104 



USRL TEST STATIONS 



telephone to the laboratory. The raft carries perman- 

 ently a 6-kva, 60-cycle regulator that will maintain 

 am voltage from 105 to 125. Canvas drops are sup- 

 plied to protect the operators in bad weather. 



Temperature gradients in the lake are checked 

 by bathythermograph records taken several times 

 throughout the test period. Since factors such as gas- 

 bubble accumulations on the bottom may affect the 

 acoustic conditions, it is advisable to make simultane- 

 ous observations on a reference system maintained 

 at the laboratory for this purpose. 



6.2.3 



High-Frequency System 



General Description 



The high-frequency system at the Mountain Lakes 

 laboratory covers the range from 100 kc to 2.2 mc. 

 While the electronic system is capable of calibrating 

 up to 3.5 mc, no standard transducers are available 

 lor those frequencies. 



The system comprises the following items: 



1 . An electric system capable of producing, meas- 

 uring, detecting, and recording signals in the fre- 

 quency range 50 to 3,500 kc. 



2. An indoor calibration tank with the necessary 

 mechanical equipment for positioning and aligning 

 the units under test. Included here also are the vari- 

 ous acoustical devices for the reduction of reverbera- 

 tion. 



3. Two sets of mechanically interchangeable trans- 

 ducers having overlapping frequency responses, with 

 one set covering the range 100 to 800 kc and the other 

 300 to 2,200 kc. 



4. Mechanical and electrical components for meas- 

 urements in the outdoor test areas. These subjects 

 will be treated in detail in the following paragraphs. 



Electric System 



The electric system consists of a heterodyne oscil- 

 lator, power amplifier, and coupling network for 

 driving a projector, and of a detector and recording 

 circuit coupled through a preamplifier for measuring 

 the output of a hydrophone. This system will make 

 continuous ink recordings of the combined response 

 of a pair of instruments throughout the frequency 

 range 50 to 3,500 kc. A block diagram of this system 

 is shown in Figure 41. The frequencies and signal 

 transmission direction are also shown on the diagram. 



Oscillator. The heterodyne oscillator consists of 

 one oscillator fixed at 15 mc and the other variable 



from 1 1.5 to 15 mc. Each feeds through its own buffer 

 to the same modulator, which is followed by a filter 

 designed to pass only the difference frequencies of the 

 oscillators. An amplifier stage follows the filter and 

 terminates in a transformer designed to supply a 

 72-ohm impedance. A standard attenuator of this 

 value is inserted between the transformer and the 

 output jack of the oscillator. 



As the block diagram indicates, a small portion of 

 the output is rectified, converted, amplified, and fed 

 into the buffer stage for the fixed oscillator. This 

 constitutes an automatic volume control [AVC] 

 which will hold the output within 0.15 db over the 

 entire frequency range, while without it, the varia- 

 tions may be 1.5 db. The maximum output of the 

 oscillator is 222 milliwatts corresponding to 4 volts 

 across 72 ohms and the harmonic level is 45 db below 

 that of the fundamental frequency. 



The mechanical construction is very similar to that 

 of the other systems. The variable-frequency oscil- 

 lator can be driven either by hand or by a synchron- 

 ous motor which allows the entire range to be cov- 

 ered in 1.5, 4.5, or 13.5 minutes. The calibrated scale 

 is a 30-foot strip of 35-millimeter film. Values may 

 be determined between the lines of this scale by in- 

 terpolation with a vernier dial. 



The oscillator frequency is adjusted to the scale 

 calibration at 76 kc by means of a high-Q tuned cir- 

 cuit and at 2 mc by means of a quartz crystal. These 

 adjustments are made by a trimmer condenser and 

 a small adjustable inductance in the circuit of the 

 variable oscillator. A frequency check is made several 

 times each day to correct for any drift. This effect, 

 however, becomes negligible after 72 hours of con- 

 tinuous operation and accordingly, the power is not 

 turned off, unless it is to be for a period of several 

 days. 



Power Amplifier and Power Level Measurements. 

 The power amplifier is the wide-band type, covering 

 50 to 3,500 kc with a gain of 35 db flat to 0.5 db. The 

 input and output impedances are both 72 ohms and 

 the maximum power available is 25 watts (174 db vs 

 10 -le watt). It should be noted that the output of this 

 amplifier can be either balanced or unbalanced with 

 respect to ground. 



As indicated on the block diagram, a power level 

 measuring set is used to measure the available power 

 at the output of the oscillator or power amplifier. 

 This instrument is a high-frequency equivalent of 

 the Western Electric 30A transmission measuring set. 



