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BELL SYSTEM TECHNICAL JOURNAL 



function of the modulator is to translate the voice frequencies along a 

 frequency scale to some assigned location in the band of frequencies to 

 be occupied by the carrier system. The carrier frequency controls 



Output 



- 18" ♦ - \&' * X -24' +130" '(30" 



Figure 30 — Schematic of modulator circuit and transmitting oscillator 



the location of the shifted voice band or sideband, and the oscillator, 

 which provides this frequency, is made an integral part of the modu- 

 lator unit. The modulating process requires a circuit element having 

 a non-linear response characteristic to produce the sideband from a 

 combination of the carrier and voice frequencies. In this circuit the 

 three-element vacuum tube is operated to give this required char- 

 acteristic. 



Since the carrier is not transmitted in the type "C" system, each 

 modulator and demodulator becomes a complete and independent 

 frequency changing unit. As previously noted, the frequency stability 

 of each oscillator must be sufhciently good so that the carriers of the 

 corresponding modulator and demodulator units will differ but slightly 

 in frequency so as not to affect unfavorably the speech which is 

 transmitted. In this connection both naturalness of the received 

 speech and intelligibility must be considered. In the type "C" 

 system satisfactory results have been obtained by holding the difference 

 between the carrier frequencies of any two associated units due to all 

 causes to within about 20 cycles. 



The usual causes of frequency variation are fluctuations in the 

 A and B battery supply and changes in temperature and humidity. 

 Vacuum tubes have to be replaced periodically and differences in 

 the tube characteristics may cause a slight variation in the frequency. 



The type of oscillator circuit was chosen to furnish the greatest 



