FREQUENCY SHIFT TELEGRAPHY 



289 



When d-c. elimination is used, operation with FS signals decentered in the 

 pass band causes little bias but it does cause a loss in signal-to-noise ratio, 

 especially at high noise levels. Due to the effect of noise peaks in causing a 

 dip toward zero in the demodulator output, the effect of noise becomes 

 exaggerated during the signal condition which is farther from midband. 

 This change in signal-to-noise condition with frequency drift is shown in 



20 



2 4 6 8 10 12 14 



CARRIER LEVEL VARIATION IN DECIBELS 



18 20 



Fig, 28. — Signal bias versus carrier level variation for AM transmission. 



INPUT 



OUTPUT 



Fig. 29. — Block diagram of dc. elimination and restoration method used to minimize 

 signal bias caused by frequency drift. 



Figs. 24 and 30. A comparison between a two-bandpass-filter discriminator 

 and a linear discriminator with d-c. elimination as regards frequency drifts 

 in the presence of noise is shown in Fig. 31. 



Radio telegraph systems operating in the H.F. region require a high degree 

 of frequency stability if narrow bandwidths are to be used. Because of 

 the several frequency conversions involved a number of different oscillators 

 or frequency sources are involved but usually the major burden of frequency 

 stability rests on the transmitter exciter and the high-frequency beating 

 oscillator of the receiver. 



Various methods of automatic frequency control may be used to hold the 



