FREQUENCY SHIFT TELEGRAPHY 



281 



Other beneficial effects resulting from the use of a Umiter are discussed 

 below under "Level Variations". 



Demodulation of Frequency Shift Signals 



It is desirable that the frequency discriminating network be of the bal- 

 anced type having two branches allowing differential combination of the 

 two rectified outputs. This minimizes the response to amplitude modula- 

 tion not eliminated by the limiter. Two general tj^es of networks have 

 been in common use for FS telegraph. One consists of two bandpass filters 

 centered about the mark and space frequencies respectively and effectively 

 dividing the total band into halves. Th'^, other consists of a two-branch 

 network each branch of which has a varying amplitude characteristic extend- 



•16 -14 -12 -10 -8 

 RMS NOISE-TO-CARRIER 



-6 -4 -2 



RATIO IN DECIBELS 



Fig. 16. — Effect of magnitude of frequency shift on distortion versus thermal noise in 

 FS transmission — 740-cycle band and 80-cycle cutoff low-pass filter. 



ing over the complete transmission band and usually well beyond. The 

 amplitude-versus-frequency characteristics of these two branches have 

 opposite slopes and are of such shape that differential combination of their 

 rectified outputs results in an approximately linear voltage-versus-frequency 

 curve, passing through zero at midband. (Fig. 8) 



In Fig. 17 is shown the characteristic of a two-bandpass-filter type of dis- 

 criminator which was used in early frequency shift terminals. The charac- 

 teristic is fairly flat near the mark and space frequencies so that this type 

 of discriminator does not produce a triangular noise spectrum. In Fig. 18 

 is shown the type of discriminator characteristic obtained by the use of two 

 narrow bandpass filters. In this case there is no broad flat region around the 

 mark and space frequencies and an intermediate type of characteristic (ap- 

 proaching the linear type) is obtained. 



With the linear type of discriminator the demodulated noise has the well 

 known triangular spectrum and, as illustrated previously, the signahng 



