270 



BELL SYSTEM TECHNICAL JOURNAL 



Apparatus 

 Typical FS Exciter for Radio Telegraph 



A typical FS exciter arrangement such as is often used with radio telegraph 

 transmitters is shown in Fig. 2. A d-c. telegraph wave, after suitable shap- 

 ing, is caused to frequency-modulate an intermediate frequency of 200 kc. 

 which, in turn, amplitude-modulates a radio frequency from a crystal- 

 controlled oscillator. The upper sideband of this latter modulation is an 

 FS signal and is selected and amplified sufficiently to drive the first amplifier 

 or multiplier stage of the transmitter. The 200-kc. oscillator is frequency- 

 modulated by a reactance modulator which, by feeding a leading or lagging 



TRANSMITTING 

 ANTENNA 



FS EXCITER 



UJ 



CRYSTAL 

 OSCILLATOR 



BALANCED 

 MODULATOR 



AMPLIFIER 



200-KC 

 FREOUENCY- 

 MODULATED 

 OSCILLATOR 



D-C TELEGRAPH 

 /SIGNAL 



<^±90*= 



REACTIVE 

 CURRENT 

 AMPLIFIER 



BALANCED 

 REACTANCE 

 MODULATOR 



KEYING 

 CIRCUIT 



WAVE- 

 SHAPING 

 CIRCUIT 



JG -—4— 



_J 



PHASE- 

 MODULATION 

 OSCILLATOR 



FREQUENCY 

 MULTIPLIERS 

 AND POWER 



AMPLIFIER 



Fig. 2. — ^Block diagram of a typical FS transmitter. 



quadrature component of current into the oscillator tuned circuit, decreases 

 or increases the frequency. By operating the reactance modulator within 

 its linear range the frequency shift wave form is made the same as the d-c. 

 telegraph wave form into the modulator. A d-c. amplifier stage, designated 

 "keying circuit", is provided to furnish a modulating wave effectively iso- 

 lated from amplitude and wave front variations of the incoming telegraph 

 signals. The d-c. telegraph signals may be polar or neutral and are often 

 obtained from a tone demodulator unit which allows keying from a remote 

 point by V.F. telegraph. The amount of frequency shift is adjusted by an 

 amplitude control in the quadrature feed-back path to the 200 kc. oscillator. 

 The shift may thus be varied continuously, or in definite steps to allow for 

 subsequent frequency multiplications, by suitable attenuation controls. 

 Controlling the shift in this manner keeps the instabilities of the reactance 



