A Simple Electronic Correlator

Abstract

Attention has been drawn in the literature to the possible utility of correlation techniques in the reception of acoustic signals. In applying such techniques, it is often necessary to find the average of the instantaneous product of two electrical signals; the black box which performs this function, which is that of a generalized detector, may be called a correlator. By use of the quarter-squares method of multiplication [AB = 14(A + B)2 − 14(A − B)2], correlators can be constructed using common triodes as the square-law devices. In practice, however, such multiplier-averagers depend strongly upon the vacuum tube characteristics which in turn require, for stability, regulated plate and heater supplies and frequent balance adjustments. If the square-law vacuum tubes in the circuit mentioned above are replaced by linear rectifiers, as many have suggested, it is shown that the resulting circuit is similar in action to a correlator. The output of this “linear rectifier correlator” is a somewhat distorted replica of the un-normalized cross-correlation function of the input signals, but the output signal-to-noise ratio for small input signal-to-noise ratios is not appreciably less than that for a multiplier-averager. The linear rectifier correlator has the practical advantage of being completely independent of power supplies, if contact rectifiers are used, and requires no balance adjustments if care is used in its construction.