MAWJEPS raiPORT 76^0 



include amplitude stabilization. Stabilization is achieved by Including 

 a tungsten filament light bulb as a part of resistor R^. Thus when E]_ 

 increases, R^ also increases, and Ap is kept more nearly constant. 



To operate the oscillator over a temperature range from to 30°C, 

 R2 must have a small positive temperature coefficient of resistance to 

 maintain aiitplitude stability. The value of this resistance and its 

 coefficient are determined by substituting a resistance-substitution 

 box (0.1^, if available) for Rg and adjusting it for oscillation and 

 sinusoidal wave form. The temperature coefficient of resistance is 

 found by temperature cycling the oscillator and noting the change in 

 resistance over the range 0-30°C. Enough positive temperature coef- 

 ficient wire such as Balco (Wilbvir Driver Co., Newark, N. J.) is wound 

 noninductively on two l/lO-watt, 1^ carbon-deposited resistors to over- 

 come the negative temperature coefficient of these resistors and supply 

 the extra positive coefficient needed to stabilize the amplitude. 



To obtain frequency stability of the oscillator with temperature, 

 capacitance C]_ must remain constant. Stability is achieved by paral- 

 leling capacitors that have a positive temperature coefficient with 

 capacitors having a negative coefficient. The combination used has 

 enabled the oscillator to be temperature cycled from 0-30°C with a 

 frequency change of less than 1 cps (z^cps - A0.01°C). 



Temperature stability is fiirther improved by use of a differential 

 amplifier, and by temperature cycling from -60 to 60°C for 8 hours 

 before Rg is determined. The cycling "ages" the components, and 

 minimizes changes with time in their characteristics . 



To avoid phase shifts through the amplifier other than in multiples 

 of 2n, large coupling capacitors are used. 



Aged thermistors aire used for the temperature sensing elements . The 

 resistance curve of a thermistor with temperature approximates the re- 

 sistance curve of a Wien-bridge oscillator with frequency. The thermistor 

 curve can be matched to the oscillator curve with a three-point match to 

 achieve maximum linearity with the following network: 



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