CARRIER SYSTEMS ON TELEPHONE LINES 



617 



One feature which is required with the demodulator, but not with 

 the modulator, is a variable control of the transmission gain of the 

 circuit. Due to the unequalized transmission of the line section 



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 Figure 36 — Demodulator characteristic — gain and frequency 



adjacent to the terminal, or other differences in the channel equiva- 

 lents, the three sideband currents normally arrive at a receiving 

 terminal with unequal strength. A potentiometer controlling the 

 gain of the demodulator permits of an equalization of the overall 

 losses on the three channels. 



In the following detailed description of the demodulator circuit 

 other minor differences between it and the modulator may be pointed 

 out: 



The sideband frequencies enter the demodulator passing to the 

 potentiometer P-1 which controls the amount of current to the input 

 transformer T-1. The position of the carrier input transformer T-2 

 is somewhat different in the demodulator circuit as compared to the 

 modulator circuit, due to the difference in the high-frequency charac- 

 teristic of the T-1 transformers. In the modulator this transformer 

 must be designed to transmit voice frequencies primarily. It has a 

 comparatively large capacity to ground which would reduce the 

 effective carrier voltage on the tube grids if it were placed in the same 

 circuit position as is the demodulator transformer. The function of 

 most of the circuit elements is evident from the previous description 

 of the modulator. The C-1 and C-2 condensers provide a low im- 

 pedance path for the carrier frequency. They are necessary here 

 because the transformer T-3 designed for high efficiency at voice 

 frequencies has considerable leakage inductance, which would present 

 a high impedance to the carrier in the plate circuit if the condensers 

 were not provided. For the maximum gain the impedance of this 



