How the Cathode-Ray Tube Works
February 1955 Popular Electronics
Even with the domination of LED, plasma, and LCD displays, there are still a whole lot of CRTs on the job. Hobbyists workbenches are filled with them for sure, but design and manufacturing facilities still have huge inventories of test equipment with CRTs, and a lot of computer equipment on the production line with CRTs sitting in racks. LED, LCD, and plasma displays all have their own claims to genius on the part of their designers, but cathode ray tube designers - and the designers of the driver circuits - deserve special recognition. Consider the physics and materials involved: glass, phosphor, magnetics, thermonics, electrostatics and electrodynamics, relativity (electrons traveling at relativistic speeds gain mass, requiring stronger deflection fields). This article from the February 1955 edition of Popular Electronics provides a look into the CRT from a layman's perspective.
February 1955 Popular Electronics
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How the Cathode-Ray Tube Works
This analysis of both electrostatic and magnetic type tubes provides better understanding of TV picture tube operation
To many, the cathode-ray tube is symbolic of the entire television industry. There is little question that if it were not for this tube, the industrial giant of modern television could never have come into being. However, while it is true that a cathode-ray tube is used in every TV receiver and that thousands of others are used in TV broadcast stations, its use is not limited to this field alone. These tubes are used in radar, in medical electronic apparatus, in test equipment, in atomic research, in industrial electronic equipment, in fact, in all phases of present-day industry and science. To understand the operation of the cathode-ray tube, then, is to be in a better position to understand the entire field of modern industrial technology.
Fig. 1. (top) The larger of these electrostatic cathode-ray tubes is made of glass and measures 5 inches. The smaller is a 1 inch metal tube.
Fig. 2. (bottom)Construction details of an electrostatic type cathode-ray tube.
Fundamentally, the cathode-ray tube (or CRT) is nothing more than a special type of indicating device. Just as a loudspeaker converts electrical signals into sound vibrations, and a meter changes electrical energy into the mechanical movement of a needle pointer, the CRT is an instrument used to change electrical signals into patterns of light.
Cathode-ray tubes are made in numerous sizes and shapes and for many special purposes. However, most cathode-ray tubes can generally be divided into two broad classes -"electrostatic" tubes and "electromagnetic" tubes. This classification is based on the means used for deflecting (moving) the electron beam which "paints" the light pattern on a fluorescent screen. Each class of tube has certain advantages as well as certain limitations, and these advantages and limitations have resulted in each type of tube being used in specific applications.
Electrostatic tubes use an electrostatic field for controlling the electron beam. This field is built up between a pair of electrodes called "deflection plates" by the application of moderate a.c. and d.c. voltages. Electrostatic tubes are generally made in small sizes, with screens from one inch to about ten inches in diameter. They have a good frequency response and are widely used in cathode-ray oscilloscopes, medical electronic equipment, industrial equipment, and in some types of radar work.
Electromagnetic tubes use a magnetic field for controlling the stream of electrons. Two pairs of coils are used for building up this magnetic field. The coils are external to the tube proper and are generally mounted in a single assembly called a "deflection yoke." Electromagnetic tubes are made with screen sizes from five inches to thirty inches in diameter. Although they have a comparatively narrow frequency response, these tubes do permit the formation of large size, sharply focused, bright images and are used in tremendous quantities in television receivers and as indicators in radar systems. Virtually all modern TV receivers employ electromagnetic tubes.
Since most people find it easier to understand the operation of the electrostatic cathode-ray tube, we shall discuss this type first. Once a clear understanding of the electrostatic tube has been acquired, the electromagnetic tube is easy to master. The basic construction of an electrostatic CRT is shown in simplified form in Fig. 2.
Fig. 3. Photographic illustration of the electron gun assembly of cathode-ray tube.
Fig. 4. In an electromagnetic type of tube, deflection coils are placed on the outside.
Fiq. 5. The photograph shows a typical deflection yoke of a modem TV receiver.
Fig. 6. This industrial multi-gun cathode-ray tube has five electron gun and deflection assemblies.
An a.c. voltage is applied to the filament of the tube, heating it to a bright glowing red. The filament, in turn, heats the cathode which is placed close to it. The cathode is covered with materials which "boil out" electrons when heated, and these electrons gather in a cloud close to the cathode.
A high positive voltage is placed on the accelerator anode and this voltage attracts the negatively charged electrons, causing them to move in a stream toward the front of the tube. The electron stream passes through a narrow hole in a cylindrical shaped electrode called the grid. Although not shaped like a conventional grid, it is given this name after the grid in an amplifier type vacuum tube because it serves the same function... it controls the number of electrons which can pass through.
