268 



BELL SYSTEM TECHNICAL JOURNAL 



phase swings, the modulator would have to be able to produce a steady phase 

 rate of charge. A phase modulator capable of performing in this way while 

 not producing undesirable phase discontinuities at the signal transitions 

 becomes rather impractical. For this reason FS telegraphy usually utilizes 

 the direct frequency modulation method. This may conveniently be accom- 



J4 t3 12 



10 9 8 7 6 5 



4 3 2 1 C 1 2 3 4 



ORDER OF SIDEBAND 



6 7 8 9 10 11 12 13 14 



(b) SQUARE-WAVE FREQUENCY MODULATION 

 SPACE -*". 1 ,-*— MARK 



DEVIATION RATIO, m =1 



765432 1 CI 23456789 10 11 12 13 14 

 SPACE — *- 1 1 ,-*— MARK- 



11 9 7 5 3 I C 1 

 1.0 I — SPACE-*" 



3 5 



■ ■llilil-i.lilill-. . 



7 9 1' 

 - MARK- 

 m = 4.7 



12 10 8 6 4 2 C 2 4 6 8 10 12 30 20 10 



ORDER OF SIDEBAND 



Fig. 1. — Amplitude of sideband components for (a) square-wave amplitude modulation 

 (b) square- wave frequency modulation, 



plished by the use of a reactance modulator which, by injecting a reactive 

 component of current into the tuned circuit of the oscillator, varies the 

 resonant frequency thereof. Such a modulator may be made linear so that a 

 frequency shift proportional to the input voltage to the reactance modulator 

 is obtained. 



To apply FS telegraph signals to a radio transmitter the regular exciter 

 oscillator is either replaced or modified by an arrangement providing a 

 source of R.F. excitation that can be shifted in frequency in accordance 



