CARRIER SYSTEMS ON TELEPHONE LINES 



613 



same for frequencies between 200 and 2,800 cycles per second. The 

 characteristics of the transformers and the impedance in which the 

 output of the modulator is terminated are the controlling factors in 

 the quality of the modulator. A typical characteristic of modulator 

 gain with frequency under ideal terminations is shown in Figure 32. 



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Figure 32 — Modulator gain — relation to frequency 



In the type "C" system the effect of the band filter impedance is to 

 cause a variation in this characteristic of approximately 5 TU at 

 2,800 cycles. 



The final requirement placed upon the modulator relates to the 

 energy level which must be handled. It should not be possible to 

 overload the modulator seriously with the amount of power produced 

 by a subscriber's set at the transmitting toll testboard level. With a 

 given modulator circuit, this requirement can be met by designing 

 the input transformer with the proper turns ratio. 



The following paragraphs give a more detailed description of the 

 actual circuit which has been developed to meet the above require- 

 ments. 



The voice-frequency circuit is through the hybrid coil to the 

 terminals of the input transformer. The resistance placed across the 

 input circuit is to improve the impedance terminating this branch 

 of the hybrid coil, and thus improve the terminal impedance looking 

 into the hybrid coil from the voice-frequency line. The condenser 

 C-1 is inserted in series with the primary winding of the transformer 

 to improve the transmission characteristic of the circuit at low fre- 

 quency. This condenser resonates with the inductance of the primary 

 winding, increasing the voltage across the primary at low frequencies 

 where the modulator input circuit tends to become less efficient. 

 The two windings of the secondary side of the transformer are separated 



