

> 

 z 



uj 



o 



8 12 



PRESSURE ( psi x 1000 ) 



Fig. 7- Series oscillator frequency vs. pressure. 



frequency shift per 1,000 psi with a frequency 

 of 9° Kcps at atmospheric pressure was observed. 

 The deviation from the best straight line was 

 +50 cps on the average. The minor shifts were 

 due to the natural instability of the circuit 

 under pressure. 



The data from the tests of the hybrid oscil- 

 lator configuration are shown in Fig. 8. The 

 frequency variation with pressure was plotted at 

 various atmospheric pressure frequencies, f , 

 ranging from 10 Kcps to 1 Mcps . The shift in 

 frequency per 1,000 psi increased as the atmos- 

 pheric pressure frequency was increased. An 

 output of 20 cps/psi was obtainable when f was 

 1.8 Mcps; however, the accuracy is better at the 

 low frequencies. In the region from to 1,000 

 psi a larger shift in frequency was noted, indi- 

 cating the I-E curve did not vary linearly. 

 However, compensation can be used to correct for 

 this nonlinearity The average shift in fre- 

 quency per 1,000 psi plotted against f Q for a 

 20,000 psi change in pressure is shown in Fig. 9 

 for the same configuration. The shift in fre- 

 quency is 15$ or better, depending on f . 



For a 200,000 cps shift in frequency from 

 1 Mcps when 20,000 psi is applied, the average 

 shift is 10 Kcps per 1,000 psi representing 20$. 

 In order to obtain the desired accuracy we must 

 measure the frequency to at least 1 Kcps . 



(b.) 



Fig. 8. Hybrid oscillator curves. 



Fig. 9- Frequency shift vs. frequency 

 at atmospheric pressure. 



However, much better resolution can be accom- 

 plished. 



Common mode rejection can be accomplished by 

 utilizing 2 hybrid tunnel diode oscillators and 

 beating the two frequencies as shown in Fig. 10. 

 The difference frequency output will result 

 because one tunnel diode is exposed to pressure 

 but the other is not, although they are in the 

 same environment . The output frequency is then 

 gated into an accumulator whose output collectors 

 indicate the stored count in binary. The dif- 

 ference frequency, being less than fj_ or f o in 

 Fig. 10, requires less accumulator capacity. A 

 serial or parallel readout can be accomplished 

 with suitable circuitry. The voltage and tempera- 

 ture problems are now common to both circuits and 

 effectively cancel out. Greater accuracy may be 

 obtained using this technique. 



The test circuitry did indicate that there is 

 some apparent hysteresis . However, the overall 

 performance characteristics indicate that this is 

 not a serious problem because the hysteresis is 

 well within the accuracy limits. Zero reference 

 shift for the single oscillator configuration is 

 due primarily to bias voltage changes and tempera- 

 ture. Common mode rejection would solve this 

 problem. 



193 



