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circuit should, of course, be a minimum at carrier and sideband 

 frequencies. At the output a low-pass filter structure F provides for 

 the suppression of the unwanted products of demodulation. 



A front view of the demodulator unit is shown in Figure 37. The 



Figure 37 — Assembly of demodulator panel. (Front view) 



panel layout and general appearance is similar to that of the modu- 

 lator. The two dials shown control the demodulator input and the 

 oscillator frequency, respectively, as indicated in these figures. 



Filters. The general function of a band filter is the selection of 

 a band of frequencies, and the protection of this band from interfering 

 frequencies located on either side. The filters determine what band 

 width is transmitted, and thus to that extent they control the quality 

 of speech which may be obtained through the carrier circuit. The 

 type "C" system transmits a band corresponding to approximately 

 200 to 2,700 cycles per second in the voice range. 



In considering the requirements imposed upon the band filters it is 

 necessary to keep in mind '■' the fact that the modulator produces not 

 only the particular sideband which is to be transmitted but also an 

 unwanted sideband of the same volume as the wanted sideband and 



^ R. V. L. Hartley, "Relation of Carrier and Side Bands in Radio Transmission," 

 Bell System Tech. Jl., V. 2, April 1923, pp. 90-112. 



