Several breadboard models of Wien-bridge oscillators were con- 

 structed; however, all of them were found to be very sensitive to 

 power-supply fluctuations. Batteries used in this instrument should 

 have an end-point voltage about 20-percent down, therefore, such a 

 degree of sensitivity on the part of the oscillator would make the 

 circuits unusable during a power fluctuation. Frequency instabili- 

 ties of several kilocycles were noted where both plate and heater 

 supplies were varied, and the errors compounded themselves. 



A new oscillator design, extremely stable with regard to power- 

 supply fluctuations, was finally evolved. In this design, the proper 

 use of negative feedback and the use of a variable-mu pentode for 

 amplitude control practically eliminated frequency shift resulting 

 from supply-voltage changes. 3y using heater-type tubes (6SK7-6V6), 

 it was possible to maintain frequency (within 20 c.p.s.) even though 

 plate-supply voltage was more than doubled. In addition, amplitude 

 and frequency were constant for more than 10 seconds after the heater 

 current was interrupted; after 10 seconds, the oscillations ceased 

 abruptly. 



Although it was necessary to redesign this oscillator for operation 

 with miniature battery-type tubes, stability was sacrificed to only a 

 very slight degree. It was necessary to provide a self -biased resis- 

 tive cathode for the second amplifier tube, which, when using battery 

 tubes, required an isolated and separate battery. Because of current 

 drawn by the rectifier in the automatic-gain -control (a.g.c.) circuit, 

 the output waveform for this particular oscillator is not so good as 

 that obtainable with a conventional T :, r ien-bridge oscillator. Howsver, 

 a few-percent distortion in this service is entirely negligible. 



The depth-frequency characteristics of the completed circuit will 

 be a function of the degree of linearity of the pressure potentio- 

 meters. The Wien-bridge oscillator has an operating frequency that 

 is about equal to l/2lfRQ when R and C are the resistance and capa- 

 citance of the component parts of the reactive arms of the bridge. 

 Variation of frequency is linear with respect to changes in R or C. 

 Rough measurements of linearity of the oscillator indicate a value 

 of more than 3 percent over the ant ire range. 



The operation of the telemeter may be described as follows: 



In referring to the schematic diagram (fig. 2), tubes V- and 

 Vp (type 1T4 and 3V4, respectively) are used as the depth oscillator, 

 described above. They are located on the same chassis as Vo and V, 

 and comprise a necessary two-stage amplifier of the R-C oscillator. 

 The phase- shifting network is mounted on onr> end-cap of the instrument 

 and consists of the two pressure potentiometers, three resistors, and 

 three capacitors. One of these resistors, R]i , and one capacitor, 

 Cg, are adjustable and function as high-frequency and low- frequency 



