186 MEASUREMENT OF PRESSURES 



reflected signals reinforce each other. (These maxima would be infinite 

 were it not for losses in the cable. It is also to be noted that the abso- 

 lute value of response is plotted without regard to phase.) At these 

 higher frequencies, therefore, the output signal bears no simple relation 

 to the applied voltage. 



In transient pressure measurements one is more interested in the 

 response to a changing transient voltage, and the sketch in Fig. 5.17 

 illustrates the response observed when a fixed potential is instantane- 



Fig. 5.17 Observed transient response of piezoelectric gauge and untermi- 



nated cable. 



ously applied to the same network. The series of oscillations corre- 

 spond to the arrival of successive reflections at intervals of 2RoC (ap- 

 proximately 1.5 /xsec. for the 660 foot cable already considered) and the 

 damping is the result of cable and other circuit losses. These oscilla- 

 tions will of course ol)scure any variations of the applied signal in com- 

 parable time intervals, and if such variations are to be observed, a 

 compromise Ijetween the condition for elimination of reflection and the 

 condition for negligible circuit leakage at long times must be sought. 

 This compromise is achieved by the use of terminating networks at the 

 ends of the cable which, roughly speaking, are equivalent to capacities 

 for slowly changing signals and approximate the surge impedance char- 

 acteristics desired for rapidlv changing signals. 



A sufficiently general network, similar to networks discussed by 

 Burch (13), to provide adecjuate compensation is indicated schemati- 

 cally in Fig. 5.18. In this network, the source of voltage Vo and the 

 capacitance Co represent the piezoelectiic gauge. The coaxial cable 

 can, to a sufficient approximation, be regarded as loss free and hence 



