ power relationship, γ ≈ 2.2~2.8. This will lead to the brightness distortion of the image.For this reason, it is necessary to carry out anti distortion processing on the three primary color electrical signals at the camera end, which is called γ correction.
In fact, the three modulation characteristic curves of the picture tube do not coincide, and the cut-off voltage (gate cathode voltage when the beam current of the electron beam is just zero) is also different.This will lead to the following problems: when R=G=B, the image color should be standard white light, but from the modulation characteristics of the picture tube, when the three cathode potentials are equal, the current of the three electron beams is not equal, so the red, green, and blue light emitted by the red, green, and blue phosphors cannot mix with the standard white light.Moreover, even if the modulation characteristics of the three electron beams coincide and the beam current is equal, the standard white light cannot be reproduced because of the different luminous efficiency of the three phosphors.This is called white imbalance.For this reason, a regulating circuit is specially set in the amplifier that excites the R, G and B primary color signals of the color picture tube.When R=G=B=black level, adjust the output voltage of the three-way amplifier to make it equal to the cut-off voltage of the three cathodes of the picture tube, which is called dark balance adjustment in white balance.When R=G=B=whitelevelAdjust thegainTo make the color of the displayed picture a standard white light, which is called light balance adjustment in white balance.soWhite balance adjustmentThere are generally five adjustment potentiometers, three of which are used for dark balance adjustment and two for light balance adjustment.
