CATHODE RAY OSCILLOSCOPE( C R O)
CATHODE RAY OSCILLOSCOPE ( C R O )
The cathode ray oscilloscope is an extremely useful and the most versatile laboratory instrument used for studing wave shapes of alternating currents and voltages as well as for measurement of voltage , current , power and frequency ,in fact ,almost any quantity that involves amplitude and waveforms. it allows the user to see the amplitude of electrical signals as a function of time on the screen. A CRO is an instrument which present signal wave - forms visually .it is also useful for comparing two signals in phase , frequency or amplitude.
A CRO can operate upto 50 MHz , can allow viewing of signals within a time span of few nanoseconds and can provide a number of wavefrms display simultaneously on the screen . it also has the ability to hold the displays for short or long time so that original signal may be compared with one coming on later.
BLOCK DIAGRAM OF CATHODE RAY OSCILLOSCOPE
CATHODE RAY TUBE ( C.R.T )
Cathode ray tube is the ' heart 'of an oscilloscope and is very similar to the picture tube of television set.
The cathode ray tube or CRT is a vacuum tube of special geometrical shape which converts an electrical signal into a visual one.
A CRT makes available a large number of electrons which are accelerated to high velocity and are brought to focus on a fluorescent screen where it produces a spot when strikes it. The electron beam is deflected during its journey in response to the applied electrical signal. As a result, the electrical signal waveform is displayed visually.
The figure below shows various parts of a cathode ray tube (CRT) .
Now we will discuss each part of the CRT in detail.
(i) Glass Envelope
It is a conical highly evacuated glass housing which maintains vacuum inside it and supports various electrodes.
The inner wall of CRT between the neck and screen are usually coated with a conducting material known as aquadag. This coating is electrically connected to the accelerating anode so that the electrons which accidentally strike the walls are returned to the anode. This prevents the walls from charging to a high negative potential.
(ii) Electron Gun Assembly
The electron gun assembly consists of an indirectly heated cathode, a control grid, a focussing anode and an accelerating anode and it is used to produce a focused beam of electrons.
The control grid is held at negative potential w.r.t. cathode. However, the two anodes are held at high positive potential w.r.t. cathode.
The cathode consists of a nickel cylinder coated with oxide coating and provides a large number of electrons.
The control grid encloses the cathode and consists of a metal cylinder with a tiny circular opening to keep the electron beam small.
By controlling the positive potential on it,the focusing anode focuses the electron beam into a sharp pin point.
Due to the positive potential of about 10,000 V on the accelerating anode which is much larger than on the focusing diode, the electron beam is accelerated to a high velocity.
In this way, the electron gun assembly forms a narrow, accelerated electron beam which produces a spot of light when it strikes the screen.
(iii) Deflection Plate Assembly
It consists of two sets of deflecting plates within the tube beyond the accelerating anode and is used for the deflection of the beam.
One set is called as vertical deflection plates and the other set is called horizontal deflection plates.
The vertical deflection plates are mounted horizontally in the tube. On application of proper potential to these plates, the electron beam can be made to move up and down vertically on the screen.
The horizontal deflection plates are mounted vertically in the tube. On application of proper potential to these plates, the electron beam can be made to move right and left horizontally on the screen.
(iv) Screen
The screen is coated with some fluorescent materials such as zinc orthosilicate, zinc oxide etc and is the inside face of the tube.
When high velocity electron beam strikes the screen, a spot of light appears at the point of impact. The colour of the spot depends upon the nature of fluorescent material.
WORKING OF A C.R.T
As the cathode is heated, it produces a large number of electrons.
These electrons pass through the control grid on their way to the screen.
The control grid controls the amount of current flow as in standard vacuum tubes. If negative potential on the control grid is high, fewer electrons will pass through it. Hence the electron beam will produce a dim spot of light on striking the screen. Reverse will happen when the negative potential on the control grid is reduced.
Therefore, the intensity of the light spot on the screen can be controlled by changing the negative potential on the control grid.
After leaving the control grid, the electron beam comes under the influence of focusing and accelerating anodes.
Since, the two anodes are at high positive potential, therefore, they produce a field which acts as electrostatic lens to converge the electron beam at a point on the screen.
After leaving the accelerating anode, the electron beam comes under the influence of vertical and horizontal deflection plates.
When no voltage is applied to these deflection plates, the electron beam produces a spot of light at the centre as shown by point O in fig below on the screen.
If the voltage is applied to the vertical deflection plates only, the electron beam and so as the spot of light will be deflected upwards i.e. point O1. Ans if the potential on the plates is reversed, the spot of light will be deflected downwards i.e. point O2.
Similarly, the spot of light can be deflected horizontally by applying voltage across the horizontal deflection plates.
APPLICATION OF C .R.O :-
1. Tracing of an actual waveform pf current or voltage.
2. Determination of amplitude of a variable quantity.
3. Comparision of phase and frequency.
4. IN televisions.
5 . In radar.
6. For finding B.H curves for hysteresis loop.
7. For engine pressure analysis.
8. For studying the heart beats , nervous reactions etc.
9. For tracing transistor curves.
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