350 BELL SYSTEM TECHNICAL JOURNAL 



one station lies between 55.5 and 58.5 kilocycles is 0.796. If the 

 assumed maximum interference at the equivalent 1,100 cycles in the 

 opposite sideband, as indicated by the dashed line in Fig. 16, is 61 TU 

 above the minimum signal and we wish to have it 50 TU below, as 

 previously stated, then we require a selectivity of 61 TU plus 50 TU 

 or 111 TU for this frequency. For other tone-producing frequencies 

 of the opposite sideband similar selectivity requirements have been 

 set up and the resultant for frequencies from 55.5 to 58.5 kilocycles is 

 shown by the solid curve in Fig. 16. 



7. Stability. As mentioned in Section 4 above, the carrier for a 

 single sideband receiver must be resupplied at the correct frequency. 

 All of the oscillators in the radio link must have sufficient frequency 

 stability to maintain the voice frequencies at the receiver output 

 correct within 20 cycles per second over long periods of time. Suppose 

 we allow 10 cycles per second variation in frequency to exist at the 

 transmitter and an equal amount at the receiver, then the variation in 

 frequency at the receiver must never exceed 0.017 per cent if the re- 

 supplied carrier is at 58.5 kilocycles. Certain advantages in stability 

 of the resupplied carrier can be obtained by the use of double demodu- 

 lation in the receiving set and these will be discussed in another paper. 



Variations in the efficiency of the transatlantic radio transmission 

 path for long wave-lengths occur with time of day and season, but 

 during any individual all-daylight transmission period the transmission 

 efficiency of the path is fairly constant. If the gain of the receiver is 

 constant, then, during this important period of the day, the minimum 

 of circuit adjustments will be required. It is hence desirable that the 

 gain of the entire receiving set be made to hold constant within ± 2 

 TU for all variations of temperature and of voltage of battery supply, 

 within the operating limits. 



It is almost self-evident that the transmission-frequency character- 

 istic through the radio receiver should not vary with temperature and 

 time. Changes of this nature should not exceed 0.5 TU within the 

 transmission band nor 5 TU outside of the transmission band. 

 Design of stable filters and vacuum-tube circuits are essential to 

 produce this result. 



The authors have endeavored, in the limited space of the preceding 

 pages, to show what radio transmission considerations must be taken 

 into account in properly designing a receiving system for a commercial 

 radio-telephone circuit. A rather detailed discussion has been neces- 

 sary to present an accurate picture of the various factors entering into 

 the production of the very essential and highly directional long-wave 

 receiving antenna system employed. 



