Download Description of an Oscilloscope Cathode Ray Tube

Description of an Oscilloscope Cathode Ray Tube
Introduction—General Description
The oscilloscope cathode ray tube (CRT) is a vacuum tube that creates and deflects an
electron beam to form a graphical image representing the electrical properties of a circuit.
An oscilloscope (Figure 1) is used to test electrical devices by displaying a graph
of the voltage between two points in a circuit over time. The CRT (Figure 2) displays
this graph. Similar CRTs are used in radar systems, televisions, and computer monitors.
This document explains to the beginning engineering or physics student the
structure and operating principle of the oscilloscope CRT.
A glass tube contains and encloses the oscilloscope CRT in a vacuum. The base
of the tube plugs into the oscilloscope, while the other end flares to a large flat area that
serves as the screen. An electron gun inside produces an electron beam that passes
through two sets of deflection plates before hitting the fluorescent screen to create an
image.
This device operates by creating and accelerating a beam of electrons with an
electric field. Another electric field between the deflection plates directs the beam to the
desired location of the display. Finally, the fluorescent screen converts the energy of the
electron beam into slowly emitted visible photons.
Four main parts make up the oscilloscope CRT: bulb, electron gun, deflection
plates, and fluorescent screen.
Figure 1: A Hitachi V-1565 Real Time Oscilloscope
Source: http://www.measurebetter.com/products/hi1565.htm
Figure 2: An Oscilloscope CRT
Source: http://www.surplussales.com/Tubes-Sock-Acc/CRTtubes.html
Figure 3: Schematic of Oscilloscope CRT
Description of Parts and Their Function
Bulb. The bulb encloses and holds the components of the CRT in a vacuum. It is a glass
tube shaped roughly like a flashlight (Figure 3). On one end of the tube is the base that
plugs into the oscilloscope and contains the leads that supply current to the components
inside. The other end is bell shaped and serves as the display, which is approximately 4
inches wide and 3 inches tall.
The bulb keeps the space around the CRT free of particles of dust and air and
holds down each of the components of the CRT.
Electron Gun. The electron gun creates the electron beam and adjusts the intensity and
width of the beam moving to the screen. It is located next to the base of the CRT and
consists of five major parts: heater, cathode, control grid, focusing anode, and
accelerating anode (Figure 3).
The heater, a rod of metal, is supplied an electric current and converts it to heat.
As the heater increases in temperature, the cathode heats up and its electrons are given
enough thermal energy to escape their molecular bonds. The cathode serves as the source
of the electron beam and is held at a negative voltage potential. The negatively charged
control grid has variable voltage and pushes some electrons back into the cathode, thus
controlling the intensity of the beam and brightness of the image on the display. The
focusing anode controls the width of the electron beam and the positively charged
accelerating anode creates the electric field needed to accelerate electrons from the
cathode to the screen. The control grid, focusing anode, and accelerating anode have
holes in their centers to allow the electron beam to pass through freely.
Deflection Plates. The deflection plates are simply pairs of oppositely charged metal
plates. There are two sets of deflection plates: vertical and horizontal (Figure 3). Each
set of plates is parallel and located at the neck of the tube. The vertical deflection plates
lie horizontally but control the vertical position of the beam. The horizontal plates are
positioned at right angles to the vertical plates and control the horizontal position of the
beam. External electric circuits are used to control and change the amount of charge on
these plates and the electric field between them.
The electron beam passes between each pair of plates, and is attracted to the
positively charged side and repelled by the negatively charged side. In this way, the
plates control the path of the electron beam and where the beam hits the screen.
Fluorescent Screen. The fluorescent screen is the display on the bulb. The most
common material used on the display is phosphorous, and it is painted on the inside of
the bulb.
Electrons emerging from the deflection plates strike the screen and the
phosphorous converts the energy in the electron beam into photons of visible light. This
results in a spot of light on the display, with brightness proportional to the intensity of the
beam.
The element on the screen is also phosphorescent, meaning that it emits energy as
light gradually instead of instantaneously. This allows us to see lines on the screen
instead of a moving dot. This line is maintained by rapid, repetitive tracing.
Conclusion and Operating Description
The four parts of the oscilloscope CRT are designed to create and direct an electron beam
to a screen to form an image. The oscilloscope links to a circuit that directly connects to
the vertical deflection plates while the horizontal plates have linearly increasing charge to
form a plot of the circuit voltage over time.
In an operating cycle, the heater gives electrons in the cathode enough energy to
escape. The electrons are attracted to the accelerating anode and pulled through a control
grid that regulates the number of electrons in the beam, a focusing anode that controls the
width of the beam, and the accelerating anode itself. The vertical and horizontal
deflection plates create electric fields that bend the beam of electrons. The electrons
finally hit the fluorescent screen, which absorbs the energy from the electron beam and
emits it in the form of light to display an image at the end of the glass tube.