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BELL SYSTEM TECHNICAL JOURNAL 



carrier input to the demodulator at the correct frequency. In the case of 

 FS signals the control may be arranged to operate only on the marking fre- 

 quency or to utilize both the mark and space conditions. It is preferable 

 to have inherent frequency stability rather than to compensate for the drift 

 at the receiving end since it is difficult if not impossible to provide an auto- 

 matic frequency control which will not reduce the transmission capabilities 



Hjco 

 zao. 



% 



cc 



-400 -300 -200 -100 100 200 300 400 

 FREQUENCY DRIFT IN CYCLES PER SECOND 



Fig. 30. — Effect of frequency drift on distortion produced b}^ impulse noise in FS 

 -ansm'ssion — 740-cycle band, 100 cycle frequency shift, 80-cycle cutoff low-pass filter. 



-500 -400 -300 -200 100 100 200 300 400 

 FREQUENCY DRIFT IN CYCLES PER SECOND 



Fig. 31. — Linear discriminator versus two-bandpass-filter discriminator when frequency 

 drift occurs in the presence of thermal noise 4 db rms noise-to-carrier ratio. 



of the system in the presence of noise and other interference. Best results 

 are obtained if the frequency stability is high enough to require but a very 

 slow correction, which usually dictates some mechanical rather than elec- 

 tronic tuning arrangement. Manual retuning may be found satisfactory 

 where stability is reasonably good provided care is taken in making the ad- 

 justments. Suitable frequency stability with the retention of flexibility in 

 frequency adjustment may be obtained by frequency sources making use of 

 a combination of crystal oscillators and high-stability variable oscillators of 

 lower frequency. 



