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BELL SYSTEM TECHNICAL JOURNAL 



the individual oscillations of the current between maximum and minimum 

 values can hardly be resolved. The envelope of the curve indicates the 

 extent to which the swing is reduced at the two ends by the increase in 

 electron beam spot size which results from deflection defocusing. In Fig. 

 8b, the focusing voltage has been changed by 12 volts and it can be seen 

 that the beam is in focus at the maximum deflection angles and out of 

 focus in the center region. With an intermediate or compromise focus volt- 

 age, the tube will quantize satisfactorily over the entire code range. Best 

 results have been obtained with the tube in the experimental pulse code 

 system by the use of a simple circuit which changes the focus voltage in a 

 linear manner with the rectified or absolute value of the input deflecting 

 signal thus compensating for the deflection defocusing of the tube. 



Fig. 9 — Typical pulse code outputs. 



The output plate is usually operated at a positive potential relative to the 

 aperture plate with attendant suppression of secondary emission from the 

 former. Output pulses of opposite polarity may be obtained by operating 

 the output plate at a negative potential and processing this plate to have a 

 secondary emission ratio greater than unity. 



The several groups of output pulses shown in Fig. 9 are illustrative of the 

 tube output. Successive digit pulse positions occur from left to right in 

 the figures. The small kink in the base line at the first and second digit 

 pulse positions in codes a and b respectively are small error pulses caused 

 by a small portion of the edge of the beam overlapping a grid wire when the 

 beam is quantized. Error pulses of this magnitude are readily eliminated 

 by slicing when the tube is used in the pulse code transmission system. 



