TRANSATLANTIC RADIO TELEPHONY 119 



The Transmitting System 



The transmitting system is shown in simplified circuit form in 

 Fig. 1. It is illustrated as grouped into three parts: The low-power 

 modulating and amplifying stages, shown below in light lines; the 

 high-power amplifiers, shown in heavy lines above and to the right; 

 and the rectifier which supplies the power amplifier with high-tension 

 direct current, shown in the upper left-hand portion of the 

 diagram. 



Referring first to the low-power portion of the system, it will be 

 seen that the voice currents (from either a telephone line or a local 

 microphone) are fed into a balanced type of modulator circuit and 

 are modulated with a carrier current of a frequency of about 33,000 

 cycles. The operation of the balanced type of modulator in sup- 

 pressing the unmodulated carrier component is explained in the 

 Colpitts and Blackwell carrier current paper referred to above. The 

 result of this modulating action is to produce in the output circuit of 

 modulator Xo. 1, modulated current representing the two side-bands, 

 for example, the upper one extending from 33,300 to 36,000 cycles 

 and the lower one from 32,700 down to 30,000 cycles. These com- 

 ponents are impressed upon a band filter circuit which selects the 

 lower side-band to the exclusion of the upper one and of any remain- 

 ing part of the carrier, with the result that only one side-band is 

 impressed upon the input of the second modulator. This second 

 modulator is provided with an oscillator which supplies a carrier 

 current of 88,500 cycles. The result of modulation between 

 the single side-band and this carrier current is to produce a 

 pair of side-bands which are widely separated in frequency, the upper 

 one, representing the sum of the two frequencies, extending from 

 118,500 to 121,200 cycles and the lower one, representing the difference 

 between the two frequencies, extending from 58,500 down to 55,800 

 cycles. In this second stage of modulation there is a relatively wide 

 separation between the two-side bands which facilitates the selection 

 at these higher frequencies of one side-band to the exclusion of the 

 other. Another important advantage is that it allows a range of 

 adjustment of the transmitted frequency without changing filters. 

 This is accomplished by varying the frequency of the oscillator in 

 the second step. In the present case, the frequency desired for trans- 

 mission is that corresponding to the lower side-band of the second 

 modulator. The lower side-band of from 58,500 to 55,800 is therefore 

 selected by means of the filter indicated. This filter excludes not only 

 the other side-band but also any small residual of 90,000-cycle un- 



