Deflection Plate Assembly
Electron beam, after leaving the electron gun, passes through the two pairs of deflection plates called the deflection plate assembly .
One pair of deflection plates is mounted vertically and deflects the beam in horizontal or X-direction and so called the horizontal or X-plates and the other pair is mounted horizontally and deflects the beam in vertical or Y-direction and called the vertical or Y-plates.
These plates are to deflect the beam according to the voltage applied across them. For example id a constant pd is applies to the set of Y-pates, the electron beam will be deflected upward if the upper plate is +ve. In case the lower plate is +ve then the beam will be deflected downward. Similarly, if a constant pd is applied to the set of X-plates , the electron beam will be deflected to the left or to the right of the tube axis according to the condition whether the left or right plate is +ve.
When a sinusoidal voltage is applied to Y-plates, the beam will be moved up and down according to the variation of plate potential. If the frequency of variation is more than 16Hz the deflection will be a vertical line in the centre of the screen. In case the sinusoidal voltage is applied to X-plates and frequency of variation is more than 16Hz the deflection will be a horizontal line. If potentials are applied to both sets of plates simultaneously, the deflection will be an oblique line. The amount of deflection is in proportion to the voltage applied to the pair of plates.