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BELL SYSTEM TECHNICAL JOURNAL 



shift exciter of the type described above, with the crystal oscillator and 

 200 kc. FS oscillator located in a temperature-controlled oven, usually has 

 a frequency stability such that the mean R.F. carrier frequency may be 

 held to within ±50 cycles up to frequencies of 20 mc over ordinary periods 

 of operation on any qne frequency. One of the advantages of this type of 

 exciter is that small inaccuracies in crystal frequencies may be compensated 

 for by adjusting the mean frequency of the 200 kc. oscillator. 



4 2 2 4 



ORDER OF SIDEBAND 



Fig. 4. — EflFect of shaping FS transitions on amplitude of radiated sidebands. 100 

 dots per second and 500 cycle frequency shift. The maximum and minimum wave shap- 

 ing conditions correspond to those used with 60 and 240 dots per second respectively in 

 Fig. 3(a). 



Receiving Terminal Arrangements 



Typical receiving terminal arrangements are shown in the block diagrams 

 of Fig. 5. Up to point "a" in the arrangements the FS and AM systems are 

 identical, being of the usual H.F. superheterodyne type. The output from 

 the second frequency-conversion stage may be either in the audio range or 

 at a considerably higher frequency such as 50 kc. Following the second 

 converter is a band-pass filter (shown at "b" in Fig. 5) which determines 

 the final over-all band width before demodulation. The two systems differ 

 only in the method of demodulation. The AM (on-off) signals are amplified 

 and rectified to give a d-c.telegraph signal. The FS signals are amplitude 

 limited and passed through a frequency discriminating network and then 

 rectified to give a d-c.telegraph signal. Beyond this point the two systems 

 are again identical. The d-c . signals pass through a low-pass filter to remove 



