22 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



most cases very little or no span pad would be used and only in rare cases 

 would the artificial line sections be needed. The small amount of slope 

 needed to supplement the span pad loss is provided in a three-step repeater 

 slope control. A total of about seven db of slope range within about 1 db 

 accuracy is available in two successive repeaters. This would permit com- 

 pensation for about three miles of inequaUty in successive line sections. The 

 slope changes are produced by switching simple networks in the amplifier 

 feedback circuit. 



The plug-in repeater consists of three separate subassemblies connected 

 together through the multicontact plug and jack connectors used in all 

 N equipment units. Of these the east-west and west-east modulator-ampli- 

 fier units are alike electrically and mirror images mechanically. The os- 

 cillator-power subassembly forms the third subassembly. These units are 

 alike in LH and HL repeaters. I'he amplifier-modulator circuits supply the 

 transmission paths through the repeater and differ between LH and HL 

 repeaters only in whether the input circuit accepts low-band frequencies 

 and amplifies high-group frequencies at the output or the reverse. The input 

 A or C filter blocks near-end crosstalk from line frequencies within the same 

 quad, which would overload and produce interchannel interference within 

 the same system, or it blocks interaction crosstalk through tertiary-voice 

 pairs from other systems in the cable. The output B or D filter suppresses 

 304 kc carrier leak and the unwanted upper sideband on the 304 kc carrier 

 of the input group frequency band. 



In addition to the slope control two additional controls are used in the 

 feedback circuit of the repeater amplifier. Resistance strapping options 

 made in the factor adjust each manufactured repeater to a nominal gain of 

 48 db to an accuracy of 1 db. A second control of flat gain is obtained from 

 a thermistor directly heated by a fraction of the total repeater output 

 power. Inasmuch as the signal tone power, when present in a channel, is 

 about 12 db below the channel carrier power, and the speech of an average 

 talker about 15 db below the power of the carrier, the total output power is 

 ahnost entirely carrier. As the fine changes in attenuation with temperature, 

 the change in the strength of the carriers at the repeater output supplies 

 more or less heat to the thermistor pellet. An increase in heat due to de- 

 creased line loss at cold temperatures causes the thermistor resistance to 

 decrease and produce more amplifier feedback and less repeater gain, thus 

 offsetting the change in line loss. The normal operating range of the ther- 

 mistor resistance is from about 1000 to 20,000 ohms. At nominal gain of 

 48 db the resistance will always be about 9000 ohms. Each pellet is con- 

 trolled in manufacture to have its nominal resistance value for a specified 

 amount of repeater output power. The temperature at which the pellet 

 operates for the standard repeater output power would vary appreciably 



