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Reprinted from Wikipedia
Cathode ray tube employing electromagnetic focus and deflection
Cutaway rendering of a color CRT
- Electron guns
- Electron beams
- Focusing coils
- Deflection coils
- Anode connection
- Mask for separating beams for red, green, and blue part of displayed image
- Phosphor layer with red, green, and blue zones
- Close-up of the phosphor-coated inner side of the screen
The cathode ray tube or CRT, invented by German physicist Karl Ferdinand Braun, is the display device that was long used in most computer displays, video monitors, televisions, radar displays and oscilloscopes. The CRT developed from Philo Farnsworth's work was used in all television sets until the late 20th century and the advent of plasma screens, LCD TVs, DLP, OLED displays, and other technologies. As a result of CRT technology, television continues to be referred to as "the tube" well into the 21st century, even when referring to non-CRT sets.
A cathode ray tube technically refers to any electronic vacuum tube employing a focused beam of electrons. This article will concentrate on the families of cathode ray tubes used as displays for television, radar, oscilloscopes etc. Another important type of cathode ray tube is the video camera tube discussed in a separate article.
The earliest version of the CRT was a cold-cathode diode, a modification of the Crookes tube with a phosphor-coated screen, sometimes called a Braun tube. The first version to use a hot cathode was developed by John B. Johnson (who gave his name to the term Johnson noise) and H. W. Weinhart of Western Electric and became a commercial product in 1922.
Cathode rays exist in the form of streams of high speed electrons emitted from the heating of cathode inside a vacuum tube at its rear end. The released electrons form a beam within the tube due to the voltage difference applied across the two electrodes, and the direction of this beam is then altered either by a magnetic or electric field to trace over the inside surface of the phosphorescent screen (anode), covered by phosphorescent material (often transition metals or rare earths). Light is emitted by that material at the instant that electrons hit it.
In television sets and modern computer monitors, the entire front area of the tube is scanned systematically in a fixed pattern called a raster, and a picture is created by modulating the intensity of the electron beam with the received video signal (or another signal derived from it). The beam in all modern TV sets is scanned with a magnetic field applied to the neck of the tube with a "magnetic yoke", a set of wire coils driven by electronic circuits. This usage of electromagnets to change the electron beam's original direction is known as "magnetic deflection".
Electron gun
The electron beam source is the electron gun, producing the stream of electrons by thermionic emission and then focusing it into a thin beam. The gun is located in the narrow, cylindrical neck at the extreme rear of a CRT and has electrical connecting pins, usually arranged in a circular configuration, extending from its end. These pins provide external connections to the cathode, to various grid elements in the gun used to focus and modulate the beam, and, in electrostatic deflection CRTs, to the deflection plates. Since the CRT is a hot-cathode device, these pins also provide connections to one or more filament-type heaters within the electron gun. When a CRT is operating, usually the gun heaters can be seen glowing orange through the glass walls of the CRT neck. It is the need for these heaters to achieve their effect that causes a delay between the time that a CRT is first turned on and the time that a display becomes visible; the CRT literally needs time to "warm up". In older tubes, this could take fifteen seconds or more; modern CRT displays have fast-starting circuits that display an image within about two seconds, using either briefly increased heater current or elevated cathode voltage. Once the CRT has warmed up, the heaters stay on continuously to keep the cathode warm. The electrodes are often covered with a thermally black layer, a patented process used by all major CRT-manufacturers to improve electron density.
The electron gun is often mounted slightly off-axis, as it accelerates not only electrons but also ions resulting from outgassing of the internal tube components and from an imperfect vacuum. The ions are heavier than electrons; therefore they are deflected less by the magnetic field from the deflection coils, and in older constructions with in-axis guns the ions were bombarding the phosphor in the center of the screen and causing its deterioration. Some very old black and white TV sets show browning of the center of the screen, known as ion burn, from this bombardment. The combination of an off-axis mounting of the electron gun and permanent magnets bending the electron beam back in the desired direction forms an ion trap; the ions are not deflected enough so they strike the neck of the tube instead of the screen and harmlessly dissipate. This system was later replaced by aluminium coating of the phosphor.
The internal side of the phosphor layer is often covered with a layer of aluminium. The phosphors are usually poor electrical conductors, which leads to deposition of residual charge on the screen, effectively decreasing the energy of the impacting electrons due to electrostatic repulsion (an effect known as "sticking"). The aluminium layer is connected to the conductive layer inside the tube, disposing of this charge. It also reflects the phosphor light in the desired direction towards the viewer, and protects the phosphor from ion bombardment.
