FREQUENCY SHIFT TELEGRAPHY 301 



advantage of being simple in that the diversity action is obtained in the 

 transmission circuits directly and no added switching circuits or adjustments 

 are required. Tests of this type of diversity selection in the field have indi- 

 cated a marked superiority over earlier FS terminal equipment. 



In connection with the use of a common Umiter care must be taken in the 

 selection of the two-channel frequencies. Frequencies having nearly 

 integral ratios such as 3 : 5 and 5 : 7 produce disturbing amplitude modula- 

 tion of the demodulated signal. The frequencies should be chosen so as to 

 avoid low integral ratios; then all amplitude modulations are negligible or 

 easily filtered out. Where frequency drift is to be allowed for, the fre- 

 quencies should be chosen so as not to approach a low integral ratio at any 

 place in the expected drift range. 



If the radio receivers associated with a space diversity FS system have 

 automatic gain control it must be a common control so the receivers will 

 change gain equally. The use of common AVC prevents overloading of 

 the receivers as the received signal strength varies. If no common AVC 

 is available the receivers should be operated in the manual gain-control 

 condition. 



Conclusions 



General Comparison of FS and AM Carrier Telegraphy 



The foregoing sections have compared the characteristics of AM and FS 

 carrier telegraph transmission under various conditions. Whether or not 

 FS would prove to be the preferable method for a specific communication 

 use depends largely on the transmission medium and the quality of trans- 

 mission desired. As regards frequency space requirements, both methods 

 provide essentially the same signaling speed capabiUty for a given band- 

 width. 



As to the abiUty to transmit through noise, FS has an advantage of 3 to 

 4 db at distortions approaching the failure point when equal bandwidths 

 are compared. At lower distortions the advantage of FS is 6 db or more 

 so that it is attractive in this respect for tandem operation of several tele- 

 graph sections where regeneration of signals is not practiced. When fre- 

 quency space permits wider bands, with correspondingly increased fre- 

 quency shifts, the signal-to-noise advantage of FS over AM increases for 

 low noise levels. Wide band FS therefore provides a means of obtaining 

 higher quality circuits if the noise level is not too great. 



The AM method is basically less susceptible to frequency variations 

 than is the FS method. However, as has been illustrated, frequency drift 

 can be compensated for by d-c. elimination so as to make FS comparable 

 to AM in this respect. 



