296 



BELL SYSTEM TECHNICAL JOURNAL 



illustrate the influence that the phase adjustment of the carrier 

 frequency has on the transmission of the lower frequencies in a system 

 of this kind. 



The upper curve shows the transmission frequency characteristic 

 of one of the carrier channels measured from terminal to terminal 

 between distortionless lines, when the phase angle of the receiving 

 carrier is adjusted for its optimum value. Under these conditions the 

 vestigial sideband and normal sideband supplement each other in their 

 effects to produce substantially flat transmission. (The insert in- 

 dicates the sustained transmission toward zero frequency when the 

 40-cycle highpass filter is omitted from the circuit.) It may be noted 

 also that with this proper phase adjustment the full band transmission 

 characteristic provided is substantially flat within a fraction of a 

 decibel from 40 c.p.s. to 15,000 c.p.s. The lower curves indicate 

 successively what happens if the phase angle of the receiving carrier 

 is adjusted different amounts from the optimum adjustment. It may 

 be noted that for a 90-degree departure the transmission of a 40-cycle 

 tone over the carrier channel would suffer more than 12 db in com- 

 parison with a 1000-cycle tone. 



Repeaters 

 As noted previously, the line circuit between Philadelphia and 

 Washington included five intermediate repeater points. A schematic 

 drawing of the apparatus installed at each point is shown in Fig. 8. 



BUILDING 



OUT 

 EQUALIZER 



BASIC 

 EQUALIZER 



REGULATING 

 NETWORK 



AMPLIFIER 



REGULATING 

 CONTROL 



Fig. 8 — Schematic diagram of repeater station apparatus. 



The amplifiers at these points, as well as those used at the transmitting 

 and receiving terminal, consisted of a new form of amplifier employing 

 the principle of negative feed-back. The principal virtues of am- 

 plifiers of this type are their remarkable stability with battery and 

 tube variations and great freedom from nonlinearity or modulation 

 effects. Each amplifier is supplemented at its input by an equalizer 

 designed to have its attenuation approximately complementary in 

 loss to that of the line circuit in a single section. The amplifiers 

 actually employed for the purpose were taken from a trial of a cable 



