To reference all the data collected with the 600-kHz system to a common 

 level, an accurate calibration of the pre-amplifier must be obtained. This was 

 done using a transmit hydrophone (Model E27) from the Naval Research 

 Laboratory. The hydrophone transmits signals of known power, and the 

 output from the 600-kHz system was recorded for different pre-amplifier gain 

 settings. The gain was set to a value between 28 and 115, and the value 

 determines the driving voltage for the amplifier. Limits on gain settings are 

 determined by the power requirements and saturation limits of the circuit. 

 Figure 8-5 shows the calibration curve for the pre-amplifier. Using this 

 curve, all the 600-kHz data can be referenced to a common level even if the 

 data were collected at different pre-amplifier settings. The curve shows strong 

 changes in gain at lower settings and levels off as the setting approaches satu- 

 ration. 



Pr o - amplifier response - 6 00 kHz 





o 

 























w *° 



c 

 ■A 



9 





















• 



fk 





















C 



8 



c l0 





















c 



s 





















1 



- 





r 



re - amp 



Lifier 



aettlns 



1 



1 







Figure 8-5. Calibration of 600 kHz pre-amplifier 



In addition to the pre-amplifier, the sensitivity of the A/D board can be 

 varied. For this project, three different settings were used, and sensitivity is 

 shown in Table 8-1. All tests indicate that the A/D board has a linear 

 response over the range of signals used. 



Table 8-1 



8-bit A/D Converter Sensitivity 



A/D Gain Setting 



Input Voltage Sensitivity 







1 mV/bit 



1 



2 mV/bit 



2 



5 mV/bit 



Chapter 8 ADCP Measurements at SUPERTANK 



143 



