EQUIPMENT AT MOUNTAIN LAKES 



83 



,ZERO SIGNAL 

 d CONDITION 



MREPI 

 PERI 

 I I f-ZERO VOLTAGE 



Figure 18. Circuit schematic of the pulse rectifier. I he 

 voltage wave form of the pulse is indicated at various 

 points in the circuit. 



become almost wholly discharged between pulses. 

 Hence, the grid signal is a saw-tooth wave, the am- 

 plitude of which can change rapidly with change in 

 the incoming pulse. 



The first section of the 6SN7 acts as an impedance 

 changer and phase inverter, the voltage at point c 

 still containing the d-c and a-c components of the 

 rectified signal at b. The a-c component of this volt- 

 age is applied to the grid of the second section of the 

 6SN7 through the RC network. The time constant of 

 this RC combination should be approximately equal 

 to that of the filter section. Bv proper adjustment, 

 the signal at d can be made equal to the a-c compon- 

 ent of the signal at c, but inverted in phase so that 

 the mixed voltage at e will be approximately equal 

 to one-half the d-c component of the voltage at c 

 with the a-c components balanced out. To facilitate 

 this adjustment, a terminal is supplied for observa- 

 tion with a cathode-ray oscilloscope. 



An increase in the intensity of the received acoustic- 

 signal causes the d-c voltage at e to rise. The thyra- 

 tron control circuit used with the pulse system re- 

 quires, however, a decrease in the d-c voltage with in- 

 crease in signal intensity and, for this reason, the 

 final tube shown is used. 



In order to be used with the pulse system, one of 

 the thyratron control circuits is modified and a single 

 coaxial jack installed to take the d-c output of the 

 pulse rectifier. A switch on the front of the panel 

 allows the operator to choose the output of either 

 the recorder circuit or the pulse rectifier. When the 

 latter is chosen, 50-ohm resistors are automatically 

 inserted to slow the pen-drive so that the recorder 

 will not hunt. This reduces the speed of the recorder 

 from 100 to some 30 db per second. 



A gain control at the inpul ol the receiver modula- 

 tor allows the sensitivity of the pulse recording sys- 

 tem to be made equal (o that ol the usual continuous- 

 wave system. However, the sensitivity of the pulse 

 system is somewhat dependent on the length of the 

 received pulse and the repetition rate. In nearly all 

 tests, however, the values of these variables will be 

 chosen and held constant throughout. 



The frequency response of the receiver modulator 

 and pulse rectifier is flat within ±0.5 db from 1 to 120 

 kc. The response at the low-frequency end is con- 

 trolled largely by the number of cycles of the signal 

 within the pulse being measured. 



Miscellaneous Features. Auxiliary Apparatus 



Methods of Connecting. Great flexibility of inter- 

 connection is obtained by the use of jack fields as 

 terminals lor the individual pieces of electric ap- 

 paratus mounted in the bays. The arrangement of 

 the jacks within the field is based on factors such as 

 accessibility, convenience in wiling, and the consid- 

 eration of cross talk. Many jacks are interconnected 

 so that commonly used combinations of apparatus 

 are established without the use of external connec- 

 tors. The jack fields also provide terminals for inter- 

 bay, inter-system, and system-to-pier lines, and for 

 frequently used coils, attenuator pads, and load re- 

 sistors. 



Connections between jacks are made with plug- 

 terminated, flexible, shielded cords referred to as 

 patch cords. Where the jack grouping permits, con- 

 nection between adjacent jacks is made with short- 

 circuited cordless plugs. The types of patch cords 

 and plugs may be seen in Figures 9 and 10. 



Grounding. Each system is provided with a funda- 

 mental circuit ground comprising a copper pipe 

 driven into the lake bottom adjacent to the pier test- 

 ing area. Four No. 0000 stranded copper cables con- 

 nect each fundamental ground to heavy copper bus 

 bars in the pier booths and apparatus bays, and to 

 copper strips mounted along the test areas which 

 are used for grounding test apparatus. Individual 

 circuit ground connections are made directly from 

 the bus bars to all panel-mounted equipment. The 

 lead sheath and outer conductors of the coaxial trans- 

 mission lines between the pier booths and the labora- 

 tory are grounded at the jacks in the pier booths. The 

 types of ground are indicated on the jack fields by 

 means of colored celluloid windows placed over the 

 designation strips. 



