What is CRO(Cathode Ray Oscilloscope)?Full Details-Knowledge Gyan

Introduction

CRO is an Electronic measuring instrument may be defined as a device. Based on electronic and electrical principles,for the measurement pf physical quantities. HIgher reliability and lower cost of rapidly developing sophisticated electronic technologies have resulted in phenomenal changes in the measurement techniques.

What is CRO(Cathode Ray Oscilloscope)?

The Cathode Ray Oscilloscope (CRO) is a very useful and versatile testing and measuring instrument for electrical and electronic circuits. Basically it measures electrical signals in the form of voltage and current varying with time.

It can directly display electrical signals as a function of time and can measure quantities such as frequency, phase shift,amplification factor,etc. It can also measure nonelectrical parameters. For this purpose nonelectrical quantities like pressure,temperature etc, are to be first converted into signal by using transducers.

Working Principle

A modern CRO is an assemblage of a number of sophisticated circuits. The main component of the CRO is a Cathode Ray tube(CRT). The CRT consists of a highly evacuated tunnel-shaped glass tube.It has an electron gun which generates a sharply focused beam of electrons of high velocities.Electrons are emitted from a cathode(K) heated by a filament (FF) supplied with adequate current.The emitted electrons pass through a small aperture drilled on the axis of a control grid(G) which has a tubular structure.The flow of electrons out of the aperture can be conntrolled by changing the control grid bias by a potentiometric arrangement.

The electron beam, on emergence out of the grid aperture, gets accelerated and focused by electrostatic lens(A1,A2,A3). The beam then passes through two pairs of deflecting plates. Usually the voltage under study is applied to the vertical deflecting plates V,V(or,Y-plates). It deflects the electron beam in virtual direction. Another voltage proportional to Time,called time base, is applied to the horizontal deflecting plates H, H( or,X-plates). Finally the electrons beam impinges on the fluorscent screen and produces visible pattern of the signal. 

A graticule with calibrated horizontal and vertical marks is fixed on the CRO screen.It helps in making measurements. The screen usually consist of a flat glass plate outside of which is optically polished and inside of which has a coating of fluorescent material(called phospors) . When no voltage is applied to X-or Y-or plates the high velocity electrons on striking the screen produces a visible spot or dot. If such a light spot sufficient intensity is left stationary on the screen for a long time it may damage the screen through overheating.This is referred to as the burning of the screen.

Block Diagram Of CRO:

For proper functioning a CRO is provided with a number of circuits: 

Vertical (Y) deflection system: The signal under test is applied to the vertical deflecting plates through an attenuator, the vertical amplifier and so called delay-line. The gain of this amplifier is set by the attenuator,usually calibrated as VOLTS/DIV.The delay line allows the input signal to arrive at the Y-deflection plates after the beam has started moving horizontally. 

Horizontal(X) deflection system: The horizontal (X) deflection plates are fed by linear sawtooth (time base) voltage through a horizontal amplifier of adequate gain. It causes the electron beam to move across the screen in a horizontal line from left to right. During thr flyback from right to left there may be a return trace which is eliminated or blanked out by applying a high negative voltage to the grid during fly back period. To get a stationary pattern on the screen the horizontal deflection must start at the same point of the input signal each time it sweeps.For this a sychronizing or triggering circuit is used. Its purpose is to synchronize the frequency of the sweep signal to the input signal.A part of the input signal is applied to the trigger circuit which generates a trigger pulse at some selected point on the input wave and starts the time base generator each time.The triigger circuit provides a link between the vertical input and the horizontal time base.

Intensity Control: Intensityof the light spot on the fluorscent screen depends on cathode current and is controlled by changing the control grid bias by a potentiometer arrangement.

Focus Control: Good focusing is obtained by changing the focal length of the electrostatic lens system.This is done by varying the potential of the focusing anode.

Position Control: The horizontal and vertical positions of the trace on the screen can be controlled by applying d.c. voltages to the deflecting plates.

Power Supply: Besides the basics circuit blocks shown. There are two power supply section. A high voltage section is used to operate the CRT. A low voltage section provides d.c. voltages of different magnitudes and is used to operate various electronic circuits of the CRO.

CRO probes:

CRO probe is a shielded coaxial cable used to connect the signal source to the CRO. It is provided with trimmer capacitor to adjust the frequency response to an optimum value. Such probes can also be made to have a 10:1 attenuation of the input signal.

Uses of CRO:-

1.Study of waveforms. 

2.Measuement of frequency:

 a. Using the CRO time base. 

b. Using Lissajous figure. 

3.Phase measurement:

 a. Using a dual-trace CRO. 

b. Using Lissajous figures.

4. Others Uses. 

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