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Unformatted text preview: Lab 2: The Oscilloscope - motion of free electrons in an electric field 1 Introduction The oscilloscope is a very versatile instrument that is used almost universally for data acquisition, and as a diagnostic tool. Its principle of operation is relatively easy to understand. However, students usually react with alarm and confusion when confronted with the numerous buttons and switches on the front panel of the device. This laboratory will hopefully assist you in overcoming any hesitation in using this device. An analogue oscilloscope consists of an electron gun encased in an evacuated narrow glass tube. The tube has a wide end face (coated with a phosphor) on which signals are displayed. In many ways, its construction resembles that of a television tube. Electrostatic fo- cusing plates and electron beam deflection plates are enclosed within the narrow tube and connected to the outside via feed-through elements. This lab will al- low students to understand the principles of such an oscilloscope. An electron beam is produced by electrically heating a metal foil (or filament). Electrons in the foil ac- quire sufficient kinetic energy to escape the foil. A large electric potential difference between the foil and a ring electrode accelerates the electrons. The focus- ing plates shape the cross section of the electron beam. The electron beam can be deflected by uniform elec- tric fields between the deflection plates before hitting the phosphor screen. These electric fields are pro- duced by applying a (voltage) signal to the inputs of the oscilloscope. When the electron beam strikes the phosphor screen, excited atoms in the phosphor con- vert the kinetic energy of the electron beam into light, producing a characteristic glow. A digital oscilloscope does not have an electron beam tube. The input waveform is digitized and displayed on the screen. You will first use a stripped down electron tube to learn how electrons are deflected in a uniform electric field. For the rest of this report, we will refer to this oscilloscope as the cathode ray tube or MT69MT108MT101MT99MT116MT114MT111MT110MT32MT103MT117MT110 MT121 MT49 MT121 MT50 MT76 MT49 MT76 MT50 MT100 MT69 MT69MT108MT101MT99MT116MT114MT111MT115MT116MT97MT116MT105MT99MT32MT100MT101MT102MT108MT101MT99MT116MT105MT111MT110MT32MT112MT108MT97MT116MT101 MT69MT108MT101MT99MT116MT114MT111MT110MT32MT98MT101MT97MT109 MT83MT99MT114MT101MT101MT110 MT80MT97MT114MT97MT98MT111MT108MT97 MT83MT116MT114MT97MT105MT103MT104MT116MT32MT108MT105MT110MT101 Figure 1: Electron beam deflection. NOTE: L 2 L 1 . CRT. The CRT is built so that the deflection plates and other electrodes are clearly visible. The exper- iments will allow you to measure the axial speed of the electron beam. Subsequently, you will learn how a simple waveform can be displayed on the screen....
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This note was uploaded on 10/25/2010 for the course PHYS 2213 taught by Professor Hor. during the Spring '10 term at Maple Springs Baptist Bible College.
- Spring '10