Q. Explain working of CRT with suitable diagram discuss the focusing arrangement. What is need of aquadag?

OR

Why are the operating voltages of a cathode ray tube arranged so that the deflection plates are nearly at ground potential?

Sol. A cathode ray oscilloscope consists of a cathode ray tube (CRT), which is the heart of the tube and some additional circuitry to operate the CRT. Main parts of a CRT are:

(i)                                     Electron gun assembly

(ii)                                   Deflection plate assembly

(iii)                                 Fluorescent screen

(iv)                                  Glass envelope

(v)                                    Base, through which connections are made to various parts.

The main parts of a CRT are shown in before going into details of working of various parts of a CRT; a summary of functions of the different parts is given:

The "Electron gun assembly" produces a sharply focused beam of electrons which are accelerated to high velocity. This focused beam of electrons strikes the fluorescent screen with sufficient energy to cause a luminous spot on the screen.

After leaving the electron gun, the electron beam passes through two pairs of "Electrostatic deflection plates". Voltages applied to these plates move the beam vertically up and down and the voltages applied to the other pair of plates move the beam horizontally from one side to another. These two movements horizontal and vertical are independent of each other and thus the beam may be positioned anywhere on the screen.

        The working parts of a CRT 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.

     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.

In smaller CRTs, connections to the various electrodes are brought out through pins in the base of the tube as shown in large and medium sized high performance tubes operate at very high voltages and these leads are usually brought out through the sides of the glass envelope.

   Electrons are emitted from the indirectly heated cathode. A layer of barium and strontium oxide is deposited on the end of the cathode, which is a cylinder-to obtain high emission of electrons at moderate temperatures. The typical values of current and voltage required by an indirectly hi=elated cathode are 600 m A at 6.3 V. High efficiency systems use 300 m A at 6.3 V. These electrons pass through a small hole in the "control grid". This control grid is usually a nickel cylinder, with a centrally located hole, coaxial with the CRT axis. This is usually a metal cup of low permeability steel, about 15 mm in diameter and 15 mm long. An aperture of about 0.25 mm is drilled in the cap of the grid for the electrons to flow through. The intensity of electron beam depends upon the number of electrons emitted from the cathode. The grid with its negative bias controls the number of electrons emitted from the cathode and hence the intensity is controlled by the grid.

       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 anodes".

 The electron beam is focused by the "focusing anode". The accelerating and focusing anodes are cylindrical in form, with small openings located in the centre of each electrode, coaxial with the tube axis. After leaving the focusing anodes, the electron beam passes through the vertical and horizontal deflection plates and then goes on to the fluorescent screen.  

 The pre-accelerating anode and the accelerating anode are connected to a common positive high voltage of about 1500 v. The focusing anode is connected to a lower adjustable voltage of 500 V. There are two methods of focusing an electron beam

(i)                                        Electrostatic focusing and

(ii)                                      Electromagnetic focusing.

  The CRO uses electrostatic method of focusing as compared to a TV picture tube which employes electromagnetic focusing.

     Electrostatic focusing. Shows an electron at rest placed in an electric field produced two parallel plates.

  The minus sign indicates that the force acts in the force acts in the opposite direction to that of the field. The above discussion is valid only if the electron is situated in a field of uniform intensity. In practice, however, the field is not uniform.

 The lateral repulsion of the electric field lines causes spreading of space between the lines, resulting in curved field lines at the ends.

  Thus the field intensity will be less at the ends also shows equipotential surfaces, indicated by solid lines.

Since the force is in a direction opposite the field and the equipotential surfaces are perpendicular to the field, the force on an electron is in a direction normal to the equipotential surfaces.

 Shows two concentric cylinders with a potential applied between them. Lateral repulsion again causes the spreading of the flux lines produci9ng a field as shown. The equipotential surfaces are shown as solid lines. It is clear from the diagram that the equipotential surfaces are curved. Let us consider the region on the two sides of an equipotential surface S as shown in the potential on the left side of the surface is V and on the right side is + V. let an electron moving in a direction AB enter the area to the left of S. This electron experiences a force which is normal to the surface S and is thus accelerated.

                 Since the force acts in a direction normal to the surface, it is the normal component of velocity that is increased after refraction while the tangential component remains the same.

Eqn. 2 is identical to the expression relating the refraction of a light beam in geometrical optics. The refraction of an electron beam follows as the bending of a light bema at a refracting surface such as an optical lens. For this reason the focusing system in a CRT is known as electron lens.

Shows the functional diagram of an electrostatic focusing arrangement, the pre-accelerating anode, which is a metal cylinder containing many baffles, collimates the electron beam which enters it through a small opening on the left hand side. The pre-accelerating anode is connected to a high positive potential. 

           The focusing anode and the accelerating anodes are co-axial with the pre-accelerating anode. The pre-accelerating and accelerating anodes are connected to the same potential while the focusing anode is connected to lower potential.

      On account of difference of potential between focusing anode and the two accelerating anodes, a non-uniform field exists on each of the two ends of the focusing anode. The equipotential surfaces, thus, form a "double concave lens".

      The electron beams entering the field at angles other than the normal to the equipotential surfaces, will be deflected towards the normal and the beam is thus focused towards the centre of the tube axis. By changing the voltage of the focusing anode, the refractive index of the electron lens is changed and therefore the focal point of the beam can be changed. The change in voltage is brought about by changing the setting of a potentiometer. This control is brought to the front panel of CRO and id marked focus.