TUBE FOR PULSE CODE MODULATION 



47 



electrons when bombarded. Electrons which strike the grid produce a cur- 

 rent in the grid circuit while electrons that miss the grid have no effect in 

 the grid circuit. The electron beam current intercepted by the grid will be 

 dependent on the y deflection or position of the electron beam. The cur- 

 rent is a maximum when the beam is centered on a grid wire and a mini- 

 mum when it is centered between two grid wires and varies with beam posi- 

 tion as indicated by the curve in the lower portion of Fig. 3. The curve is 

 constructed for the case in which the beam diameter is slightly greater than 

 the space between two grid wires. The current to the grid never becomes 

 zero for any beam position. It may be thought of as having a ''d-c. com- 

 ponent" B. Amplifier 2 in Fig. 2 introduces a bias which cancels this "d-c. 

 component" so that the feedback voltage is symmetrical about zero. 



• INPUT 

 VOLTAGE 



AMPLIFIER 



plates: 



APERTURE OUTPUT 



ELECTRON DEFLECTOR 

 GUN PLATES 



AMPLIFIER 



2 



Fig. 2 — Coding tube with quantizing grid and circuit schematic. 



The grid current is amplified and the voltage developed is applied in feed- 

 back relation to the F deflection plates as shown in Fig. 2. The case in 

 which a positive feedback potential deflects the beam in the same direction 

 as that for a positive signal voltage will be considered. 



The beam deflecting voltage is equal to the sum of signal and feedback 

 voltages and the beam position is a linear function of the deflection voltage 

 so we may write 



e + Dy = 6/ 



(1) 



where e is the input signal voltage, D the deflection constant, y the beam 

 deflection or position and e/ the feedback voltage. The feedback voltage 

 6/ is a periodic function of beam position y. Equation 1 therefore defines 

 equilibrium beam positions for input signal voltage e. Equilibrium beam 

 positions in accordance with Equation 1 are determined graphically in the 



