54 BELL SYSTEM TECHNICAL JOURNAL 



at the grid by the impinging primary beam for the quantizing action rather 

 than the directly intercepted electron current as heretofore assumed. The 

 secondary electron collector located in front of the quantizing grid is main- 

 tained at a positive potential and collects most of the secondaries from the 

 grid. There is, of course, a correspondence between the secondary electron 

 current and the fraction of the beam current intercepted by the grid wires. 

 The quantizing circuit is made by connecting the feedback path to the 

 secondary collector and the quantizing action proceeds as described hereto- 

 fore. This method has the following advantages over the direct primary 

 current method: (1) the collector current as a function of beam position is 

 much more regular; (2) the swing between maximum and minimum current 

 is considerably larger because of the secondary emission multiplication at 

 the grid surface; and (3) the capacitance between collector and ground is 

 lower than the capacitance between the closely spaced grid and aperture 

 plate. 



With secondary electron current feedback, the aperture plate is operated 

 at a positive potential relative to the grid to suppress secondary electrons 

 from the aperture plate. The proportion of the secondary emission from 

 the grid collected by the aperture plate is small compared with that collected 

 by the secondary collector. High velocity secondaries originating at the 

 aperture plate are, however, able to penetrate the retarding field and strike 

 the grid. These energetic secondaries produce low-velocity secondaries at 

 the grid which flow to the secondary cojlector electrode. This alters the 

 character of the secondary collector current in accordance with the spacial 

 pattern of the apertures in the code plate. The surface of the aperture plate 

 is carbonized to reduce the emission of high- velocity secondaries. The spa- 

 cial variation of the quantizing current is reduced to less than 10% of 

 the total quantizing current swing with this treatment. 



The periodic nature of the quantizing current with beam position must be 

 as uniform as possible as regards both the ''a-c" and ''d-c" components. 

 The maximum swing should also be as large as possible to permit the use 

 of the lowest possible impedance in the feedback path to obtain a wideband 

 characteristic for fast coding. 



The factors which determine the maximum current swing are electron 

 beam current, secondary emission coefficient of the grid wires, electron 

 beam spot size, grid wire diameter and spacing. The quantized beam falls 

 approximately halfway between the center of a grid wire and the midpoint 

 between wires. The beam must be small enough that its edge does not 

 extend into the region beyond the wire on which it ''leans" by an appre- 

 ciable amount. This is obtained with the electron beam focused to a spot 

 slightly smaller than that for maximum quantizing current amplitude. 



