FREQUENCY SHIFT TELEGRAPHY 



111 



shown the over-all signaling frequency response to FS and AM signals for 

 a nominal band width at point "a" of about 740 cycles. It will be noted 

 that the FS method is but slightly inferior and that both systems fail at a 

 frequency of approximately one-half the band width. 



E^eci on Noise: 



The effect of thermal noise on distortion for bandwidths of 115, 230, and 

 740 cycles is shown in Figs. 11 and 12. The rms. noise-to-carrier ratios 



< 



Q 150.8 



?^ 

 ZiO 

 Q-iiJO.6 



r< 



^go.4 

 FO 



iiJQO.2 



too 200 300 400 500 

 DOTTING SPEED IN DOTS PER SECOND 



Fig. 10. — Overall frequency response of a 740-cycle band to AM and FS signals. 



z 



UJ 



U 



a:40 



UJ 

 Q. 



Z 



30 



-20 -18 -16 -14 -12 -10 -8-6-4-2 2 4 6 



RMS NOISE-TO- CARRIER RATIO IN DECIBELS 



Fig. 11.— Peak distortion versus thermal noise for FS transmission— 80-cycle cutoflF 

 low-pass filter. 



indicated in the figures were measured in the 3300-cycle band of the radio 

 receiver in all cases. The actual noise-to-carrier ratio existing in the trans- 

 mission band used was lower and may be obtained from the following table. 

 The values of correction are from rms. measurements. 



Pass Band at Point "a" 

 1920 cycles 

 740 

 230 

 115 



Correction to be made 

 -2.3 db 

 -6.5 

 -11.6 

 -14.3 



