FREQUENCY SHIFT TELEGRAPHY 303 



more careful adjustment and more attention than does an FS system. This 

 is partly due to the amplitude limiter in the FS system which results in a 

 constant amplitude of signal from the discriminator and partly due to the 

 fact that an FS signal is no more subject to noise interference during the 

 spacing condition than during the marking condition. Therefore the operat- 

 ing point on the demodulated wave may be set and left for long intervals 

 even though transmission conditions vary widely. This greater ease in 

 maintaining good adjustment of the equipment probably accounts for some 

 of the apparent improvement in changing from an AM to an FS system. 



It should be noted that a system may fail either because of level variations 

 well above the noise level or because of the signal becoming submerged 

 in noise. If a system fails because it can accept only moderate level varia- 

 tions, an increase in transmitted power will provide no improvement since 

 the level variations will remain the same as before. On the other hand, a 

 system which can accept very wide variations in level will show improve- 

 ment upon increasing transmitted power up to the point where no failures 

 occur due to an unfavorable signal-to-noise ratio. 



The over-all improvement obtained in changing from AM to FS radio 

 telegraph is sometimes expressed as a ratio of transmitted powers required 

 to give equivalent transmission results over the two systems. Such a ratio 

 fluctuates widely depending upon the prevailing conditions. With little 

 fading the improvement ratio will be mainly due to the better signal-to- 

 noise obtained with FS and may be less than 5 db. Under severe fading 

 conditions no amount of power may give good results with AM while FS 

 may be satisfactory. ' Thus the power ratio would become infinite. By 

 making a long-time comparison an average power ratio figure may be 

 found which gives equal average error rates in the printed copy from each 

 system. Such tests^ between a triple space diversity AM system and a 

 double space diversity FS system have indicated a power ratio of 11 db 

 in favor of the latter when the error rate was 0.1 to 0.5 per cent. 



When the two systems are thus made equal by adjustment of transmitted 

 power, more errors due to the signal becoming submerged in noise occur in 

 the FS system to compensate for a larger number of errors in the AM system 

 due to rapid level changes. Often the reason for changing a radio telegraph 

 system from AM to FS is to increase the reliability of the circuit and not 

 just to save transmitted power. To insure a definite improvement in such 

 cases the carrier level should not be decreased more than about 6 db. 



References 



1. Certain Topics in Telegraph Transmission Theorv, H. Nvquist, Trans, of A.I.E.E. 



Vol. 47, April 1928, pp. 617-644. 



2. Frequency Modulation, Balth. van der Pol, Proc. ofl.R.E., Vol. 18, July 1930, p. 1202. 



