CHAPTER 4 - CHAPTER 4 OSCILLOSCOPES 4.1 Introduction The...

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CHAPTER 4 OSCILLOSCOPES 4.1 Introduction The cathode ray oscilloscope generally referred to as the oscilloscope, is probably the most versatile electrical measuring instrument available. Some of electrical parameter can be observed with the oscilloscope are: 1. dc and ac voltage 2. indirect measurement of ac and dc current, 3. time 4. phase relationships 5. frequency 6. a wide range of waveform evaluations such as rise time Many nonelectrical physical quantities such pressure, strain, temperature and acceleration can be measured by using a transducer to convert the physical parameter to an equivalent voltage. The oscilloscope consists of the following major subsystems: 1. Cathode ray tube or CRT 2. Vertical amplifier 3. Horizontal amplifier 4. Sweep generator 5. Trigger circuit 6. Associated power supplies The heart of the instrument is cathode ray tube. 56
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4.2 The cathode ray tube The cathode ray tube used in an oscilloscope is very similar to the picture tube in the television set. Figure 4.1 shows a cross section of a CRT. Figure 4.1: Cathode ray tube with the major components identified The major components of a general purpose CRT are: 1. Evacuated glass envelope 2. Electron gun assembly 3. Deflected plate assembly 4. Accelerating anodes 5. Phosphor coated screen Screen size diameters of approximately 1 to 25 in or larger. Most laboratory quality oscilloscopes use a CRT that has a circular screen approximately 5in in diameter. All electrical connections except the high voltage connection are made through the base of the CRT. The glass envelope is evacuated to a fairly high vacuum to permit the electron beam to traverse the tube easily. The electron gun assembly consists of the triode section and the focus section which are shown in Figure 4.1. The purpose of the electron gun assembly is to provide a source of electrons, converge and focus them into a well defined beam, and accelerate them to 57
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toward the fluorescent screen. The electrons that make up the beam are given off by thermionic emission from the heated cathode. The cathode is surrounded by a cylindrical cap that is at a negative potential. This cap, which has a small hole located along the longitudinal axis of the CRT, as shown in Figure 4.2, acts as the control grid. Because the control grid is at a negative potential, electrons are repelled away from the cylinder walls and, therefore, stream through the hole where they move into the electric fields of the focusing anodes. Figure 4.2: Triode section of the CRT The focus lens consists of the first anode, focus ring and astigmatism aperture or second anode. The purpose of this section is to converge and collimate the beam to obtain the minimum size and best defined spot on the phosphor screen of the CRT. The section of the CRT just beyond the electron gun assembly is the deflection.
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This note was uploaded on 03/04/2011 for the course EET 307 taught by Professor Rahimrazak during the Spring '11 term at University of Malaya.

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CHAPTER 4 - CHAPTER 4 OSCILLOSCOPES 4.1 Introduction The...

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