CARRIER SYSTEMS ON TELEPHONE LINES 611 



frequency stability with variations in power supply, particularly in 

 the plate battery. Fluctuations in the filament current are reduced 

 by the use of ballast resistors to a point where they do not appreciably 

 affect the oscillator frequency. 



In order to maintain stability with temperature and humidity 

 changes, it was necessary to develop circuit elements (primarily the 

 inductance in the oscillating circuit) which were not greatly affected 

 by these variables. As a result the oscillators vary less than 10 

 cycles per second at the highest carrier frequency with power vari- 

 ations within the limits of plant maintenance, and have a frequency 

 temperature coefficient of approximately .002 per cent per degree 

 Fahrenheit. This corresponds to about one cycle per second per 

 degree Fahrenheit in the highest frequency units used in the type 

 "C" system. The temperature difference between offices containing 

 terminal equipment seldom exceeds 20 degrees Fahrenheit. 



An extreme change in frequency of 20 cycles per second may be 

 encountered with different tubes. Maintenance experience has shown 

 that it is usually unnecessary to check the synchronization of the 

 modulator and demodulator carrier frequencies more often than once 

 a week, unless tubes are replaced or some other unusual circuit change 

 occurs. 



The modulating tubes are placed in a push-pull arrangement, and 

 the carrier voltage is applied to both grids in phase and the suppression 

 of the carrier frequency secured by a differential connection of the 

 output transformer windings. It is difficult to completely suppress 

 the carrier and a limit is set upon the amount which can be allowed on 

 the high-frequency line without causing interference between systems. 

 This limit requires that the carrier flowing out from the modulator 

 should not exceed approximately 500 microamperes. With varying 

 conditions of power, the balance of the modulator cannot be maintained 

 absolutely constant, so that to insure meeting this requirement under 

 the worst conditions it is necessary to adjust the balance under 

 normal conditions to a point where the carrier has been reduced to 

 about 150 microamperes at the output of the modulator. The side- 

 band current flowing at this place in the circuit is ordinarily of the 

 order of 2,000 microamperes. Adjustment of the carrier balance is 

 made by changing the condenser across one half of the input circuit 

 and by selecting tubes. 



A further requirement imposed on the modulator unit is that of 

 gain stability. In order to maintain sufficiently constant transmission 

 over a circuit there must be a high degree of inherent stability in all 

 those units whose variations are not included in the indications of 



