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OUTLINE The CATHODE RAY OSCILLOSCOPE( CRO) Introduction, Oscilloscope Block Diagram, Review on periodic Signals, Cathode Ray Tube (CRT), Waveform Display, Various Controls on the Front Panel. Digital Oscilloscopes Review on Analog-to-digital conversion (ADC) Digital Oscilloscopes

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In the First Year laboratory the most important and complex piece of equipment to be used is the Neotronics OS-620 Dual Trace Oscilloscope. In this piece of software all of the important characteristic applications and features of the CRO will be explained, I could list all of the features mentioned in the manual provided by the manufacturer but I don't really understand all of them myself . The Oscilloscope is useful for making measurements in both the A.C. and D.C. domain. It allows us to view the input waveforms and hence verify their validity. (i.e if we expect a square wave but recieve a triangle wave it is obvious that our circuit is not functioning correctly). The device works essentially by plotting or tracing the amplitude of the input waveform against time on a Cathode Ray Tube (CRT) display. The waveform is plotted many times per second so it is important that the signal is in some way periodic so that the signal seen on the display remains static. INTRODUCTION

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CRT Oscilloscope Block Diagram A basic CRO consists of : • Cathode Ray Tube • Time base (sweep) generator • Horizontal Amplifier • Trigger Circuit • Vertical Amplifier • Delay Line

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1. Time Base Generator A Time Base Generator is used to generate the saw tooth voltage required to deflect the beam in the Horizontal section. The circuit used to generate the saw tooth is called the Continuous Sweep Generator. 2. Horizontal Amplifier The Horizontal Amplifier is used to amplify the saw-tooth voltage, before it is applied to the Horizontal section 3. Trigger Circuit A Trigger Circuit is used to convert the incoming signal into trigger pulses, so that the input signal and the sweep frequency can be synchronized.

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4. Vertical amplifier It is a wide band amplifier used to amplify or attenuate the input signal to make it seen properly on the screen. This process is controlled by volt/div control that is used to select the suitable vertical voltage that causes deflection of one division. Note the volt/div is called the oscilloscope sensitivity. 5. Delay Line •A delay line is used to delay the signal in the vertical section. •The signal in the Horizontal has to pass through the ‘Trigger pick off’ circuit, ‘Time Base’ Generator and Horizontal Amplifier before it finally reaches the Horizontal Deflection Plates (HDP). The signal therefore takes longer to reach the HDP as compared to the Vertical section. This gives an error, in which the initial portion of the input signal is lost. The distortion can be overcome by using a delay line in the Vertical section.

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Idea of Operation of CRT 1. Electron Gun Electron Gun is used to generate a beam of electrons. The screen of a CRT is formed by depositing a coating of a phosphor materials on the inner side of the tube face . When the electron beam strikes the screen, electrons within the strike area are raised to a higher energy level. Then they loss this energy in the form of light as they return to their normal energy level. The glow may persist for a few milliseconds or even longer, depending on the material employed. The color of the glow produced at the screen may be blue, red, green or white If no deflecting fields are present, the electrons travel in a straight line from the hole in the accelerating anode to the center of the screen, where they produce a bright spot. 2. Screen

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3. Vertical and Horizontal Deflection Plates During the way to the screen, the electrons beam passes between two pairs of deflecting plates (horizontal and vertical plates). If there is no potential difference between both pairs of plates, the electrons strike the screen on the origin. An potential difference between the horizontal pair of plates deflects the electrons horizontally, and an potential difference between the vertical pair deflects them vertically.

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As shown in the figure, when the upper vertical plate potential is +E/2 and the lower potential is –E/2, the (negatively charged) electrons will be attracted towards the upper (positive) plate. So the bright spot will be deflected vertically upwards. If the potential difference between the vertical plates is reversed, the bright spot will be deflected vertically downwards. The deflection height depends on potential difference.

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