CATHODE RAY TUBE 



that if the EHT is 2-4 kV, 0-33 mm deflection is produced for a potential 

 difference of 1 V between the deflector plates. 



Satisfactory operation of these tubes is dependent upon the maintenance 

 of the correct mean potential in the region between the pairs of deflector 

 plates with respect to the potential of the third anode. Each pair of plates 

 should be fed in push-pull so that the mean potential remains the same, and 

 the mean potential should be arranged to be equal to, or not far from, the 

 third anode potential. Cathode ray tubes are prone to certain defects, some 

 of which are due to failure to pay sufficient attention to this point. 



O o o o O 



Figure 32.12 



Astigmatism — This is a refusal upon the part of the spot to focus properly. 

 On manipulating the focus control both the height and width of the spot 

 can be made to pass through minima, but the minima do not occur at the 

 same settings of the control; hence the spot passes through the stages 

 shown in Figure 32.12. The trouble is caused by an unsatisfactory relation- 

 ship between the mean potentials of the two pairs of deflector plates. 



Trapezium distortion — This is a deflection distortion caused by failure to 

 drive the pair of deflector plates further from the tube cathode in push-pull. 

 If the tube has X plates further from the cathode, and an attempt is made to 

 use it to draw on the screen a rectangular grid like Figure 32.13a, what in 



(a) 



(b) 



Figure 32.13 



(C) 



fact appears is Figure 32.13b; if the Y plates are further from the cathode 

 and are asymmetrically fed, Figure 32.13c results. 



Trapezium distortion does not follow the use of asymmetric drive to the 

 deflector plates nearer the cathode, but other faults may, such as a variation 

 in the goodness of the focus in different parts of the screen. 



Briefly, the mechanism of trapezium distortion is that the unbalanced 

 drive to the deflector plates causes the mean potential to differ from that of 

 the final anode, setting up an additional weak electrostatic lens. The beam 

 undergoes deflection as it passes the first pair of plates, and is then subjected 

 to refraction in the same plane according to the power of the lens formed by 

 the second pair. 



Pin-cushion distortion — With electrostatic cathode ray tubes, the effect of 



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