CATHODE RAY OSCILLOSCOPE (What is CRO , its working principle and structure)?

CATHODE RAY OSCILLOSCOPE

INTRODUCTION

— The cathode-ray oscilloscope (CRO)  is for measurements , and analysis of waveforms 

— CRO is generally an x-y plotter; on a single screen it can display different signals applied to different channels.

— A moving luminous spot over the screen displays the signal. 

— The central unit of the oscilloscope is the cathode-ray tube (CRT), and the remaining part of the CRO consists of the circuitry required to operate the cathode-ray tube.

Block diagram of a cathode-ray oscilloscope:

COMPONENTS OF THE CATHODE-RAY OSCILLOSCOPE

The CRO consists of the following:

•       ( i) CRT

•       ( ii) Vertical amplifier

•       ( iii) Delay line

•       ( iv) Horizontal amplifier

•       (v) Time-base generator

•       ( vi) Triggering circuit

•       (vii) Power supply 

FUNCTIONAL UNITS

a)       Electron Gun

b)       Evacuated Tube

c)       Deflecting System

d)       Time Base

e)       Trigger Circuit

    ELECTRON GUN

Electron Gun

a)      Heating Filament

b)     Cathode

c)       Grid

d)     Electron lens

Fluorescent Screen:

—  Phosphor is used as screen material on the inner surface of a CRT. Phosphor absorbs the energy of the incident electrons. The spot of light is produced on the screen where the electron beam hits.

—  The bombarding electrons striking the screen, release secondary emission electrons. These electrons are collected or trapped by an aqueous solution of graphite. 

—  Collection of the secondary electrons is necessary to keep the screen in a state of electrical equilibrium.

—  The type of phosphor used, determines the color of the light spot. The brightest available phosphor isotope, P31, produces yellow–green light with relative luminance of 99.99%. 

Electron Gun

•         In the electron gun, electrons are emitted, converted into a sharp beam and focused upon the fluorescent screen.

•         The electron gun consists of an indirectly heated cathode, a control grid, an accelerating electrode and a focusing anode.

•         The electrodes are connected to the base pins. The cathode emitting the electrons is surrounded by a control grid with a fine hole at its centre.

•         The accelerated electron beam passes through the fine hole.

•         The negative voltage at the control grid controls the flow of electrons in the electron beam, and consequently, the brightness of the spot on the CRO screen is controlled.

EVACUATED TUBE

•         Vacuum space

•         Screen

•         Graphite inner wall

DEFLECTING SYSTEM

•         X-plates

•         Y-plates

•         X and Y Deflection Amplifiers

•         X-shift Control

•         Y-shift Control

•         Sensitivity Control

DEFLECTION SYSTEM

•        Electrostatic  deflecting system consists of a pair of horizontal and vertical deflecting plates.

•        Let us consider two parallel vertical deflecting plates P1 and P2. The beam is focused at point O on the screen in the absence of a deflecting plate voltage.

•        If a positive voltage is applied to plate P1 with respect to plate P2, the negatively charged electrons are attracted towards the positive plate P1, and these electrons will come to focus at point Y1 on the fluorescent screen.

The deflection is proportional to the deflecting voltage between the plates. If the polarity of the deflecting voltage is reversed, the spot appears at the point Y2, as shown in Fig. (a).

•          To deflect the beam horizontally, an alternating voltage is applied to the horizontal deflecting plates and the spot on the screen horizontally, as shown in Fig. 14-3( b ).

•          The electrons will focus at point X2. By changing the polarity of voltage, the beam will focus at point X1. Thus, the horizontal movement is controlled along X1OX2 line.

TIME BASE :

•          Sawtooth potential difference

•          Time period control

Display waveform on the screen:

•          Figure shows a sine wave applied to vertical deflecting plates and a repetitive ramp or saw-tooth applied to the horizontal plates.

•          The ramp waveform at the horizontal plates causes the electron beam to be deflected horizontally across the screen.

•          If the waveforms are perfectly synchronized then the exact sine wave applied to the vertical display appears on the CRO display screen.

Triangular waveform:

Similarly the display of the triangular waveform is as shown in Figure.

TIME-BASE GENERATORS:

•          The CRO is used to display a waveform that varies as a function of time. 

•          As the beam velocity is a function of the deflecting voltage, the deflecting voltage must increase linearly with time.

•          A voltage with such characteristics is called a ramp voltage. If the voltage decreases rapidly to zero—with the waveform repeatedly produced, as shown in Fig. 14-6—we observe a pattern which is generally called a saw-tooth waveform.

•          The time taken to return to its initial value is known as flyback or return time.

Trigger Circuit

•          Maintain a stable trace

•          Trig level control

•          Trigger time base

•          automatic triggering

POINTS TO REMEMBER:

•         1 . CRO is used to study waveforms.

•         2 . CRT is the main component of a CRO.

•         3. Prosperous P31 is used for the fluorescent screen of a CRO.

•         4 . A CRO has the following components :

–     ( a) Electron gun

–     ( b) Deflecting system

–     ( c) Florescent screen

•         5. Lissajous figures are used to measure frequency and phase of the waves under study.

•         6. A time-base generator produces saw-tooth voltage.

•         7 . An oscilloscope amplifier is used to give input signal