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BELL SYSTEM TECHNICAL JOURNAL 



this was supplied from a shielded vacuum tube oscillator which on 

 comparison with a standardized piezo-electric oscillator was found 

 to possess the required stability. The double detection type receivers 

 were used for no other reasons than their availability and their con- 

 venience for quantitative work. The beating down oscillator within 

 the sets and the intermediate frequency step passed through in the 

 sets by received signals do not figure in the following discussion of 

 test methods but, of course, in each case the necessary set tuning 



Radio 

 Transmitter 



J 



Masker 

 Oscillator 



Oscillograph 

 I ° — 1 For Wave Shape 

 Oscillograms 



Tone from 

 Telephone Circu 



Z> 



Fig. 6 — Diagram of system used for single frequency tests 



adjustments were made. To avoid confusion it is w^ell to think of these 

 receivers as being replaced by high frequency amplifiers and simple 

 detectors since the local beating oscillator referred to in later pages is 

 the separate shielded oscillator described above which is used to beat 

 the signals down to audio frequencies. 



In this work the moving coil type oscillograph was used throughout 

 for the purpose of making photographic signal records. As indi- 

 cated in Fig. 6 two oscillographs with elements connected in series 

 were employed; one for the purpose of making a continuous record 

 of the variation in the amplitude of the signal using a slow moving 

 photographic paper tape and the other to obtain the wave shape of 

 the signal by means of the usual high speed photographic film drum. 

 An element of one oscillograph was also used at times to record on 

 the film drum the wave shape of signals rectified at the transmitting 

 antenna and sent over the telephone lines. 



Fig. 7 is the interior view of the test station at Riverhead showing 

 the general arrangement of the oscillographs and accessory apparatus. 

 This oscillograph equipment formed about the only fixed portion 

 of the apparatus, other portions being changed from time to time 

 for different tests. These arrangements will be described later in 

 connection with the records which they were used to obtain. 



In considering these various records perhaps we had best look 

 first at the simpler ones and then proceed in a more or less orderly 



