82 



USRL TEST STATIONS 



negative rectangular pulses when triggered. In addi- 

 tion, there is a relaxation oscillator capable of being 

 synchronized with various subharmonics of the 60-c 

 Filament supply. Short, sharp, positive impulses from 

 this oscillator arc used to trigger stages A and B 

 simultaneously. The rectangular pulse from A con- 

 trols the transmitter modulator, and its length de- 

 termines the length of the modulator signal. 



The negative rectangular pulse from B is differen- 

 tiated, yielding a sharp negative impulse at the begin- 

 ning and a sharp positive impulse at the end. Multi- 

 vibrator C is triggered by the latter (C responds 

 only to positive impulses) after A and B are triggered 

 and at a time determined by the length of the rec- 

 tangular pulse from stage B. The rectangular pulse 

 now generated in stage C is used to control the active 

 receiving time of the receiver modulator. 



The recurrence rate of this sequence can be set at 

 60, 30, 15, or 3 times per second by means of a selec- 

 tor switch. The pulse length of the multivibrators is 

 controlled by the time constants of the associated 

 resistance-capacitance [RC] circuits. Each stage has 

 two such controls. A calibrated smooth change of 

 resistance covers a time ratio of 20, and three fixed 

 condensers give three decades of pulse length. With 

 this arrangement it is possible to cover pulse dura- 

 tions from 0.1 to 30 milliseconds with overlapping 

 scales for the whole range. 



Transmitter Modulator. The transmitter modula- 

 tor is essentially a stage of push-pull amplification 

 with a cathode resistor which serves also as the cath- 

 ode resistor of a 6L6 tube. The voltage drop across 

 this resistor, due to the current drain of the 6L6, is 

 made sufficient to bias the amplifying lubes of the 

 push-pull stage beyond cutoff and thus keep them 

 from passing any signal. The surge from the genera- 

 tor is amplified, and the resulting large negative pulse 

 is applied to the grid of the 6L6, which stops con- 

 ducting and allows the push-pull amplification to act 

 normally for the pulse period. An output transformer 

 is used with this push-pull stage in order to eliminate 

 the d-c components owing to the amplifying tubes 

 passing from a nonconducting to a conducting state 

 and back again during the pulsing sequence. These 

 components may be observed on a cathode-ray oscillo- 

 scope when there is no signal frequency being ampli- 

 fied. They are balanced by adjusting the screen-grid 

 potentials of the amplifying tubes. An output trans- 

 former is used that has an essentially fiat frequency 

 characteristic from 1 lo 150 kc. 



The transmitter modulator has input and output 

 impedances of 135 ohms. It operates from a d-c B sup- 

 pi) of 275 volts and an a-c filament supply of 6.3 volts. 

 The gain of the unit is 10 db and the maximum un- 

 distorted power output is 145 db vs 10~ 16 watt. The 

 power output between pulses is more than 70 db 

 below the maximum undistorted pulse output. The 

 transients due to imperfect d-c balance are 50 db be- 

 low the same maximum. 



Receiver Modulator and Pulse Rectifier. The op- 

 eration of the modulator section of the receiver 

 modulator and pulse rectifier unit is very similar to 

 that of the transmitter modulator but the operational 

 characteristics are different. It has a high input im- 

 pedance designed to work with the amplifier of the 

 recorder circuit. It is capable of discriminating 

 against the highest signal output of the cathode-fol- 

 lower stage in the preceding amplifier. Hence, any 

 portion of the incoming signal may be selected with- 

 out interference from the rest of the signal. This se- 

 lection is controlled by adjustments on the pulse 

 generator. 



The conversion of the recurrent pulses from the 

 modulator into a d-c voltage suitable for operating 

 the power level recorder is not simple. This voltage 

 produced must satisfy two requirements: 



1. Its a-c component must be smaller than the 

 change in the d-c voltage inherent in the resolution 

 of the recorder. In other words, its magnitude will 

 determine the resolution obtainable without appre- 

 ciable instability, though the final limit is set by the 

 nature of the recorder circuit. 



2. It must be capable of changing about its equilib- 

 rium value at least as fast as the pen-drive motor can 

 change the level of the signal into the pulse rectifier. 

 If this condition is not met the recorder system will 

 hunt, though this may always be avoided by decreas- 

 ing the motor speed. 



The circuit producing the voltage which meets 

 these requirements is shown schematically in Figure 

 18. The operation is as follows: The receiver modu- 

 lator is adjusted to pass a short ptdse (0.3 to 0.6 milli- 

 second) of the acoustic signal to be measured. This 

 input to the pulse rectifier at point a and the re- 

 sultant rectified voltage at point b are indicated on 

 the drawing. The condenser C is made small (0.005 

 fil) in order that it may be charged to full value with- 

 in the duration of the pulse. The resistor R is chosen 

 so that 1/RC is approximately ecpial to the pulse 

 repetition frequency. This allows the condenser to 



