TUBE FOR PULSE CODE MODULATION 49 



position p5. If the beam is perturbed to the left from position />4 it will 

 continue to move to the left until it reaches position p3. 



The number of possible equilibrium beam positions for an input signal 

 sample depends on the maximum values of the feedback voltage and the 

 slope of the deflection characteristic. It is necessary that only one equilib- 

 rium beam position be available for a particular small range of signal voltage. 

 This can be achieved as follows: if a signal voltage e is established with 

 the feedback circuit inoperative, the beam will be at a position p, Fig. 3. 

 When the feedback circuit is activated with the signal voltage held at e, the 

 beam will move from p to p5. With this procedure, signals in the range 

 from ei to 62 will result in equilibrium beam positions between points / and 

 r on the curve. Thus, for signal voltages within the range from ei to 62 the 

 beam will fall in the small spacial interval m, whereas beam positions for 

 input signals in the same range without feedback would vary from grid 

 wire 5 to grid wire 6. Likewise, signal voltages between e^ and 63 will cause 

 the beam to assume positions between 5 and / in the spacial interval n. 

 The electron beam may be thought of as "leaning" on one side of a grid 

 wire for a finite signal voltage range and on the same side of the next grid 

 wire for an adjacent signal voltage range. 



If the feedback voltage is of the opposite polarity to that assumed above, 

 the quantizing action proceeds in the same manner except that the quan- 

 tized beam positions lie at the left of the wires. The beam may be thought 

 of as "leaning" on the opposite side of the grid wire. 



The proper quantizing action is obtained by establishing and holding the 

 signal voltage with the feedback circuit inoperative and then activating this 

 circuit. The feedback circuit may be deactivated and activated by either 

 (a) blanking and deblanking the electron beam, or (b) defocusing and 

 focusing the electron beam by applying the proper voltage change to the 

 beam control or focusing electrodes of the gun, respectively. 



Since the grid wires are parallel to the horizontal rows of aperture holes, 

 the feedback action constrains the beam to sweep out the code group initially 

 selected even though the sweep axis is tilted slightly with respect to the 

 grid wires and aperture plate. The maximum swing of the feedback volt- 

 age at the deflection plates should be about three or four times the value of 

 the voltage required to deflect the beam from one code group to the next in 

 order to provide ample protection against the beam jumping from one code 

 group to the next code group during the sweep. 



The Experimental Coding Tube 



The seven-digit experimental tube developed for pulse code transmission 

 system trials utilizing the electrode lineup shown schematically in Fig. 2 is 

 pictured in Fig. 4. The electron gun assembly and the target plate assembly 



