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A deflection yoke is a kind of
magnetic lens thumb thumb A subtype of a magnetic lens ( quadrupole magnet) in the Maier-Leibnitz laboratory, Munich A magnetic lens is a device for the focusing or deflection of moving charged particles, such as electrons or ions, by use of the magnetic ...
, used in
cathode ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms ( oscilloscope), ...
s to scan the
electron beam Cathode rays or electron beam (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to el ...
both vertically and horizontally over the whole screen. In a CRT television, the electron beam is moved in a
raster scan A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image ...
on the screen. By adjusting the strength of the beam current, the brightness of the light produced by the phosphor on the screen can be varied. The cathode ray tube allowed the development of all-electronic television. Electromagnetic deflection yokes are also used in certain radar displays.


Magnetic compared to electrostatic deflection

Another way of deflecting an electron beam is to put two pairs of electrodes inside the CRT, after the electron gun structure.
Electrostatic deflection Electrostatic deflection refers to a way for modifying the path of a beam of charged particles by the use of an electric field applied transverse to the path of the particles. The technique is called electro''static'' because the strength and di ...
is common in
oscilloscope An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...
displays, because it is easier to drive deflection plates at high frequencies, compared to driving the large
inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of th ...
of a deflection yoke. Compared with electrostatic deflection, magnetic deflection has fewer obstructions inside the tube and so allows for a larger-diameter electron beam, producing a brighter image. This is an advantage for a raster-scan display, which must cover the whole screen instead of one narrow trace as in an oscilloscope. Additionally, magnetic deflection can be arranged to give a larger angle of deflection than electrostatic plates; this makes the CRT and resulting television receiver more compact. The angle of magnetic deflection, for a given deflection current, is inversely proportional to the square root of the CRT accelerating voltage, but in electrostatic deflection, the angle is inversely proportional to the accelerating voltage (for a particular value of deflection plate voltage). This has the practical effect that high accelerating voltages can be used without greatly increasing the power of the deflection amplifiers. Donald G. Fink (ed), ''Electronic Engineer's Handbook'', McGraw Hill, 1975, , pages 11–40 through 11–44 While a magnetic deflection yoke can be used to provide a random-access
vector display A vector monitor, vector display, or calligraphic display is a display device used for computer graphics up through the 1970s. It is a type of CRT, similar to that of an early oscilloscope. In a vector display, the image is composed of drawn li ...
image, the high inductance of the yoke windings requires powerful amplifiers that may be expensive to produce with high bandwidth. In a raster scan display used for television or computer displays, the energy stored in the deflection system is recovered and used to develop the high voltage (10,000 volts or more) used to generate the electron beam in the CRT.


Structure

The deflection yoke has two sets of coils, perpendicular to each other and to the neck of the cathode ray tube. The coils are bent into a rough saddle-shape, to conform to the neck of the CRT and to provide a linear magnetic field distribution. In a color television receiver, three electron beams are all scanned in unison by the single deflection yoke. In a television receiver, the deflection coil may include ferrite segments to help direct and concentrate the magnetic field.


See also

*
Quadrupole magnet Quadrupole magnets, abbreviated as Q-magnets, consist of groups of four magnets laid out so that in the planar multipole expansion of the field, the dipole terms cancel and where the lowest significant terms in the field equations are quadrupole. ...
* Magnetic deflection *
Electron beam processing Electron-beam processing or electron irradiation (EBI) is a process that involves using electrons, usually of high energy, to treat an object for a variety of purposes. This may take place under elevated temperatures and nitrogen atmosphere. Poss ...
*
Analog television Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog ...
* Analog television#Displaying an image * Nipkow disk *
Image dissector An image dissector, also called a dissector tube, is a video camera tube in which photocathode emissions create an "electron image" which is then swept up, down and across an anode to produce an electrical signal representing the visual image. It e ...
*
Scan line A scan line (also scanline) is one line, or row, in a raster scanning pattern, such as a line of video on a cathode ray tube (CRT) display of a television set or computer monitor. On CRT screens the horizontal scan lines are visually discernible ...


References

Cathode ray tube Electromagnetic coils Television technology {{Electronics-stub