Cathode Ray Tube (CRT)

Cathode ray tube acts as the heart of the cathode ray oscilloscope (CRO). Below figure shows the basic construction and parts of cathode ray tube (CRT).

Principle of CRT

CRT works on the principle of thermionic emission i.e. emission of electrons from a heated surface. The various parts of CRT are:
1. Electron Gun
2. Pre-accelerating and accelerating anode
3. Horizontal deflecting plate
4. Vertical deflecting plate
5. Fluorescent screen
6. Glass envelope
7. Aquadag coating

The parts of CRT are described as below:

1. Electron Gun

The source of focused and accelerated electron beam is the electron gun. The electron gun, which emits electrons and forms them into a beam consists of a heater, a cathode, a grid, a pre-accelerating anode, a focusing anode and an accelerating anode. Electrons are emitted from the indirectly heated cathode which has a layer of barium and strontium oxide on the end of the cathode. These electrons pass through a small hole in the control grid, which is a nickel cylinder. The intensity of the electron beam depends upon the number of electrons emitted from the cathode. The grid controls the number of electrons emitted from the cathode and hence the intensity.

2. Pre-accelerating and Accelerating Anode

The electrons, emitted from the cathode and passing through the hole in the control grid are accelerated by the high positive potential which is applied to the “Pre-accelerating” and accelerating anode. The electron beam is focused by the “focusing anode”. The pre-accelerating and accelerating anodes are connected to a common high positive voltage of around 1500 V. The focusing anode is connected to a voltage of around 500 V.

Focusing of an electron beam can be done by two methods:

(i) Electrostatic focusing technique and
(ii) Magnetostatic focusing technique.

(i) Electrostatic Focusing Technique: In below figure, the force acting on the electron (at rest) placed between the two parallel plates in an electric field is given by

F = -eE Newton

This force acts opposite to the direction of electric field intensity (E )

Below figure shows the functional diagram of an electrostatic focusing technique. Here, the preaccelerating and accelerating anodes are applied with a common voltage while the focusing anode is connected to a lower voltage. Due to the difference of potential between the focusing anode and the two accelerating anode, a non-uniform electric field exists on each of the two ends of the focusing anode. The equipotential surfaces, thus form a “double concave lens”. Below figure shows a double concave electron lens.

Electrostatic focusing technique is used in laboratories.

(ii) Magnetostatic Focusing Technique: One of the other two methods by which the focusing of electron beam can be done is magnetostatic focusing technique. Magnetostatic focusing technique is used in television picture tube. However, computer CRT uses both electrostatic and magnetostatic focusing technique.

3. Horizontal Deflecting Plate
The horizontal plate is applied with a sawtooth waveform. This waveform is produced by the UJT relaxation oscillator. This UJT is inbuilt in the CRT.

The horizontal scale of the beam is produced by the potential applied to the horizontal deflecting plates. The beam moves from left to right across the CRT screen. It is deflected to the right by the increasing amplitude of the ramp voltage and the fact that the positive voltage attracts the negative electrons.

Horizontal deflecting plates are mounted vertically and produces a horizontal deflection.

During retrace time of the waveform, a blanking circuit produces a signal so that the cathode grid is applied with high negative potential which steps or blocks the electrons entering inside the CRT.

4. Vertical Deflecting Plate
Vertical deflecting plate pairs are mounted horizontally and produces an electric field in the vertical plane. The test signal whose waveform is to be observed on the screen is applied to the vertical deflecting plates.

A “synchronizing circuit” is used to synchronize both the horizontal and the vertical signal by adjusting the time period of the sawtooth waveform.

The electron beam after the vertical deflecting plates hits the screen and a phosphorous coating converts the heat into light energy.

5. Fluorescent Screen
The front of the CRT is called the face plate. Some CRT uses face plate made up of fibre optics. Face plate is coated with phosphor added with traces of other elements called activators. Light is emitted during phosphor excitation which is known as fluorescence. The electron beam on striking with the phosphor gives off light and heat. The luminance of the phosphor is a measure of its brightness.

6. Glass Envelope
The whole CRT parts are enclosed in an evacuated glass envelope so that the emitted electrons are able to move about freely from one end of the tube to the other.

7. Aquadag Coating
Aquadag coating is used to collect the secondary electrons emitted when the electron strikes the screen. Aquadag is an aqueous solution of graphite. Collection of secondary electrons is necessary to keep the CRT screen in a state of electrical equilibrium.

Note:

Focus control in CRT: Focus in CRT is done by adjusting the focusing anode potential.
Position control in CRT: Small DC potential is applied to the deflecting plates to keep the position of the beam at the centre of the screen. For accurate positioning, at the middle of the screen “Astigmatism” is used.
Intensity control: Intensity of the beam is controlled by varying the cathode grid negative potential.
Calibration circuit: The calibration circuit of oscilloscope uses a square waveform calibrating voltage.

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