For use in an oscilloscope, this general design is modified somewhat so that rather than tracing out a raster, the electron beam is directly steered along an arbitrary path while its intensity is kept constant. In time-domain mode, the usual mode, the horizontal deflection is proportional to time (measured out by a "sweep oscillator" in the oscilloscope), and the vertical deflection is proportional to the measured signal(s). In the less-common X-Y mode, both the horizontal and vertical deflections are proportional to measured signals.
In addition, the electron gun is centered in the tube neck; the problem of ion production is either ignored or mitigated by using an aluminized screen.
Tubes designed for oscilloscope use are longer and narrower than tubes designed for raster scan use, greatly reducing the maximum deflection angle required. This allows for the use of electrostatic deflection instead of magnetic deflection. In this case, deflection is done by applying an electrical field via deflection plates built into the tube's neck, allowing the electron beam to be steered much more rapidly than with a magnetic field, where the inductance of the electromagnets imposes relatively severe limits on the maximum frequency in the signal that can be accurately represented. The limited deflection angle also removes any need for dynamic focusing of the electron beam (which would also be difficult to accomplish at the required high deflection speeds). Finally, the limited angle greatly eases the difficulty of ensuring that the beam deflection produced is a linear function of the deflection voltages applied.
Even electrostatic deflection has its limits, though. One problem is that the deflection plates appear as a fairly large capacitive load to the deflection amplifiers, requiring large current flows to charge and discharge this capacitance rapidly. A second, more subtle problem is that when the electrostatic charge switches, some electrons are already part-way through the deflection plates and will only be partially effected by the change in charge. This produces the effect where even if the charge on the plates switches instantaneously, the electron beam hitting the screen slews along the screen at a much slower pace.
Extremely high performance oscilloscopes avoid these problem by subdividing the vertical deflection plates (and, sometimes, the horizontal deflection plates) into a series of plates electrically joined by a delay line terminated in its characteristic impedance. The timing of the delay line is set to match the velocity of the electrons as they fly towards the screen. In this way, a change of charge "flows along" the deflection plate along with the electrons that it should affect, and the beam (as seen on the screen) slews almost instantly from the old point to the new point. In addition, because the entire deflection system operates as a matched-impedance load, the problem of driving a large capacitive load is mitigated.
A few tubes designed for use in so-called dual beam oscilloscopes contain an electron gun that produced two electron beams. The horizontal deflection of these beams was usually shared while the vertical deflection plates were independent (allowing a time-domain display to show two signals absolutely simultaneously). A few tubes also offered independent horizontal deflection plates.
Many modern oscilloscope tubes then pass the electron beam through an expansion mesh. This mesh acts like a lens for electrons and has the effect of roughly-doubling the deflection of the electron beam, allowing the use of a larger faceplate for the same length tube envelope. The expansion mesh also tends to increase the "spot size" on the screen, but this tradeoff is usually acceptable.
Oscilloscope CRTs designed for the fastest use then pass the electron beam through a micro-channel plate just before the electrons reach the screen. Through the phenomenon of secondary emission, this plate greatly multiplies the number of electrons reaching the phosphor screen, allowing even an extremely fast-moving electron beam to produce enough light to be visible to the naked eye.
The phosphor screen of oscilloscope tubes is also different from the screen of display tubes. Because the display may be a single-shot event, the phosphor chosen usually has a much longer persistence than is chosen for a CRT displaying a moving picture. Also, its color is usually chosen for maximum efficiency. For oscilloscope displays viewed by the human eye, this usually leads to the iconic P31 green trace. This phosphor produces the best trade-off between visibility, photographability, and resistence to burning by the electron beam. For displays meant to be photographed, the the deep blue trace of P11 phosphor is sometimes chosen while for extremely slow displays, very-long persistence phosphors such as P7 produce an amber or yellow afterimage.
The phosphor screen of most oscilloscope tubes also contains a permanently-marked internal graticule, dividing the screen using Cartesian coordinates. This internal graticule allows the easy measurement of signals with no worries about parallax error. Less-expensive oscilloscope tubes do not contain an internal graticule; instead, an external graticule of glass or acrylic plastic is used. In either case, the graticule can often be illuminated for use in a darkened room.
Oscilloscope tubes almost never contain integrated implosion protection. External implosion protection must always be provided, either in the form of an external graticule or for tubes with an internal graticule, a plain sheet of glass or plastic. The implosion protection shield is often colored to match the light emitted by the phosphor screen; this improves the contrast seen by the user
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