This Note will explain the mechanics and electronics of the Cathode Ray Tube or CRT. The CRT was designed in the 1920's and although several changes have been made to the original design, the changes to the basic structure have been relatively minor. It is ironic that although the vacuum tube industry has been reduced to a mere shadow of what it was in the early days of electronics, the CRT, the sole volume survivor of the vacuum tube days, appears as though it will be with us for many years to come.
The basic elements of the CRT are shown in Figure 1. The three most important systems are 1) the electron gun - a series of metallic structures responsible for generating a beam of electrons, 2) an external deflection systems used to aim the electron beam, and 3) the phosphor-coated faceplate that glows, or more properly, phosphoresces when struck by an electron beam. We will examine these structures one at a time.
The Electron Gun
This structure is composed of two subsystems: the electron emitting portion and the focu vs and acceleration subsystem. The electron gun accelerates a focused stream of electrons, which are used to create or paint an image on the CRT screen. A small coil of tungsten wire called the filament is heated by passing an electrical current through it. In much the same way as an incandescent lamp functions, the filament glows. It is this warm orange glow of the filaments that is visible in the neck of the operating CRT that provides first pass indication of proper operation when servicing CRTs.
|With no filament glow, there is no electron beam. The reason for this is that the filament is composed of a special material that emits electrons when heated in much the same way that a pan of water emits vapor molecules when heated. The emitted electrons, which come bubbling off the hot filament, will be used to create the electron beam. The hot filament uniformly heats a surrounding cylindrical cathode. This provides a better uniform surface than the twisted filament wire and results in a "smoother" flow of emitted electrons. Figure 2 shows the cathode structure.
As shown in Figure 3, a series of anodes (sometimes called grids) compose the focus and the accelerating part of the electron gun. Located closest to the heated cathode is a round disc, approximately the size and the shape of a dime with a small hole in the center. Because of its closeness to the cathode, this control grid, when charged to a small (3-5 volt) negative potential, will deflect electrons back toward the cathode and prevent the beam from entering the focus and accelerating subsystem. When charged positively, the grid attracts the electrons, which are constrained to flow in a small beam through the hole. The electrons are attracted through the hole by the presence of a short positively charged first accelerating anode. The positive charge helps to pull the electrons through the hole in the control grid, however after passing through the hole, the electrons are attracted by a much more positive (approximately 500 volt) final accelerating anode. Because of the higher voltage, the electrons accelerate (pass) beyond the first accelerating anode as they move more rapidly toward the final accelerator. Before doing so, the electrons must pass through another cylindrical device called the focus anode. This anode is charged with a relatively large (250 volt) negative charge and the electrons must focus into a small beam since the center axis of this anode is the most distant point for the electrons. Any electrons outside of the center are deflected toward the center by the large negative charge (as shown in Figure 4).
The Phosphor-Coated Faceplate
As the fine beam of electrons exits the focus area and moves toward the final accelerating anode, they are within range of a much higher (10,000-25,000 volt) positive voltage, which is impressed upon the screened faceplate of the CRT. This high voltage is sufficiently large to cause the electrons to bypass the accelerating anode and accelerate directly to the screen; having been accelerated by the two accelerating anodes to a velocity which will not allow them to stop.
The inside of the CRT faceplate is coated with a thin layer of conductive material (often aluminum-coated) which is connected to the high voltage power supply. This high voltage is ultimately responsible for attracting the electron beam to the faceplate. This inside coating of the faceplate is coated with yet an additional layer of chemicals called phosphors that emit light when struck (excited) by an electron as shown in Figure 5. The type of chemical used will determine the color of the emitted light.
Another important characteristic of the phosphor is the persistence. Persistence is the length of time that the phosphor continues to glow after the electron beam is removed. By using a long persistence phosphor in some applications, annoying flicker can be eliminated whereas in other applications, particularly those with rapidly changing images, a short persistence phosphor is desirable.
|The Deflection System
In either case, a bright electron beam focused in the center of the screen is of little value. It is now necessary to describe how an image is painted on the screen. The electron beam will have to be deflected from one side of the screen to the to paint a line. By deflecting the beam from the top of the screen to the bottom after each line is painted across, it is possible to draw an entire image, line by line. Figure 6 demonstrates this concept. In essence, the electron beam will be deflected to the upper left hand corner of the screen, deflected across the screen to paint a line (Step 1 of Figure 6), retraced to the lift (Step 2 of Figure 6) and then deflected across from left to right to paint another line (step 3). This process is repeated until the entire picture is drawn. During the retrace, the electron beam must be shut off. This is achieved by putting a negative voltage on the control grid, which force the electrons back toward the cathode. The actual movement of the beam (while it is turned on) is controlled by deflection circuitry, which is the last major system in the CRT.
The deflection circuitry for PC monitors (and television receivers) is based on principles of magnetics. It is a well-known phenomenon that electrons (and therefore electron beams) can be deflected by other electrons or by magnetic field. In the PC monitor, two coils of wire known as the yoke are wrapped around the neck of the CRT (see Figure 7). A close look at the construction of these coils shows that they are wound at a ninety-degree angle to one another. One coil is wound in an up-down direction (relative to the CRT faceplate) while the other has a side to side orientation. Consequently, passing current through either coil has the effect of creating a magnetic field, which, because of the location of the yoke, will deflect the electron beam either up and down, or from side to side.
|In actual operation, a current which resembles a sawtooth, as shown in Figure 8, is passed through each coil and the resultant changing magnetic field causes the electron beam to deflect in proportion to the magnetic field generated. The effect of the horizontal sawtooth current is to cause the beam to deflect from the top to bottom. Notice that the sawtooth rises slowly and returns to zero very quickly. In this manner, the beam crosses the face of the screen slowly (while the image is being drawn), yet retraces (returns) very quickly. If the horizontal sawtooth occurs more frequently than the vertical sawtooth and the two are synchronized together, then an image is painted line by line across the screen as shown in Figure 9.
The frequency of the horizontal sawtooth is referred to as the horizontal scan rate, while the corresponding vertical sawtooth frequency is called the vertical scan rate. The vertical scan rates are generally fifty to sixty times per second whereas the horizontal rates are in the order of fifteen to 75 kilohertz. Because the magnetics in the yoke are "tuned" to these scan rates; it is difficult to operate a specific monitor at a scan rate different than the one for which it is designed. An exception to this rule is the multiscan type of unit, which is covered in a separate application note.
The pattern of light painted across the CRT screen when the beam is simultaneously swept horizontally and vertically is called a raster and hence the name raster scan. The circuitry for sweeping the beam across the entire screen is achieved by having two oscillators that generate the sawtooth shaped drive signals to the yoke.
|Synchronized with these oscillators is a circuit that applies a varying voltage level to the control grid to vary the intensity of the electron beam as it moves from place to place across the screen. This varying intensity is what creates the actual image on the screen. Figure 10 shows how turning the beam on and off repeatedly will cause a series of bars to be painted on the screen. The reader should study this figure carefully. The signal that controls the voltage on the control grid is the video information signal and is taken from the broadcast signal in the case of television or is taken from screen memory in the graphics controller card in the case of the PC monitor.
The video information signal controls the intensity of the beam as it crosses the screen painting the image. The only remaining function to be performed is the synchronization of the video information signal relative to the position on the screen. This function is achieved by adding sync pulses to the video information signal.
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