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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.