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DELL SYSTEM TECHNICAL JOURNAL 



It is further improved by applying the filter after the frequency is 

 stepped down rather than before. To illustrate this improvement 

 assume that there is present an interfering signal at 60,000 cycles, 1,500 

 cycles off from the edge of the received telephone band. This is a 

 frequency difference of about 2]/ 2 per cent; but after each of these 

 frequencies is subtracted from 90,000 cycles, the difference of 1500 

 cycles becomes almost 5 per cent. This enables the filter to effect a 

 sharper discrimination against the interfering signal. Furthermore, 

 the filter is not required to be of variable frequency as would be the 



Fig. 8 



case were it employed directly at the received frequency since by 

 adjusting the frequency of the beating down oscillator the filter is in 

 effect readily applied anywhere in a wide range of received frequencies. 

 The receiving method, therefore, enables the filter circuit, and indeed 

 also the intermediate frequency amplifiers, to be designed for maximum 

 efficiency at fixed frequency values without sacrificing the frequency 

 flexibility of the receiving set. 



A photograph of the receiving set used in the transatlantic measure- 

 ments is reproduced in Fig. 8. The signals are received on a square 



