TRANSATLANTIC RADIO TELEPHONY 



127 



accomplished merely by permitting all of these components to pass 

 through the detector tube. The detecting action whereby the voice- 

 frequency currents are derived, is accomplished by a remodulation 

 of the carrier with each side-band. 



With the present eliminated carrier method of transmission the 

 side-band is unaccompanied by any carrier with which to remodulate 

 in the receiving detector. It is necessary, therefore, to supply the 



SCHEMATIC OF SINGLE SIDE BAND RECEIVER 



kiUrm«<fiaU 



M PR? 



Fig. 7 



detector with current of the carrier frequency obtained from a local 

 source. Thus, in the present experiments, if a current of the original 

 carrier frequency, 55,500 cycles, is supplied to the detector it will 

 remodulate or "beat" with the received side-band of, say 55,800 to 

 58,500 cycles and a difference-frequency band of 300 to 3000 cycles, 

 i.e., the voice frequency band will result. 



The arrangement actually used, however, is not quite so simple as 

 this. It is shown schematically in Fig. 7. Reception is carried out 

 in two steps, the received side-band being stepped down to a lower 

 frequency before it is detected. The stepping down action is accom- 

 plished by combining in the first detector the incoming band of 55,800 

 to 58,500 cycles with a locally generated current of about 90,000 

 cycles. In the output circuit of the detector the difference-frequency 

 band of 34,200 to 31,500 cycles is selected by a band filter and passed 

 through amplifiers and thence to the second detector. This detector 

 is supplied with a carrier of 34,500 cycles which, upon "beating" 

 with the selected band, gives in the output of the detector the original 

 voice-frequency band. 



The object of thus stepping down the received frequency is to secure 

 the combination of a high degree of selectivity with flexibility in 

 tuning. The high selectivity is obtained by the use of a band filter. 



