MEASUREMENT OF PRESSURES 



185 



nal signal and the properties of the entire circuit, followed by a series of 

 signals resulting from multiple reflections and traversals of the cable. 

 These reflections are eliminated if the cable is terminated in a value of 

 impedance characteristic of the cable and equal, for cables with negli- 

 gible series resistance and leakage conductance, to the surge impedance 

 U^ = Vl/C. For practical cables, this surge impedance is a resistance 

 of the order 50 to 100 ohms. It is evident that this requirement for no 

 distortion by reflections is not directly compatible with the necessity of 



000 



FREQUENCY (KC) 



Fig. 5.16 Simulated frequency response of a piezoelectric gauge and unter- 



minated cable. 



high d-c resistance of the order of megohms across the piezoelectric 

 gauge. 



The type of behavior when a piezoelectric gauge is connected to a 

 long cable without any termination at the receiving end is shown in 

 Fig. 5.16. The cable used was 660 feet long with a capacity of 17 X 10"^ 

 ixf at a frequency of 10 kc. The characteristics of the gauge were simu- 

 lated by an oscillator as source of constant voltage (20 volts) connected 

 in series with a condenser of 100 mm/ representing the capacity of a gauge, 

 and the output voltage was measured by a vacuum tube voltmeter. It 

 is evident that for frequencies below about 20 kc. the output is inde- 

 pendent of frequency and it has a value determined by the capacity of 

 the cables. At higher frequencies, however, the response has a series of 

 voltage maxima, characteristic of open-circuit resonances, for which the 



