178 



BELL SYSTEM TECHNICAL JOURNAL 



save the cost of selection, twelve discs were used per arm to obtain 

 the better balance resulting from the averaging of the characteristics 

 of a large number of discs. There is some sacrifice in efficiency due to 

 using the large number of discs but in this application it was of minor 

 importance. 



The averaging obtained by using twelve discs in each arm is helpful 

 in several other respects. First, the normal impedance, transmission 

 and balance do not vary greatly from unit to unit. Secondly, although 

 each disc has a negative coefficient of resistance vs. temperature and 

 there is a variation in the coefficient among discs, the average coefficient 

 of twelve discs chosen at random will be very nearly equal to the 

 average of any other twelve discs chosen at random and the balance 

 between arms will, therefore, remain practically constant with tem- 

 perature, even though the impedance and efficiency vary. A similar 

 advantage is obtained in the case of aging and a good balance is ob- 

 tained throughout the life of the equipment. 



TO 

 DEMODULATOR 



CWT\ 



Fig. 14 — Schematic diagram of oscillator circuit. 



Oscillator 



As mentioned previously the same carrier (suppressed) frequency is 

 used for transmission in both directions, thus requiring a single 

 oscillator instead of two as in the Type D system. The principal 

 requirement for an oscillator for this use is that its frequency remain 

 stable under the variations in power supply voltage and temperature 

 which will occur. The new design, which is shown schematically in 

 Fig. 14, provides a degree of stability such that no operating adjust- 

 ments will be required due to these factors. Relatively high stability 

 with changes in temperature is obtained by balancing the positive 

 capacitance-temperature coefficient of the copper-oxide load and the 

 mica tuning condenser against the negative coefficient of the paper 

 tuning condenser. 



