CARRIER SYSTEMS ON TELEPHONE LINES 



625 



because of its property of balancing out second order effects, which in 

 a single tube or unbalanced circuit are the largest of all the modulation 

 products at the usual loads. 



The currents from the three channels enter the carrier amplifier 

 shown in Figure 42. The circuit consists of two stages; the first 

 stage of two tubes, the second of four of higher power rating. The 

 gain is controlled in 2 TU steps by the adjustable potentiometer in 

 the input. The gain frequency characteristics for different potenti- 

 ometer settings are substantially flat within a small fraction of a 

 TU over the range of any channel. 



The amplifiers for the two directions are of slightly different design, 

 each amplifier being arranged for a flat characteristic over its own 

 group of frequencies. It has been stated that the load capacity of 

 the amplifier is limited ^° by the modulation products which increase 

 with the load. Figure 43 shows the amount of second and third 



90 



t- 



3 

 f^80 



D 



o 



z 



u 



•^ 60 



z 



D 50 



^40 



U 

 CD 



„ 10 20 30 40 50 60 



AMPLIFIER OUTPUT-MILS INTO 600 OHM CIRCUIT 



Figure 43 — Amount of second and third harmonics as function of carrier 

 repeater output 



harmonics produced in a typical repeater with varying single frequency 

 output. By connecting the tubes in push-pull instead of in parallel, 

 the second harmonics have been reduced by about 15-20 TU. Other 

 products of modulation as well as the second and third harmonics 

 increase with the output and thus the power which can be taken from 

 the amplifier under the operating conditions is limited as these effects 

 are likely to result in interchannel interference. 



When the alternating voltage applied to one grid is positive with 



i"F. C. Willis and L. E. Melhuish, "Load Carrying Capacity of Amplifiers," 

 Bell System Tech. Jl., V. 5, October 1926, pp. 573-592. 



