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DISPLAY SYSTEM DESIGN PROBLEMS 



The electrostatic tube is shown in Fig. 12-9. The cathode, control grid, and 

 anodes are cylindrical in shape, and their axes coincide with the axis of the 

 tube. The voltages which are applied to the first and second anodes 

 accelerate the electrons and cause them to be focused into a narrow beam. 

 Focusing is usually controlled by adjusting the first anode voltage; some 

 focusing action can be effected by varying the voltage of the second anode. 



Beam Deflection. Deflection of the cathode ray beam can be accom- 

 plished by either a magnetic or an electric field. In the magnetic field 

 deflection system the electron beam is deflected by a magnetic field which 

 results from passing current through a set of deflection coils. The coils are 

 multilayer windings on either a square or a circular iron core as shown in 

 Fig. 12-10. Because of the large air gap and the low magnetizing force that 



(a) Circular Deflection Yoke (b) Square Deflection Yoke 



Fig. 12-10 Types of Deflection Yokes. 



is usually used, the iron core serves more to shape the magnetic field than 

 to increase the sensitivity. The two coils of Fig. 12-lOa are connected so 

 that their developed fluxes are in opposition around the iron core but add 

 along the vertical diameter; thus horizontal deflection occurs. The square 

 yoke of Fig. 12-lOb may be used to provide both horizontal and vertical 

 deflection in a single unit. The two horizontal coils produce a horizontal 

 field (and vertical deflection) while the two vertical coils produce a vertical 

 field (and horizontal deflection). The deflection produced by a magnetic 

 field is normal to the direction of the field. The magnitude of deflection is 

 a linear function of the magnetic field strength, which is in turn proportional 

 to the coil current. 



Deflection of the electron beam by an electric field is accomplished by 

 developing an electric potential between two pairs of plates located just 



