EQUIPMENT AT MOUNTAIN LAKES 



71 



Committee [NDRC]. The Underwater Sound Refer- 

 ence Laboratories have designed and constructed 

 most of the mechanical equipment and have devel- 

 oped the high-power equipment, the pulsing system, 

 the polar recording system, and certain additional 

 features needed for special testing. 



62 CALIBRATION AND TESTING 



EQUIPMENT AT MOUNTAIN LAKES 



The calibration and testing systems of USRL are 

 described on the basis of their present status. It should 

 be emphasized, however, that developments in sonar 

 gear and the constantly improving techniques in test- 

 ing are making new demands, in many cases beyond 

 the capacities of existing equipment. It must there- 

 fore be constantly changed and improved to maintain 

 the standards required of a reference laboratory. 



For example, system I, installed in June 1942, in- 

 corporated a narrow band-pass filter which discrimin- 

 ated against noise and other interference. This was at 

 the time a distinct improvement but after the devel- 

 opment of the ptdse method, which requires a wider 

 transmission band, system 1 was inadequate. For this 

 reason, and also because of the increasing importance 

 of noise analysis, system 1 is now limited in its appli- 

 cations, and a continuously increasing proportion of 

 the work is handled by system 2. 



6.2.1 Electrical Components of Systems 



The essential parts of systems 1 and 2 are described 

 in a sequence which traces a typical signal from the 

 generator to the projector and from the hydrophone 

 to the recorder. 



Test Signal Generators 



The primary signal generators are beat-frequency 

 oscillators covering the 15-c to 150-kc range with a re- 

 sponse uniform within 0.3 db. A visual indication of 

 the frequency setting is provided by a calibrated scale 

 on a strip of 35-mm motion picture film 30 feet long, 

 coupled through a sprocket chain to the air condenser 

 controlling the frequency. The length of this scale in- 

 dicates the degree of frequency resolution. The shape 

 of the condenser plates is such that the scale gives ade- 

 quate frequency resolution throughout the entire 

 range. 



A synchronous motor drive provides the lock-in be- 

 tween oscillator and recorder when frequency-re- 



Figl're 5. View of raft. 



spouse traces are taken, although the dial may be 

 operated manually. 



The heterodyne oscillator assembly contains three 

 separate circuits. Two of them operate as a beat-fre- 

 quency oscillator, one fixed at 650 kc and the other 

 variable from 500 to 650 kc. The difference frequency 

 of the heterodyned outputs furnishes the signal range 



to 150 kc. This arrangement is identical in both sys- 

 tems, but the third circuit is fixed at 678 kc in system 



1 and at 717 kc in system 2 for use in tuning the de- 

 tector circuit described later in this section. 



Frequency stability has been obtained by design 

 features such as (1) mounting the three oscillator cir- 

 cuits in the same chassis to have the same ambient 

 temperature, (2) separating the component parts with 

 networks and buffer amplifiers, and (3) using suitable 

 shielding and filters. To correct for the slight drift in 

 frequency that still may occur, means for adjusting 

 the carrier frequencies are provided and the fre- 

 quency scale can be checked by aligning with the 60-c 

 power supply and with a 100-kc crystal shunted across 

 the oscillator output. Since the adjustment is based 

 on the difference frequency, no attempt is made to ob- 

 serve the actual frequencies of the carrier oscillators. 



The oscillator furnishes a maximum output level 

 of 150 db vs 10— 10 watt adjustable over a 40-db range. 

 Harmonics in the output voltage are at least 40 db be- 

 low the fundamental, and a minimum signal-to-noise 

 ratio of 50 db is realized. 



In system 2 a thermal noise signal may be generated 

 by using the noise generator in conjunction with the 

 heterodyne oscillator. The noise generator includes a 

 voltage regulator tube functioning as a wide-band 



