N-1 CARRIER TELEPHONE SYSTEM 425 



limit non-linear characteristic is obtained. At the left of Fig. 5 is a view of 

 the compressor vario-losser assembly and extending to the right the com- 

 ponents from which it is constructed. Similar construction is used for the 

 expandor and channel modulator units. The vario-losser units function by 

 virtue of the fact that their a-c impedance can be varied and closely con- 

 trolled by a d-c bias. Consequently, when made a part of a suitable network 

 and controlled by a d-c bias proportional to the signal level, the compressor, 

 which comprises four varistor elements, can be made to increase its attenua- 

 tion as the signal increases; while in a different network, the expandor, 

 which comprises six varistor elements, can be made to decrease its attenua- 

 tion with increasing signal. The close degree to which the compressor and 

 expandor characteristics must complement each other makes it necessary to 

 use varistor elements that are very precisely controlled as to their a-c im- 

 pedance at specific values of bias current. This is accompHshed by careful 

 selection of elements which comprise only a fraction of the total distribution 

 of characteristics produced and then grouping these selected units into as- 

 semblies as illustrated. These selected groups must then pass transmission 

 requirements which are directly related to the compandor performance. 



The channel modulator is also composed of selected germanium varistors 

 but, unlike the vario-lossers, the modulators do not all have to be substan- 

 tially alike. It is sufficient that the four elements comprising any one modu- 

 lator be alike to control the carrier leak. One modulator may then differ 

 considerably from another in impedance. 



Copper oxide instead of germanium varistors are used in the group modu- 

 lators at terminals and repeaters because their lower impedance level and 

 somewhat lower noise figure give better performance in these circuits. 



Thermistors 



A thermistor, which introduces a large change of resistance with tempera- 

 ture, is used to regulate the gain of the repeaters and group amplifiers. 

 The thermistor element is a tiny pellet of semi-conducting oxides which is 

 equipped with lead wires, a glass coating and an insulated heater. This 

 whole assemblage, which is less than a tenth of an inch in diameter, is 

 covered with a bright gold coating and enclosed in an evacuated glass tube 

 to reduce heat losses. A network consisting -of a thermistor disc and two 

 wire wound resistors, and tailor-made on the basis of precision measure- 

 ments on the individual thermistor and heater, is included in the assembly 

 to serve as a contactless thermostat for the power sensitive thermistor 

 pellet so that the resistance of the latter is wholly under the control of 

 transmission currents. The thermostat network also serves to adjust the 

 pellet to standard characteristics, thus avoiding impracticable close toler- 

 ances on the basic dimensions and heat treatment processes during manu- 



