78 



USRL TEST STATIONS 



tics as those in the detector circuit. 



The continuous indication of tuning is essential, 

 particularly for frequency response measurements at 

 distances greater than about 3 meters. For such a 

 length of path through water there is a significant 

 delay in transmission. In order that the detector be 

 tuned correctly for the incoming signal, it must lag 

 behind the oscillator by an amount which is a func- 

 tion of the frequency sweep rate and the travel time 

 of the sound through the water. The tuning indi- 

 cator, which gives a continuous reading, permits com- 

 pensating adjustments to be made during the test 

 period. 



Linear Level Recorder 



Electromechanical recorders are used with each of 

 the systems to provide continuous and permanent 

 records of the response of the devices under examina- 

 tion. Each recorder consists of an amplifier which 

 maintains an arbitrary equilibrium voltage at its out- 

 put terminals by controlling, through a motor drive, 

 the position of a sliding contactor on a strip attenua- 

 tor at the input. A pen attached to this contactor re- 

 cords its position on a strip of moving paper. The 

 speed of the paper drive is synchronized with the 

 frequency sweep of the oscillator, so that the paper 

 may have a fixed frequency scale. 



The electronic circuit of the level recorder used 

 with system 1 includes a special strip attenuator, a 

 second attenuator for presetting the gain, and an 

 amplifier tuned to 28 kc, followed by a half-wave rec- 

 tifier circuit. The normal d-c output of this circuit is 

 about 100 volts at equilibrium and is impressed on 

 the grids of a pair of d-c amplifiers. These isolate the 

 a-c thyratrons which follow. The anode current of 

 each thyratron is passed through one of the windings 

 of a small dual armature motor, the field of which is a 

 permanent magnet. The thyratron and the d-c am- 

 plifier circuits are so arranged that an increase in the 

 d-c voltage decreases the normally negative grid 

 voltage of one thyratron with respect to its cathode, 

 causing it to fire (allow the passage of current) and 

 thereby drive the motor in one direction. Conversely, 

 a decrease in the d-c voltage produces the same effect 

 on the second thyratron, causing it to drive the motor 

 in the opposite direction. 



A continuous silk cord, after a few turns around 

 the motor shaft, runs over three pulleys and back to 

 the shaft. The pulleys are so placed that a section of 

 the cord extends the length of the attenuator strip 



and parallel to it. On guides, also parallel, is mounted 

 a carriage with an arm, making contact on the strip. 

 This carriage is clamped to the cord so the contact 

 may be moved to any point on the attenuator by the 

 rotation of the motor. 



The recorder seeks at all times to maintain the 

 equilibrium d-c voltage at which the thyratrons are 

 extinguished, by changing the setting of the contact 

 on the input attenuator strip. The maximum rate 

 at which the recorder can respond to changes in im- 

 pressed level is approximately 100 db per second. 



The resolution of this system is determined by the 

 marginal d-c bias on the thyratrons and may be ad- 

 justed to within less than 0.1 db. The effective over- 

 all stiffness of the electronic and mechanical system 

 in the region of balance is determined largely by an 

 injected a-c bias, used primarily to control overshoot- 

 ing. 



The strip attenuators are wound for a total at- 

 tenuation of 50 db at 5 db per inch. They are 

 mounted horizontally, directly over the recording 

 paper which is a continuous strip with perforations 

 along each edge. The paper moves over a roller with 

 matching sprocket teeth that is driven by a small 

 synchronous motor through an adjustable gear train 

 allowing rates of 2, 6, or 18 inches per minute or per 

 hour. A friction clutch with a double ratchet attach- 

 ment permits the paper to be advanced or rewound 

 on the supply spool by means of a hand crank. The 

 paper drive motor is tied in to the oscillator drive 

 motor so that both may be operated by a single 

 switch. 



The frequency resolution of the recorder is a func- 

 tion of the frequency sweep rate of the oscillator and 

 the speed at which the recording paper travels. Nor- 

 mally, the oscillator and recorder are driven at the 

 same relative speed (usually the intermediate one) 

 to maintain the proper relationship between oscilla- 

 tor frequency and the frequency calibration of the 

 paper. Under these conditions the individual charts 

 for to 150 kc are approximately 32 inches long. 

 However, the frequency resolution may be improved 

 ninefold by setting the oscillator sweep rate at mini- 

 mum and the paper drive at maximum. The system 

 is operated most frequently in this manner from 

 to 460 c with special recording paper. To cover this 

 frequency range requires a chart 32.5 inches long. 



The level recorder of system 2 differs from that of 

 system 1 chiefly in that its amplifier is designed for a 

 flat frequency response from 100 c to 150 kc, and in 



