FREQUENCY SHIFT TELEGRAPHY 



273 



carrier ripple and higher frequency noise components and are then ampUtied 

 to a suitable level to operate automatic recording or printing apparatus. 

 The d-c. signals may also be used to modulate an audio frequency so as to 

 pass the signals to a remote point by multichannel voice-frequency carrier 

 telegraph methods. 



ANTENNA 



R-F 

 AMPLIFIER 



FIRST 

 FREQUENCY 

 CONVERTER 



I-F 

 AMPLIFIER 



SECOND 

 FREQUENCY 

 CONVERTER 



(b) 



E 



BAND- PASS 

 FILTER 



LI N EAR 

 AMPLIFIER 



3^ 



(a) 



RECTIFIER 



LOW-PASS 

 FILTER 



D-C 

 AMPLIFIER 



n 



D< 



TONE 

 KEYER 



TELEGRAPH 

 SIGNALS 



V-F. 



Fig. 5. — ^Block diagram of a typical receiving arrangement for either AM or FS tele- 

 graph signals. 



The radio receiver portion of the terminals up to point "a" should be 

 designed to have low noise and good selectivity. Extreme H.F. oscillator 

 stability is necessary for either system if narrow band width operation is to 

 be maintained without constant attention. An over-all frequency stabiHty 

 of ±50 cycles is desirable for the receiving terminal over a period of 6 to 

 8 hours. Sufficient selectivity and ampUfier capacity should be provided 

 at all points to prevent overloading by unwanted signals or loss of automatic 



