FREQUENCY SHIFT TELEGRAPHY 



271 



modulator a constant percentage of the frequency shift, which would not 

 be the case if the shift were adjusted by varying the amplitude of the modu- 

 lating wave. The use of a balanced instead of an unbalanced reactance 

 modulator minimizes variation of the mean frequency and also allows the 

 shift to be varied without affecting the mean frequency. 



The frequency-shift signal transitions are wave-shaped, to restrict side- 

 band radiation, by means of a low-pass filter in the d-c. telegraph signal path 

 to the reactance modulator. The low-pass filtering is made adjustable to 

 accommodate a range of signaling speeds. Frequency-versus-time wave 



PxPd 



Q + 



® 



240 



23 -CYCLE 

 DOTS WITH 

 200 CYCLE 

 SUPERIMPO- 

 SED PHASE 

 MODULATION 



Fig. 3. — Frequency shift keyer output wave forms, (a) Low-pass filtering adjusted 

 to produce similar wave shapes at dotting speeds of 60, 100, 200, and 240 cycles, (b) 200 

 cycle phase modulation superimposed on a 23 dots per second signal. 



forms from an exciter of the type described are shown for several keying 

 speeds in Fig. 3a. The effect of wave shaping on the sideband components 

 in the R.F. output of such an exciter is shown in Fig. 4. 



Phase modulation may readily be added to the signal in this type of exciter 

 by superimposing the desired sine wave modulating frequency on the tele- 

 graph signal wave input to the reactance modulator as indicated in Fig. 2. 

 Figure 3b shows the keyer output wave form with superimposed phase 

 modulation. The use of this tjrpe of phase modulation is considered later. 



To obtain optimum results in FS radio telegraph transmission and to 

 allow close spacing of channels, a high degree of frequency stability is neces- 

 sary. An over-all frequency stabiHty of it 100 cycles is desirable in a system 

 using a value of frequency shift between 500 and 1000 cycles. A frequency 



