612 



BELL SYSTEM TECHNICAL JOURNAL 



the pilot channel. The modulator and demodulator are the only 

 units in this category. 



In the modulator and demodulator the principal factors tending to 

 cause instability of gain are the variations in plate and filament 

 battery where these occur and changes in the tubes during their life. 

 A characteristic behavior of the modulator described below has been 

 used to advantage in minimizing this instability. The gain of the 

 modulator for varying values of the carrier voltage passes through a 

 maximum near the point where the grids of the modulator tubes are 

 driven to a positive potential, with respect to the filament. The 

 output from the carrier oscillator will increase with increasing plate 

 potential, and due to the above characteristic may be made to com- 

 pensate somewhat for the tendency of the gain of the modulating 

 tubes to increase. Figure 31 shows the change in gain of the modu- 



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Figure 31 — Modulator gain — relation to plate potential 



lator circuit when the plate potential of either the oscillating or 

 modulating tubes is changed independently. With these arrange- 

 ments the total variation in the modulator or demodulator gain, due 

 to the fluctuations of power supply, does not usually exceed ± .25 

 TU. The possible variation due to tube differences is somewhat 

 larger, approximately ± .7 TU. This is not serious since tubes are 

 ordinarily replaced when a system is out of service, and the gain can 

 be readjusted before the system is restored to operation. 



Another requirement which the modulator must meet is one of 

 transmission quality or equality in transrhission gain at various 

 frequencies in the voice band. The modulator should not limit the 

 band of frequencies which are to be transmitted over the system. 

 In other words, the gain of the modulator should be substantially the 



