Preprocessing of Analog-Filter Outputs for Computer-Aided Data Acquisition

Abstract

Experimental analog circuits for squaring, integrating, and taking the square root of an analog filter output have been evaluated with particular emphasis on the problems encountered with high crest-factor signals. The true root mean square value of signals having crest factors as high as 10:1 can be accurately determined, but higher (30:1) crest factors present problems because of d-c offset at the integrator input. The problem of crest factor variation with filter bandwidth can be handled by feedback from the computer used to acquire the data. The analog system is compared with a simulated digital system that samples the filter output at random time intervals, squares the amplitude, and accumulates the results. The digital approach appears to have several advantages provided that the number of samples taken is chosen to be large enough to obtain a statistically stable spectral estimate.