THE L3 SYSTEM — AMPLIFIERS 



903 



Misalignment Adjustment 



We can distinguish three major effects which will contribute to mis- 

 alignment and consequent degradation of system signal-to-noise ratio. 

 One is design error — the degree to which the design gain of the ampUfier, 

 because of the finite number of elements and other limitations, fails to 

 match the Hne loss. Second is the cumulative effect in the individual 

 amplifier of manufacturing deviations of the elements. Third is the aging 

 of the components of the amplifier, of which the tube aging mil of course 

 be the dominant short term effect. The signal-to-noise performance of 

 the system can be improved by reducing the misahgnment, if this is 

 done without resorting to measures which would introduce systematic 

 instead of random gain deviations. 



If we study the shapes of gain change introduced by the more im- 

 portant deviation contributors, particularly the element variations, we 

 find as might be expected that in general the effects are small at low 

 frequencies and increase sharply near the upper edge of the band if the 

 thermistor is held constant. In the system, the regulation around the 

 main pilot will automatically act to reduce the deviation at 7 mc to zero, 

 adding a square root of frequency curve that results in a bow shaped 

 deviation as illustrated by Fig. 15. Examining the signal-to-noise effects 

 of the degradation caused by misalignment, we conclude that it is most 

 desirable to reduce as much as possible the misahgnment at 4 mc, the 

 television carrier frequency in the combined system. The output beta 

 circuit has, therefore, been designed to give var3dng amounts of this 

 shape, the total range being dzO.6 db at 4 mc in 0.2 db steps. The suc- 

 cessive steps of this gain adjustment are simple multiples of each other, 

 symmetrical in the two directions of adjustment, so that we put into 



(CIRCUIT AND 

 TRANSFORMER 

 PARASITICS ) 



^--TEST POINT 

 (T FOR MEASURING 

 CATHODE VOLTAGE 



Fig. 14 — Input beta circuit elements — line amplifier. 



o 



+.B 



