46 BELL SYSTEM TECHNICAL JOURNAL 



perimental pulse code system uses a seven-digit code plate which provides 

 for coding 128 amplitude values. 



The vertical distances between the centers of successive code sweep posi- 

 tions (horizontal dashed lines in Fig. lb) are made equal so that the codes 

 are spaced by equal input voltage steps. A continuous range of input sig- 

 nal amplitudes will result in a continuous range of horizontal sweep positions. 

 With an infinitely small electron beam, input signal amplitudes in the range 

 from to ji will produce a single output pulse group and input signal sam- 

 ple amplitudes from ji to y2 will produce another output pulse group. This 

 process of dividing the total input amplitude range into finite steps and ar- 

 ranging that input voltages falling within each step produce one and only 

 one output pulse group is called quantization. 



The tube of Fig. 1 will only quantize effectively if the electron beam is 

 infinitely small and the sweep and aperture plate axes are aligned exactly. 

 With a finite beam size, there will be sweep positions for which the beam 

 straddles and sweeps out a combination of two adjacent codes. 



Precise quantization and a uniform pulse output are required. The 

 problems of quantizing, alignment and uniform pufee output have been 

 solved by the use of a wire grid, called the quantizing grid, located in front 

 of the aperture plate. 



Quantization of Beam Position by Feedback 



The quantizing grid consists of a horizontal array of grid wires aligned 

 parallel to the code sweep or X axis of the aperture plate. The grid spacings 

 and alignment are such that a wire lies between each adjacent pair of code 

 groups as viewed from the various incident angles of the deflected electron 

 beam. The quantizing grid, by means of an electrical feedback path to the 

 signal deflection plates, divides the input signal range into a number of 

 equal steps and positions the electron beam to the proper level for the code 

 corresponding to the voltage step within which the signal amplitude sample 

 falls. The quantizing grid wires also constrain the electron beam during 

 the formation of the output code pulses, so that it must sweep out the code 

 initially selected. In general, wires or shaped electrodes of any sort located 

 where the electron beam can impinge thereon and connected in feedback 

 relation to the deflection system constrains the electron beam to move in 

 patterns prescribed by these electrodes and are thus called beam guides. 



The coding tube with quantizing grid and feedback circuit is shown 

 schematically in Fig. 2. The electrode line-up, reading from left to right 

 in the figure, consists of an electron gun, deflection system, secondary elec- 

 tron collector, quantizing grid, aperture plate and output plate. 



For the present purpose, a consideration of the collector and output plate 

 electrodes is omitted and it is assumed that the grid does not emit secondary 



