262 BELL SYSTEM TECHNICAL JOURNAL 



Obtaining High Amplifications 



The attenuation of cable pairs being inherently high at carrier 

 frequencies, high amplifier gains are called for, otherwise the cost of 

 the carrier circuits goes up very materially. Since as the power 

 carrying capacity of the repeaters is increased a point is soon reached 

 where it becomes very expensive to go further, high amplifications 

 must be secured by letting the transmitted currents become very 

 weak before amplifying them. A natural limit to this is found in the 

 so-called thermal or resistance noise ^ generated by all conductors. 

 Similar natural and largely insuperable noises are introduced by the 

 vacuum tubes in the amplifiers. Other sources of noise are: 



1. Telegraph and signaling circuits worked on other pairs in the same 



cable with the carrier circuits. 



2. Radio stations. 



3. Noise from power systems, particularly electric railways. 



The latter two disturbances originate outside the cable so that they 

 are subject to the shielding effect of the lead sheath which increases 

 rapidly with increasing frequency. Generally speaking, in a new 

 cable both of these and also the noises from other circuits in the same 

 cable may be relegated by location and design to comparatively minor 

 importance. On existing cables, however, they may require special 

 treatment. In all cases, however, the lower levels at the upper 

 frequencies, which largely determine the repeater spacings, are estab- 

 lished primarily by the thermal noise in the conductors and by the 

 corresponding noises in the vacuum tubes. In the Morristown 

 installation the amplifications were kept small enough and the levels 

 high enough so that noise was not an important factor. 



Experimental Results 



A large number and wide variety of tests have been made using the 

 setup at Morristown. These were generally of too technical a 

 character to be of interest in a general paper such as this one. It will 

 be of chief interest to note that no serious difftculty was experienced in 

 setting up the 850-mile four-wire 4 to 40-kc. circuit with the necessary 

 constancy of transmission loss at different frequencies, although the 

 equalizer arrangements which made this possible presented intricate 

 and difificult problems of design. Nine separate carrier telephone con- 



' "Thermal Agitation of Electricity in Conductors," by J. B. Johnson, Phys. Rev., 

 Vol. 32, p. 97, 1928, and "Thermal Agitation of Electric Charge in Conductors," by 

 H. Nyquist, Phys. Rev., Vol. 32, p. 110, 1928. 