The electron stream next passes through a group of two or three cylindrical electrodes which have different d.c. voltages applied to them. The accelerating anode may be one of this group. The d.c. voltages applied as well as the shape and size of these electrodes set up an electrostatic field which narrows the electron stream and focuses it into a sharp beam. Because of this action, one of the electrodes may be termed the focusing anode.
Next, the sharply focused electron beam passes through a pair of flat electrodes arranged in a horizontal plane with respect to the tube. If a d.c. voltage is applied to these plates, the electron beam will be attracted toward the more positive plate and repelled from the negative plate, bending either up or down, depending on how the d.c. voltage is applied. If an a.c. voltage is applied, the beam will move up and down alternately. Since the beam moves in a vertical direction, these electrodes are called the vertical defection plates. The horizontal defection plates are a similar pair of electrodes, but arranged at right angles to the vertical plates, and serve to move the beam either to the left or right.
After passing through the deflection plates, the electron beam goes on to strike the front of the tube, which has been covered with chemicals which glow when struck by the electrons. This action is termed "fluorescence" and, therefore, the film of chemicals on the face of the CRT is called a fluorescent screen. The choice of chemicals used in making the fluorescent screen determines the color of the glow and how long the glow continues after the electron beam strikes the screen. This latter characteristic is called the persistence of the screen.
Most of the cathode-ray tubes used in oscilloscopes have a medium persistence green screen. Zinc orthosilicate is frequently used for such screens. Television receivers generally employ a medium persistence white screen. Various mixtures may be used to produce a white screen, including a combination of zinc sulphide and zinc beryllium silicate. For high speed photography, a CRT using a short persistence blue screen is desirable. Such screens may be made from calcium tungstate.
Since the purpose of a cathode-ray tube is to obtain a pattern of light on a screen, the fluorescent screen is often considered one of the most important parts of the tube. This is shown by the fact that a tube's size is given in terms of its screen diameter. A "seven inch tube" has a screen with a diameter of approximately seven inches.
In a cathode-ray tube, the assembly of electrodes which produces the stream of electrons, not including the deflection plates, is called the "electron gun." The electron gun of a typical CRT is shown in Fig. 3.
An electromagnetic CRT is somewhat simpler. in construction than an electrostatic tube since it does not have deflection plates. However, it still has an electron gun assembly, although the focusing anodes may be missing. The beam of electrons sent out by the electron gun is deflected by a magnetic field set up, in turn, by two pairs of curved coils mounted around the neck of the tube close to the bulge of the "funnel." See Fig. 4. The coil assembly, or deflection yoke, of a typical television receiver is shown in Fig. 5. Note how the two sets of coils are mounted at right angles to each other.
Electromagnetic cathode-ray tubes which do not have focusing electrodes in their electron gun assembly employ an external magnet to produce a magnetic focusing field. Such magnets may be either electromagnets or permanent magnets, or a combination of both. Thus, electromagnetic tubes may be subdivided into two smaller classes; those employing electrostatic focusing and magnetic deflection and those employing both magnetic focusing and magnetic deflection.
The shape of the light pattern formed on the screen of a CRT depends on the type of electrical signals applied to the deflection elements (whether coils or electrostatic plates). If steady d.c. voltages are applied, a dot will appear on the screen, with its exact position determined by the relative sizes of the applied deflection voltages. If a.c, signals are applied, a line or pattern will be formed, with its shape and size determined by the electrical waveforms and amplitudes of the a.c. signals. In a television receiver, the light pattern forms a raster made up of a series of horizontal lines.
Varying the voltage applied to the grid electrode in the electron gun assembly will change the number of electrons that can pass through and strike the screen and hence the instantaneous brightness of the glow. In a television receiver, the video signal is applied to the grid-cathode circuit of the CRT and changes the evenly glowing raster into a pattern of light and dark segments which, in turn, makes up the picture.
The vast majority of present day cathode-ray tubes employ a single electron gun assembly and a single set of deflection elements. However, tubes have been made with a number of electron guns, including some of the tubes designed for color television. Cathode-ray tubes have also been made with several complete electron guns plus deflection elements. Such tubes arc virtually several independent cathode-ray tubes with a single screen. One such tube is shown in Fig. 6.
Special purpose cathode-ray tubes, color television picture tubes, TV camera tubes, and other types of cathode-ray tubes will be discussed in other issues of POPULAR ELECTRONICS. END