FREQUENCY SHIFT TELEGRAPHY 



285 



Magnitude of Frequency Shift in Relation to Bandwidth 



A frequency-shift transient in a band of given width has a wave shape 

 much like that of an ampHtude transient in the same band provided the shift 

 is symmetrical and not over 50% of the bandwidth.^ If the frequency shift 

 approaches the total width of the band the transient is of such shape as to 



20 



-20 -18 -16 



-14 -12 -10 -8 -6 -4 -2 

 RMS NOISE-TO-CARRIER RATIO IN DECIBELS 



Fig. 21.— Effect of the low-pass filter cutoff frequency on distortion in the presence of 

 thermal noise — 740-cycle band. 



50 



^40 

 cc 



Q. 



2 30 



20 



-26 -24 -22 



■20 -18 -16 -14 -12 -10 -8 - 

 RELATIVE NOISE LEVEL IN DECIBELS 



Fig. 22. — Effect of the low-pass filter cutoff frequency on distortion in the presence of 

 impulse noise — 740-cycle band. 



cause distortion and to make the system more susceptible to noise. A very 

 small shift results in a low amplitude of demodulated signal, which is more 

 readily distorted by noise and biased by frequency drifts. It is of interest, 

 however, that the signal-to-noise ratio of the demodulated signal is not 

 proportional to frequency shift for high noise conditions. As described 

 before, noise peaks tend to reduce momentarily to zero the output from a 

 balanced discriminator. The amplitudes of the dips or holes are thus about 

 one-half the demodulated signal amplitude for any value of shift. This 

 tends to maintain a constant signal-to-noise condition and this characteristic 



