TRANSATLANTIC RADIO TELEPHONY 349 



phone receiver is about the most serious frequency of interference to 

 received speech on a telephone circuit, and that frequencies above and 

 below 1,100 cycles are of somewhat less importance. Signals at the 

 equivalent 1,100-cycle frequency produce a type of interference which 

 good engineering practice requires should be reduced at least 50 TU 

 below the desired signal. (This amount of interference is equal to 

 about 500 noise units at the — 10 TU transmission level.-') 



To satisfy this requirement, the relative desired signal should be 

 50 TU greater than the relative interference at the output of the 

 demodulator. Assigning a minimum magnitude to the beating oscilla- 

 tor voltage of 90 TU above the minimum desired signal voltage on 

 the grid of the demodulator reduces this type of interference suffi- 

 ciently. (Using a balanced demodulator arrangement, this value 

 might be reduced some 20 TU). 



Since any two signals at frequencies entirely outside the band 

 accepted by the demodulator are suppressed some 80 TU, we need 

 not consider their second-order modulation products. 



If we use double demodulation in a radio receiver as is assumed 

 in the first part of this section, then we must consider other products 

 of modulation with the beating oscillator for frequencies distant from 

 the accepted band. Space will not permit us to more than mention 

 these, but since the frequencies to be suppressed are distant from the 

 frequencies to be received, their suppression is relatively simple. 



In the final demodulator of a radio receiver we must tolerate a 

 certain amount of distortion due to the intermodulation of input fre- 

 quencies. By limiting the band width into the final demodulator to 

 the same width as the desired output band, the distortion due to inter- 

 modulation with interference lying outside the desired band is elimi- 

 nated. By supplying a large amount of carrier to the final demodu- 

 lator and by using a balanced demodulator the amount of noise and 

 distortion due to intermodulation of frequencies lying inside the 

 desired band is reduced. 



If the desired signal band extends from 58.9 to 61.1 kilocycles and 

 is an upper sideband corresponding to voice frequencies from 400 to 

 2,600 cycles then, in effect, we must supply a carrier at 58.5 kilocycles 

 to produce the proper voice frequencies in the output circuit. This 

 carrier frequency will also demodulate the frequencies below it in such 

 a way as to produce audible signals and for this reason ample protec- 

 tion must be supplied against the opposite sideband it stations are 

 likely to exist in that range. Calculations show that this is the case; 

 for if 100 stations are distributed at random over the 190-kilocycle 

 range between 10 and 200 kilocycles, then the probability that at least 



