CRT (Cathode ray tube)


The main part of oscilloscope is CRT. It is an electronic device that produce visual re presentation of electric effect. A cathode ray tube mainly consists of the following:

  1. Electron gun
  2. Electrostatic deflection
  3. Envelop0e and screen

The main component of oscilloscope cathode ray tube (CRT). This is also used in television, radar, and computer displays. The tube is popularly called as picture tube, there are two systems used, i.e electromagnetic and electrostatic. The electromagnetic one uses electromagnets and the deflection is obtained by varying the field strength of the deflection coils. This type is not used in test equipment’s (CRO) and commonly used in radar and television the electrostatic CRT obtains deflection by varying the potential in the deflection plates. This creates electrostatic fields proportional to amplitude of the voltage used. This is the CRT used in electronic instrument.

  • Electron gun –

An electron gun is one which produces electrons, accelerates and conditions them into a thin beam. A gun consists of a heater, cathode, control grid and three anodes. The heater is made up of Tungsten filament and produces heat necessary for thermionic emission. The cathode consists of cylinder of nickel with Barium oxide coated on the cap on one end. Electrons are produced from the end of cathode by thermionic emission. A heat shield is provided around the cathode to concentrate the electron into a beam. The grid or the control electrode is a cylinder enveloping the cathode has an opening in the center of one surface, By varying the potential on the grid the number of electrons forming the beam is controlled.

Adjacent to control grid is the pre-accelerating anode, which has an opening in the end facing the control grid and has another opening to pass the electron beam. The electron beam enters the pre-accelerating anode is scattered because the anode has given with high +ve voltage.

The beam coming out of the pre-accelerating anode is focused by using the focusing anode (i.e. the focusing anode is given with a -ve voltage which repels the scattered electrons and makes the scattered beam into a sharp beam). The accelerating anode accelerates the beam which is made to strike the screen. Since the screen is coated with fluorescent material, it makes a bright spot at the point of hit.

The potential of the pre-accelerating anode is around 300 volt and the potential of focusing anode is about-300 volt. The anode -3 is the most +ve part of the electron gun and it is about 300 volt +ve or about 1000 volt with respect to anode 2.

  • Electrostatic deflection system –

The deflection system consists of two sets of plates, the movement of electron beam is controlled by these deflecting systems, The vertical plates move the beam up and down. while the horizontal plate moves it right and left. The two sets of plates are physically separated fair enough to prevent interaction of fields. This enables vertical and horizontal movements to be independent of one another. Signals are applied to opposite plates of a pair in a push-pull fashion. A d.c. potential on the plates is balanced to place the dots exactly on the center of screen. An a.c. signal of any shape on the horizontal plates will produce a horizontal trace. The length of the trace will be proportional to the signal amplitude.

  • Envelope and Screen –

The CRT employs an evacuated glass envelope and the viewing screen is coated with flouroscent material on the inside front of tube. The flouroscents materials or compounds of light metals such as zinc, cadmium, beryllium and calcium with non-metals such as oxygen, silicon and Sulphur. Normally P6 type phosphor is used for display in colour. The electrons strike the screen with considerable force and cause the material flourescence (bright spot). The emitted light produces a bright spot which is visible from the outside of the screen. The screen can produce either blue or yellow traces on the screen, depending upon the material coated.

As the electrons bounce from the screen, they are collected by the aquadag coatings and return to the power supply. The aquadag is a metal coating which covers all the inside of the forward portion of the CRT as shown in the figure. A very high +ve voltage is connected to the aquadag by an external connector.

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