MWffiPS REPORT 7630 



blocking oscillator that is connected to a sending transducer. The 

 resulting sound pulse is reflected twice to reduce errors due to water 

 motion, and then picked up on a receiving transducer that is the input 

 for a high-gain pulse-shaping amplifier (Fig. k) . The amplifier 

 retriggers the blocking oscillator, and a repetition frequency results, 

 which is higher than the free-running rate. Thus the water path acts 

 as the delay line where the variation in sound velocity through the 

 water changes the delay and hence the "sing-around" frequency. The 

 frequency also depends on the path length and the circuit delays . The 

 configuration of the path length makes it impossible to measure this 

 length to any desired degree of accuracy. Also, because of selective 

 attenuation the received pulse rises slowly in comparison with the 

 sent pulse; hence, an unknown time delay is introduced during which the 

 received pulse is below the noise level. 



Consequently, the instrument must be calibrated in a liquid for 

 which the velocity of sound is known accurately, and the liquid must be 

 similar to that in which the instrument will be used. Thus, if the 

 instrument is to be used in sea water, it may be calibrated in distilled 

 water in which the sound velocity is known as a function of temperatiore . 

 Two recent determinations of the velocity of sound in distilled water 

 have been published by Greenspan-Tschiegg and Wilson (Ref. 7 and 8). 

 Both determinations used similar methods. However, over the temperature 

 range of 0-50°C, there is a difference of 0.26 m/sec in spite of claims 

 of 0.05 m/sec and 0.095 m/sec maximum errors. 



The free-running frequency of the blocking oscillator is about 

 5 kilocycles. A "sing-around" frequency of about 6 kilocycles is 

 achieved by increasing the path length to 2k. J cm. Negative pulses are 

 developed across a 12-ohra resistor in the emitter lead of the blocking 

 oscillator to switch an Eccles-Jordan circuit. This circuit produces 

 square waves and divides the frequency in half (5 kilocycles). This 

 frequency (5 kilocycles) is in the center of Telemetry Band 8. A small 

 Band 8 low-pass filter is used to transmit a sine-wave output. 



The sound- velocity circuit may shift in frequency due to the input 

 triggering on a precursor. To prevent shifting, MBS has found that the 

 sending crystal should be polarized with the inner electrode made 

 negative, and the receiving crystal should be reversed with the inner 

 or small electrode made positive. Also adjustment of the reflectors is 

 made with the transmitters disconnected from the rest of the circuit, 

 and the output of the amplifier is observed on an oscilloscope. This 

 procedure is repeated until it can be seen that there is enough signal 

 to saturate the amplifier and yet enough attenuation to remove unwanted 

 reflections. The first signal must be clean with no precursor. 



