EQUIPMENT AT MOUNTAIN LAKES 



107 



lull-scale deflection on the recorder. The noise level 

 under these conditions is off the lower end of the 

 recorder scale, which means that it is below 30 db vs 

 10 _lli watt into 72 ohms. 



Recorder. The recorder circuit used in this system 

 is practically identical with those of the lower fre- 

 quency systems and operates only at 97 kc. 



Frequency-calibrated paper is available but only 

 for one relative speed of oscillator to recorder. How- 

 ever, if it is necessary to make records at other speeds, 

 calibrated transparent scales may be used. 



Indexing circuits for marking uncalibrated paper 

 at predetermined points on the oscillator scale are 

 available. Indexing, however, is done only when a 

 special paper is being used, or when directivity pat- 

 terns are being taken. 



Noise Generator. As shown in the block diagram, 

 the noise generator may be substituted for the fixed 

 oscillator at the entrance to the first buffer stage in 

 the high-frequency system. The noise signal is a 

 6.6-kc band centered at 15 nic and, as this is hetero- 

 dyned with the variable-frequency oscillator, the re- 

 sulting output is the same width centered at the 

 frequency /. 



The random noise signal is first generated by a gas 

 discharge tube (RCA 150-30), amplified and fed 

 through a 1.1-kc band-pass filter centered at 455 kc. 

 It is then used to modulate a 2.095-mc signal from a 

 crystal oscillator, and the products in the band cen- 

 tered at 2.5 mc are selected, doubled, and then tripled 

 in frequency. This gives as an output a band of noise 

 6.6 kc wide, centered at 15 mc. The maximum signal 

 level is approximately 0.5 rms volts across 72 ohms. 



The addition of a narrow band-pass filter and a 

 local 455-kc oscillator in the noise generator makes 

 it a very useful tool for accurately aligning the fixed- 

 frequency oscillator at 15 mc. This is done by first 

 adjusting the variable oscillator to one of the film 

 scale calibration points with the noise generator and 

 filter circuit substituted for the fixed oscillator. The 

 fixed oscillator is then replaced in the circuit and its 

 trimmer capacitors adjusted until the same calibra- 

 tion point is attained. 



Power Requirements and Power Supplies. The en- 

 tire electric system is powered from the regulated 

 115-volt, 60-cycle source available in the laboratory. 

 The filament supplies to all the tubes are constant 

 voltage transformers which give additional regula- 

 tion to the electronic circuits. This type of trans- 



Ficure 42. Electrical equipment of high-frequency system. 



former may be used, since the harmonics arising from 

 this regulation do not interfere with the measure- 

 ments. This becomes obvious from the fact that the 

 lowest frequency detectable with this system is 50 kc; 

 this simplifies considerably the various power and 

 distribution requirements. 



The plate supplies are three units of regulated d-c 

 power which can supply up to 450 milliamperes at 

 300 volts and are identical with the units used on the 

 intermediate-frequency systems. 



For the low-level signal circuits (hydrophone pre- 

 amplifiers and wide-band amplifier) a special supply 

 was constructed with higher regulation than the 

 above. This unit furnishes 1 50 milliamperes at 140 

 volts with a stabilization ratio of approximately 

 1:10,000 and an internal resistance of about 1 ohm. 

 A similar power supply is available for use with the 

 outdoor test equipment and is built into the same 

 portable case with the external meter circuit. 



Transmission Line and Jack Fields. The photo- 

 graph of the electric system (Figure 42) shows the 

 transmitting jack field below the oscillator and the 

 receiving jack field below the detector. Transmission 



