108 



USRL TEST STATIONS 



lines connect each of these fields with the correspond- 

 ing junction box at each end of the tank. There are 

 also crossties between the boxes as well as between 

 the jack fields. 



With the exception of the control lines for index- 

 ing, reporting, and directing, all lines and crossties 

 are standard rubber-covered coaxial cable and termi- 

 nations are all coaxial jacks (Western Electric Type 

 464A). This type of line and terminal is also used on 

 the d-c power leads to the hydrophone preamplifiers, 

 since in some instances pickup in the power leads will 

 be impressed on the signal leads by stray coupling 

 capacities in the preamplifiers. Connections to the 

 jack fields and to the instruments are made with 

 patch cords terminating in standard coaxial plugs 

 (Western Electric Types 337A and D 122403). The 

 control lines are all standard twinex cable terminat- 

 ing in standard telephone jacks (Western Electric 

 Type 218A). The use of such standard jack fields and 

 patch cords greatly enhances the flexibility of the 

 system. 



Lines from other systems also terminate in the jack 

 fields. This allows interchange of equipment between 

 systems for special testing. Another feature of the 

 arrangement is that the lines to other systems can be 

 extended to the pier installations by means of their 

 jack fields, thus making measurements in the out- 

 door test convenient. 



The characteristic impedance of the lines is about 

 72 ohms and the line loss is negligible in most cases. 

 For example, the loss on a complete loop from the 

 high-frequency room out to the end of the pier and 

 back again (about 425 feet) was about 1 db at 2.2 mc 

 and only 2 db at 3.5 inc. However, it must be 

 emphasized that, unless the lines are properly ter- 

 minated, standing waves will occur and lead to er- 

 roneous results. 



Ground Loop and Cross Talk Problems. Since 

 ground loop effects and cross talk depend on the 

 components of particular apparatus and their ar- 

 rangement in the circuit, it is almost impossible to 

 specify conditions which will apply to all circuits. 

 This is particularly true at the high frequencies and 

 the low signal levels encountered in this system. How- 

 ever, some initial precautions are mentioned here. 



The first is the use of good shielding which must 

 be extensively employed to obtain satisfactory re- 

 sults. In all the electronic circuits heavy copper shield 

 cans must be used to isolate individual stages in each 

 amplifier, oscillator, etc. It has even been found nec- 



essary to use copper shield cans around the metal 

 tubes in order to decrease radiation. 



The same precautions have to be used on all ex- 

 ternal wiring. In most cases, the coaxial cable used 

 for transmission lines has two shields, one wound on 

 top of the other, for more adequate results and all 

 connections are made by coaxial jacks and plugs. It 

 is important to remember that it is just as necessary 

 to shield the high-level lines as the low-level ones. It is 

 even necessary to shield the power leads, particularly 

 those to the low-level circuits. 



Adequate filtering (decoupling) must be used in 

 each individual stage for all electronic equipment 

 and extended, in most cases, even to filtering the fila- 

 ment supplies. The latter is particularly true in the 

 case of cathode-follower circuits. In addition, ex- 

 treme care must be exercised in the choice of ground 

 conditions in each individual stage. All of these pre- 

 cautions are necessary because of lead inductance and 

 stray capacities. In fact, at these frequencies some 

 commercial capacitors will appear inductive and 

 some commercial inductances, capacitative. Resis- 

 tors must be especially designed to operate at these 

 frequencies. 



Even with all the above precautions, trouble due 

 to ground loops may still arise. This problem is over- 

 come by a variety of precautions. The first and most 

 important is to determine a good ground and connect 

 all other grounds to it. In this case, the tank is se- 

 lected and the heavy copper straps in the bays of 

 electronic equipment are connected to it by No. 2 

 copper wire. The tank is connected to the fundamen- 

 tal ground at Pier 2 by No. copper wire. Once the 

 grounds are established, it is necessary to connect all 

 shielding to them in such a manner as to minimize 

 ground loops. 



Since the ground loop conditions change as the 

 experimental setups change, it is necessary to con- 

 struct the system with the greatest flexibility in 

 grounding. This is done by isolating from ground 

 the shields of transmission lines and similar equip- 

 ment and carrying the connections from the shields 

 through the patch cords to the particular piece of 

 equipment terminating the line. The shields could 

 be connected to ground at this point or isolated by 

 means of a doubly shielded transformer. In general, 

 it was found best to ground the line at one end only, 

 and leave the other end floating. However, all these 

 matters are subject to experimental conditions. 



The subject of ground loops and cross talk would 



