CARRIER SYSTEMS ON TELEPHONE LINES 581 



The type of oscillator is essentially the same as that used in the type 

 "C" carrier systems for producing the carrier frequencies. It is 

 controlled by condensers which include an adjustable air condenser for 

 tuning to the particular frequency desired. 



Two indicators are located at the repeater, one for each direction of 

 transmission. Each indicator circuit consists of a vacuum tube 

 rectifier operating from coupled tuned circuits into a d.-c. milliammeter 

 having a special scale calibrated in transmission units. The filament 

 and plate currents and bias potentials are obtained from the standard 

 130- volt battery. The advantage of using the same battery for the 

 several functions is that it makes possible the stabilization of the 

 rectifier output with power variations. An adjustable grid bias 

 voltage is obtained from the negative drop of the filament circuit with 

 an opposing 3-volt dry cell battery connected in series. With this 

 arrangement normal variations in the 130- volt source cause only a 

 negligible change in the indicator meter readings. 



At the receiving terminal, in addition to the indicator circuit which 

 is the same as at the repeater, an alarm circuit is provided as noted 

 above. A sensitive marginal relay is connected in series with the 

 indicator meter. When this relay operates, it starts the delay circuit 

 by removing ground from the grid condensers of the alarm tube. 

 The leakage through the grid resistances then causes the condenser 

 potential, which is the grid potential of an auxiliary rectifier tube 

 operating from the same power source, to decrease slowly, resulting 

 eventually in a rise in the current of the plate circuit of the alarm 

 rectifier tube. If the marginal relay remains operated for a given 

 length of time, the alarm tube plate current will rise to a value 

 necessary to operate the alarm relays. For shorter periods of opera- 

 tion, the normal highly negative grid potential of the rectifier tube is 

 restored and no alarm is operated. The timing of the delay circuit is 

 adjusted by the values of the grid leak resistances and condensers. 

 A delay of about 15 seconds is usually employed, which effectually 

 prevents false operation due to occasional transients such as speech 

 interference. The adjustment of the contacts on the alarm relay is 

 ordinarily such as to cause an alarm to be given at limits of ± 1.5 TU 

 variation. 



General Transmission Considerations 



Lines. The typical open-wire telephone line consists of a number 

 of 10-foot crossarms spaced two feet apart on poles whose height 

 varies from 30 feet upward depending on local conditions. The poles 

 are spaced at an average interval of 130 feet. Each crossarm carries 

 10 wires. The wires are normally spaced at 12-inch intervals, except 



