Summary of Progress on Evaluation of Frequency Modulated 

 System for Measuring Water Temperature, 

 Salinity, Depth, Transparency, etc. 



Instrumentation Division 



In the investigation of a device for converting a temperature change 

 to a frequency change, one of the most promising designs studied 

 (judging from technical reports) was that used in the Scripps telere- 

 cording BT. This unit utilized a Wien bridge type of oscillator built 

 with two subminiature vacuum tubes and one transistor on the output 

 stage. Thermistors were used as the sensing elements. 



This circuit was breadboarded in the Instrumentation Division, and 

 stability tests on the device showed a drift of 2 or 3 c.p.s. at room 

 temperature at an oscillation frequency of about 1,000 c.p.s. In the 

 attempt to improve on this stability another circuit was breadboarded. 

 This circuit included a transistorized vibrating wire transducer ampli- 

 fier made into an oscillator and showed approximately the same 

 stability. However, during an experiment with an improved volume 

 control (the insertion of another transistor to increase the feedback) 

 a marked improvement in the frequency stability accompanied the 

 improved amplitude stability. This circuit operated for a few days 

 and drifted only 1/10 c.p.s. during this period. When the circuit was 

 subjected to an environmental change, from room temperature to 

 freezing, a shift of about 5 c.p.s. occurred. A thermistor was added 

 to the circuit for temperature compensation, and this shift was reduced 

 to 1 c.p.s. over the same temperature change. 



Another stage of amplification was added to increase the output 

 signal and provide for cable loading. Tests showed that load changes 

 on this amplifier resulted in frequency shifts. For this reason a buffer 

 stage was added ahead of this last amplification stage to isolate the 

 oscillator from the reflected load changes. 



The latest model, which is being readied for sea tests, oscillates at 

 about 6,000 c.p.s. and has a sensitivity of about 130 cycles per degree 

 of temperature. The drift is about one part in 1,000 over an eight-hour 

 period. 



Further development is now in progress on a second unit which 

 will include another vibrating wire transducer for pressure measure- 

 ment in terms of frequency. Also under study is a circuit that will 



Encl. (1) to Code 5401-BEO/bah memo of 23 Jul 1958 



B-5 



