r 



STAFF EXCITATION 



r 



SIGNAL CONDITIONING 



~1 



IKC CONSTANT 



VOLTAGE 



OSCILLATOR 



CONSTANT 



CURRENT 



AMPLIFIER 



POWER 

 SUPPLY 



AC - DC 



CONVERTER 



LOW PASS 

 FILTER 



OUTPUT 



POWER 

 SUPPLY 



STAFF 



WATER LEVEL 



Fig. 5- Block diagram of wave and tide monitor. 



STAFF EXCITATION 



Twin "T" Oscillator 



The basic oscillator consists of an amplifier 

 with approximately ^0 db raw gain with a selec- 

 tive negative feedback loop and a positive feed- 

 back loop. The twin T network in the negative 

 feedback loop results in a peaking amplifier, 

 the frequency of peaking being determined by the 

 twin T network. Stable components have been 

 employed in the network to insure peaking sta- 

 bility with temperature and aging processes. 

 The gain of the amplifier at the peaking fre- 

 quency approaches its raw gain of k-0 db. There- 

 fore, care has been taken in the design of the 

 amplifier with local feedback in the gain stages 

 to insure good stability of the kO db raw gain. 

 The positive feedback loop adjusts the gain 

 around its loop to unity at the frequency deter- 

 mined by the twin T network. 



Incorporated in the positive feedback loop is 

 a temperature compensated automatic gain control 

 circuit. Without applying this technique, the 

 oscillator would have to be severely overdriven 

 to maintain oscillation over the operating con- 

 ditions encountered. For the application at 

 hand the oscillator has to meet a rigid ampli- 

 tude stability requirement, better than i0.1$>, 

 while maintaining an overall distortion of below 



0.57°- Since an overdriven oscillator cannot 

 meet either one of these requirements, especially 

 the latter, the use of an automatic gain control 

 is mandatory. 



The block diagram for the oscillator (Fig. 6) 

 will be used to explain the operation of the 

 automatic gain control circuit. The output, E Q , 

 is rectified, filtered and applied to the control 

 diode, CR1, by means of resistor B g . Since the 

 DC bias current through a diode also determines 

 its AC impedance the magnitude of the feedback 



AA/V 



R, 



CR I V 



-AAAn 



AUTOMATIC 



GAIN CONTROL 



CIRCUIT 



Fig. 6. Twin T oscillator 



96 



