FREQUENCY SHIFT TELEGRAPHY 



279 



tage of 2.5 to 4.5 db at a distortion of 35% to 40%, corresponding to the 

 selector failure point of the usual teletypewriter. From a signal-to-noise 

 standpoint it is thus seen that the gain in changing to the FS method is 

 approximately equal to the resulting increase in average transmitted power 

 of about 3 db. A comparison at a lower distortion such as 15% shows 

 an advantage of 4 to 6 db. At a still lower distortion the 740-cycle band, 

 because of the higher deviation ratio, shows an improvement of over 10 db. 

 In this region the slopes of the curves make accurate comparisons impossible 

 due to the masking effect of other, sources of distortion. These large im- 

 provements at low noise levels are similar to those associated with wide- 

 band FM broadcast systems. However, in carrier telegraph transmission 

 the criteria are so different that the difference between a nearly perfect 



50 



I- 

 z 



UJ 



^40 



Q. 

 ?30 



10 



-26 -24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 



RELATIVE NOISE LEVEL IN DECIBELS 



Fig. 13. — Peak distortion versus impulse noise for FS transmission — 80-cycle cutolf 

 low-pass filter. 



circuit and one of small distortion is not of great importance except when a 

 large number of telegraph sections are to be operated in tandem. From a 

 practical standpoint the improvement in signal-to-noise is not more than 

 about 6 db for equal band widths. 



In Figs. 13 and 14 similar characteristics are shown for the case of impulse 

 noise. The noise level values of these cur\'es are purely relative since no 

 attempt to measure the peak noise was made. The comparisons between 

 AM and FS, and between different band widths, agree closely with those 

 for thermal noise. 



EJfect of Limiter on Signal-to-Noise Ratio 



The limiter in the FS system is a high-gain nonlinear amplifier which 

 delivers to the frequency discriminating networks an essentially square wave 

 having transitions coinciding with the passage of the instantaneous voltage 

 of the input carrier signal through zero. The limiter thus passes only the 



