THE L3 SYSTEM — EQUALIZATION AND REGULATION 



873 



but with the biases and feedbacks differing in order to provide an EI 

 characteristic that corrects for sensitivity changes of the thermistor 

 with operating current. This maintains the overall loop feedback rela- 

 tively constant over the 1 to 20-ma current range. The thermistor is 

 directly heated by the plate current of the dc amplifier in order to obtain 

 single time-constant performance of the thermistor. The thermistor 

 transmission, plate current changes as an input and pilot level as an 

 output, is the main frequency characteristic of the regulator loop. 



LINE THERMOMETER REGULATORS 



It is possible to dilute the regulation system with less costly, less ac- 

 curate regulators without undue loss of overall performance. This is 



•190 o 



MANUAL 

 PILOT 

 LEVEL 



CONTROL 



AMBIENT 

 COMPENSATION 



REGULATING 

 THERMISTOR 



BURIED 



THERMOMETER 



THERMISTOR 



Fig. 28 — Thermometer regulator schematic. 



accomplished by the use of thermometer regulators at alternate regu- 

 lating points. These consist of a thermometer thermistor buried in the 

 ground, electrically in parallel with the regulating thermistor. The 

 circuit is quite simple as indicated by Fig. 28. Ground temperature 

 changes vary the resistance of the thermometer thermistor thereby 

 changing the current and resistance of the regulating thermistor. The 

 manual control is used to effect initial alignment of the system. The 

 regulating sensitivity is designed to slightly overcompensate for cable 

 loss changes in order to somewhat ease the burden on the following 

 dynamic regulator. 



AMBIENT TEMPERATURE COMPENSATION 



Both types of line regulators require the assistance of ambient tem- 

 perature compensation of the regulating thermistor. Conventional com- 

 pensation circuits would hold the thermistor resistance within about 

 20 per cent but this would produce an error of one db at a thermometer 



