280 



BELL SYSTEM TECHNICAL JOURNAL 



frequency or phase changes of the signal. Noise voltages which are small 

 compared to the signal cause approximately linear phase modulation of the 

 signal and this is passed through the Hmiter. The amount of frequency 

 deviation thus imparted to the carrier by a given component of noise is 

 proportional not only to its amplitude but also to its frequency separation 

 from the carrier. This gives rise to the so-called ''triangular noise spec- 



50 



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z 



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IT 

 111 

 £L 



?30 



z 

 o 



-26 



-20 -18 -16 -14 -12 -10 -8 -6 

 RELATIVE NOISE LEVEL IN DECIBELS 



Fig. 14. — Peak distortion versus impulse noise for x\M transmission- 

 low-pass filter. 



-cycle cutofif 



50 



40 



S 20 

 O 



■16 -14 -12 -10 -8 

 RMS NOISE-TO-CARRIER 



-6 -4 

 RATIO IN 



-2 



DECIBELS 



Fig. 15. — Effect of the limiter on distortion versus thermal noise for FS transmission — 

 740-cycle band, 350-cycle frequency shift, 80-cycle cutoff low-pass filter. 



trum"'' when a hnear frequency discriminator is used. If the limiter is 

 removed from the frequency-shift terminal the noise components in phase 

 with the carrier as well as those in quadrature therewith are allowed to reach 

 the frequency discriminating network. For a balanced type discriminator 

 this increases the demodulated noise for small amounts of noise about ?> db. 

 In Fig. 15 is shown the effect of removing the limiter from the circuit. The 

 limiter is seen to have little effect on the failure point, but an improvement 

 of 2 to 4 db is shown in the 5 to 12% distortion region. 



