ELECTRONIC SYSTEM 



The electronic apparatus is required to transform the changes in capacitance which 

 appear in the gaging element into proportional current or voltage variations to drive the recor- 

 der. The equipment as designed provides a sensitivity range of 0.60 to 25.0 in. (trough to 

 crest wave amplitude) for full-scale (30-mm) recorder deflection. A precision attenuator is 

 built in which allows selection of 10 fixed sensitivity positions— 0.60, 1.0, 1.5, 2.5, 4.0, 6.0, 

 10, 15, 25, and 40-in. peak-to-peak amplitude for full-scale recorder deflection. Calibration 

 is accomplished by switching the calibrating condensors which are chosen by the setting of 

 the attenuator first to one side of the bridge and then to the other. This simulates equal in- 

 cremental variations in water level about the zero reference point, i.e., the water level at 

 which the bridge was initially balanced by the operator. 



The circuits employed are a modification and adaptation of those em.ployed in the reso- 

 nant-bridge carrier system which has been used for some time for the measurement of extreme- 

 ly small changes in capacitance. The system consists of a doubly resonant capacitance 

 bridge which is driven by the output of a 10-kc oscillator-amplifier circuit, an attenuator, a 

 2-stage voltage amplifier and a dual demodulator, the output of which is a d-c voltage whose 

 magnitude and polarity are proportional to height of the water surface from any arbitrary zero 

 point chosen by the operator (usually, but not necessarily, the quiescent water level in the 

 absence of waves). A block diagram of the complete system is shown in Figure 3. The 10-kc 

 oscillator-amplifier circuit (block 1) supplies the excitation to the resonant bridge (block 2) 

 which incorporates provision for manually balancing out the resistive and reactive components 



10 kc 

 Oscillator and 

 Power Amplifier 



Gaging 

 Element 



Resonant 



Copacitive 



Bridge 



Voltoge 

 Amplifier 



Null 

 Detector 



Demodulator 



Sanborn 

 Recorder 



Direct 

 Coupled 

 Amplifier 



3 2 4 7 8 



Figure 3 - Block Diagram of the Type 145-A Dynamic Wave-Height Recording System 



