EQUIPMENT AT MOUNTAIN LAKES 



77 



COAXIAL 

 JACKS 



Figure 12. Circuit schematic of coupling amplifier. This amplifier provides extremely high input impedances for 

 operation in balanced or unbalanced circuits. 



Design considerations in the construction of these 

 required the selection of a mid-frequency of 97 kc. 

 The detector tuning oscillator of the signal generator 

 of system 2, therefore, supplies a frequency of 747 kc. 

 A rotary switch permits the rapid selection of any 

 of the three band-pass filters, which have widths of 

 10, 300, and 6,000 c, centered at 97 kc. Following the 

 filter circuits are three stages of amplification termi- 

 nating in a 135-ohm output circuit. 



Monitor Converter 



System 2 is provided with a converter circuit com- 

 prising a modulator and a local oscillator which may 

 be varied continuously from 94 to 100 kc. The prim- 

 ary function of this circuit is to permit aural monitor- 

 ing of supersonic frequencies by converting the 

 normal 97-kc signal output of the detector circuit, 

 to an audio frequency range of to 3,000 c. 



Indicator 



The use of narrow-band crystal filters in the de- 

 tector circuit makes it necessary to center the output 



of the second modulator precisely on the mid-fre- 

 quency. The adjustment of the detector-tuning oscil- 

 lator to accomplish this is referred to as "tuning the 

 detector" and it is correct when maximum detector 

 output is obtained. It has been found expedient to 

 provide for a continuous visual indication of this 

 adjustment. 



This indication is produced by taking a portion 

 of the detector output signal from a constant voltage 

 source in the recorder circuit and modulating it with 

 a signal from a crystal-controlled oscillator tuned to 

 the mid-frequency of the crystal filter in the detector 

 circuit. The resultant difference signal is rectified and 

 impressed on an electron-ray tube. The shadow angle 

 of this tube opens and closes at the difference fre- 

 quency. This is a direct indication of the deviation 

 in cycles per second from the center frequency of the 

 crystal filter. The tuning adjustment may thus be 

 maintained within a fraction of a cycle at all times. 

 Frequency drifts of the oscillator with respect to the 

 filters are minimized by the use of oscillator-stabiliz- 

 ing crystals having the same temperature characteris- 



