TUBE FOR PULSE CODE MODULATION 



55 



Optimum performance has been obtained with the electron beam focused 

 to a radius^ of about 5 mils for the grid spacings previously specified. 



The principal irregularities in the periodic quantizing current are caused 

 by variations in secondary emission coefficient of the grid and aperture 

 plate and deflection defocusing of the electron beam. The secondary 

 emission from the grid surface can be made sufficiently uniform by careful 

 processing. This is illustrated in Fig. 7 which is a trace of an oscilloscope 



iNfttfittNtfHti******* 



**^**Hf,tt 



:i|l|H*SM|*M^,n*#**MM*|M.^ji||_^ 



Fig. 7 — Variation of quantizing current with beam deflection. 



Fig. 8 — Deflection defocussing. 



presentation of the secondary collector current as the beam is deflected by a 

 linear sweep at right angles to the grid laterals. The trace was limited to 

 cover a sufficiently small number of wires to show details of its shape. 

 Figure 8a illustrates the effect of deflection defocusing. The sweep was 

 expanded to cover all of the grid wires and the electron beam focused for 

 maximum ampUtude in the center region. The curve is so compressed that 



6 Distance from the center of the beam to a point at which the electron current density 

 is 5% of its value at the center. 



