FREQUENCY SHIFT TELEGRAPHY 



295 



signal through selective fading becomes less probable. The spectra gen- 

 erated by sinusoidal phase modulation of 1.0, 1.4, and 2.0 radians are shown 

 in Fig. 34. Most of the energy is seen to be concentrated in the carrier and 

 hrst order sidebands. Less than 1.0 radian of modulation results in too 

 little amplitude of the sidebands, while more than 1.5 radians results in too 

 wide spread of energy outside the first order sidebands. The center three 



4 3 2 1 C I 2 3 4 



ORDER OF SIDEBAND 



Fig. 34. — Frequency spectra for sinusoidal phase modulation. 



components are equal at about 1.4 radians. In the case of FS the phase 

 modulation frequency appears as a variation in amplitude of the signals from 

 the discriminator. For AM no additional ampUtude variation is caused by 

 the phase modulation if there is no selective attenuation in the medium, 

 but if such exists the phase modulation frequency or a multiple thereof 

 appears in the rectified signal. To permit these unwanted amplitude varia- 

 tions to be removed by the low-pass filter so as not to break up the signals, 

 the phase modulating frequency should preferably be ten or more times the 

 maximum signaling frequency. 



