The oscilloscope Cathode Ray Tube and other Internal components
My Data <ul><li>From: </li></ul><ul><li>Mahmoud Abdel Aziz Ali Ahmed </li></ul><ul><li>3 rd year Comm. </li></ul><ul><li>Class(5) </li></ul><ul><li>B.No.(25) </li></ul><ul><li>To: </li></ul><ul><li>Proff. Dr/Sayed Kamel </li></ul><ul><li>Electronic Measurements Course </li></ul><ul><li>Communication &Electronics Department </li></ul><ul><li>Faculty of Engineering </li></ul><ul><li>Zagazig University </li></ul>
Agenda <ul><li>1-Overview. </li></ul><ul><li>2-Internal Components. </li></ul><ul><li>3-Cathode Ray Tube in general. </li></ul><ul><li>Thermionic emission </li></ul><ul><li>Electron Gun </li></ul><ul><li>The Anode </li></ul><ul><li>The Deflection Plates </li></ul><ul><li>The Fluorescent Screen </li></ul><ul><li>4-How to use oscilloscope. </li></ul><ul><li>Voltage and Time Control </li></ul><ul><li>Cables </li></ul><ul><li>Patterns </li></ul><ul><li>5-Digital Oscilloscope. </li></ul><ul><li>6-Embedded Oscilloscope. </li></ul>
OVERVIEW An oscilloscope is easily the most useful instrument available for testing circuits because it allows you to see the signals at different points in the circuit. The best way of investigating an electronic system is to monitor signals at the input and output of each system block, checking that each block is operating as expected and is correctly linked to the next. With a little practice, you will be able to find and correct faults quickly and accurately.
THERMIONIC EMISSION <ul><li>Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the forces restraining it. The charge carriers can be electrons or ions, and in older literature are sometimes referred to as "thermions". After emission, a charge will initially be left behind in the emitting region that is equal in magnitude and opposite in sign to the total charge emitted. But if the emitter is connected to a battery, then this charge left behind will be neutralized by charge supplied by the battery, as the emitted charge carriers move away from the emitter, and finally the emitter will be in the same state as it was before emission. The thermionic emission of electrons is also known as thermal electron emission . </li></ul>
ELECTRON GUN The ELECTRON GUN is roughly equivalent to the cathodes of conventional tubes. The cathode of the electron gun in the CRT is required not only to emit electrons, but also to concentrate emitted electrons into a tight beam. In the electron tubes that you have studied, the cathode was cylindrical and emitted electrons in all directions along its entire length. This type of cathode is not suitable for producing a highly concentrated electron-beam. The cathode of the CRT consists of a small diameter nickel cap. The closed end of the cap is coated with emitting material. This is shown in figure. Because of this type of construction, electrons can only be emitted in one direction. Notice that the emitted electrons shown in figure are leaving the cathode at different angles. If these electrons were allowed to strike the screen, the whole screen would glow. Since the object of the electron gun is to concentrate the electrons into a tight beam, a special grid must be used. This special grid is in the form of a solid metal cap with a small hole in the center. The grid is placed over the emitting surface of the cathode and charged negatively in relation to the cathode. The dotted lines represent the direction of cathode emitted electron repulsion. Since all emitted electrons leave the cathode (point C), their paths can be identified. An electron attempting to travel from point C to point B (downward) will instead follow the path from point C to point E to point P. Consider an electron leaving from C in the direction of point A (upward). Its path will be curved from point C to point P by electrostatic repulsion. These curving electron paths are due to the negative potential of the grid coupled with the high positive potential of the anode. The potential of the anode attracts electrons out of the cathode-grid area past point P toward the screen. The grid potential may be varied to control the number of electrons allowed to go through the control-grid opening. Since the brightness or intensity of the display depends on the number of electrons that strike the screen, the control grid is used to control the brightness of the CRT.
THE ANODE The FOCUSING ANODE is charged a few hundred volts positive with respect to the cathode. Electrons emitted by the cathode are attracted to the focusing anode. This is the reason that they travel through the small hole in the grid. The second electrode, called the, ACCELERATING ANODE is charged several thousand volts positive in relation to the cathode. Any electrons approaching the focusing anode will feel the larger electrostatic pull of the accelerating anode and will be bent through the opening in the focusing anode and will travel into the area labeled D. You might think that once an electron is in this region, it is simply attracted to the accelerating anode and that is the end of it. This does not happen. Because the accelerating anode is cylindrical in shape, the electrostatic field radiating from it is equal in all directions. Thus, an electron is pulled in all directions at once, forcing the electron to travel down the center of the tube. Then, the electron is accelerated into the accelerating anode. Once it passes the mid-point (point E), it feels the electrostatic attraction from the front wall of the accelerating anode, which causes it to move faster toward the front. Once the electron reaches point F, equal electrostatic attraction on either side of the opening squeezes it through the small opening in the front of the anode. From there, it is joined by millions of other electrons and travels in a tight beam until it strikes the screen (point S).
THE DEFLECTION PLATES ELECTROSTATIC DEFLECTION uses principles you are already familiar with. Namely, opposites attract, and likes repel. Here you see an electron traveling between two charged plates, H 1 and H 2 . As you can see, before the electron reaches the charged plates, called DEFLECTION PLATES , its flight path is toward the center of the screen. In view B, the electron has reached the area of the deflection plates and is attracted toward the positive plate, H 2 , while being repelled from the negative plate, H 1 . As a result, the electron is deflected to the right on the inside of the screen. You, the viewer, will see the spot of light on the left side of the CRT face (remember, you are on the opposite side of the CRT screen). This is shown in view C.
THE FLOURESCENT SCREEN The inside of the large end of a CRT is coated with a fluorescent material that gives off light when struck by electrons. This coating is necessary because the electron beam itself is invisible. The material used to convert the electrons' energy into visible light is a PHOSPHOR . Many different types of phosphor materials are used to provide different colored displays and displays that have different lengths of PERSISTENCE
The function of an oscilloscope is extremely simple: it draws a V / t graph, a graph of voltage against time, voltage on the vertical or Y-axis, and time on the horizontal or X-axis. As you can see, the screen of this oscilloscope has 8 squares or divisions on the vertical axis, and 10 squares or divisions on the horizontal axis. Usually, these squares are 1 cm in each direction Many of the controls of the oscilloscope allow you to change the vertical or horizontal scales of the V / t graph, so that you can display a clear picture of the signal you want to investigate. 'Dual trace' oscilloscopes display two V / t graphs at the same time, so that simultaneous signals from different parts of an electronic system can be compared.