February 1955 Popular Electronics
 Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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Even with the domination
of LED, plasma, and LCD displays, there are still cathode ray tubes (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.
See also How the Cathode-Ray Tube Works,
Picture Tubes,
TV X-Rays,
TV X-Rays Are Back.
How the Cathode-Ray Tube Works
This analysis of both electrostatic and magnetic type tubes provides
better understanding of TV picture tube operation
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.
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
tech-nology.
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.
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.
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.
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 am-plifier 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 at-tracted 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
Posted April 11, 2024
(updated from original post
on August 17, 2011)
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