106 



USRL TEST STATIONS 



quencies because the input capacity is the controlling 

 impedance. 



A calibrating resistor has been included in these 

 preamplifiers to obtain the coupling loss of the cir- 

 cuit, which is required in order to evaluate the open- 

 circuit voltage of the hydrophone. 



Wide-Band Amplifier. The wide-band amplifier 

 shown on the diagram has a gain of 40 db and is flat 

 within 0.25 db from 50 kc to 3.2 mc. At 3.5 rac the 

 response is down 3 db. Both the input and output im- 

 pedances are 72 ohms. 



The purpose of this amplifier is to increase the 

 signals that are too small to record, even with the 

 full gain of the detector. 



Since the amplifier is unbalanced, it is associated 

 with a combination of 72:72-ohm coils which allows 

 changing the condition to ground from unbalanced 

 to balanced and vice versa. By means of terminations 

 on the coaxial jack-strips, the coils and the amplifier 

 may be connected to any portion of the circuit, 

 though they are normally used as shown in the dia- 

 gram. 



Detector. The action of the detector is best ex- 

 plained by tracing through a signal from the input. 

 The signal frequency is first amplified and then im- 

 pressed upon a modulator which receives another 

 signal from the variable-frequency oscillator, the 

 value of which is 15 rac-/. 



One of the modulation products ( 1 5 mc — / + / = 1 5 

 mc) is selected, filtered, amplified, and led to a second 

 modulator. The second signal to this modulator is 

 from a local oscillator with a frequency of 15.097 mc, 

 which gives 97 kc as one of the modulation products 

 of this stage. This signal is carried through a band- 

 pass filter centered at 97 kc and 6 kc wide to succeed- 

 ing stages of amplification. 



The reason for bringing the 15 mc — / signal to the 

 detector is that it ties the detector so completely to the 

 oscillator that it will detect no signal more than 3 kc 

 on either side of the one to which the oscillator is 

 set. The principal advantage of this plan is that since 

 most of the amplification takes place in a channel 6 

 kc wide, a much better signal-to-noise ratio is ob- 

 tained. One other advantage is relative ease of record- 

 ing at a single frequency (97 kc) as compared with 

 wide-band recording. The final stages consist of am- 

 plifiers and buffers for the 97-kc signal. A meter with 

 a logarithmic scale may be connected beyond these 

 stages to indicate the signal level. This is particularly 



useful when aligning and positioning the instru- 

 ments in the tank. 



The purpose of the buffer stages is to isolate vari- 

 ous circuits from each other. One buffer output goes 

 directly to the rms detector and recorder. The others 

 are used for monitoring and for making frequency 

 adjustments. 



To correct for drift in the 15.097-mc oscillator, pro- 

 vision has been made for checking and maintaining 

 the 97-kc carrier in the center of the 6-kc pass band. 

 A visual indicator of frequency drift such as that 

 used in the other systems is not practical because of 

 the relatively larger drifts in this system and because 

 the eye is unable to perceive flickers above 30 per sec- 

 ond. As mechanical shock alone may change the 

 frequency as much as 100 c, tone discrimination by 

 the ear is the method used. The 97-kc signal is led to a 

 heterodyne listener consisting of a 94- to 100-kc oscil- 

 lator, a modulator stage, and a loudspeaker. If the 

 listener is set at 97 kc, no tone will be heard from the 

 speaker when the carrier is 97 kc, but any drift from 

 this will be observed as a beat-frequency tone. A 

 separate quartz-crystal oscillator of 100 kc is avail- 

 able for calibration of the heterodyne listener. 



As is the case with all balanced modulator circuits, 

 spurious signals will arise if there is lack of proper 

 balance. As the first modulator stage in the detector 

 is of this design, some provision must be made for 

 indicating the point of balance. In this particular cir- 

 cuit, the modulator unbalance results in an ampli- 

 tude modulation of the 97-kc carrier, which is 

 greatest when the signal in the detector input is 97 

 kc. Using a cathode-ray oscilloscope to observe the 

 97-kc carrier envelope, the balance controls are ad- 

 justed for minimum amplitude modulation of the 

 carrier. Audible monitoring of the carrier can be used 

 in conjunction with the oscilloscope and provides an 

 extremely sensitive indication of the balance point. 

 The tone at this point will be noticed to lose its 

 "mushiness." 



The external meter circuit shown in the block 

 diagram is somewhat similar to the meter circuit in 

 the detector but has the advantage of being portable, 

 which allows its use for aligning the acoustic instru- 

 ments in the pier test areas away from the electric 

 system. 



The detector input is 72 ohms unbalanced and the 

 gain of the detector plus recorder is such that 75 db 

 vs 10 _1G watt into 72 ohms is required to produce a 



