Acoustic surface wave filter device with asymmetric transducers and

Abstract

In order to obtain greater flexibility in the design of a linear-phase electrical filter comprising an acoustic surface wave device including a pair of apodized transducers intercoupled by means of a multi-strip coupler, for example to obtain more equal contributions by the two transducers to the gain versus frequency characteristic of the filter, the transducers are apodized differently but have mutually complementary non-linear phase characteristics. First a transfer characteristic corresponding to the collective contribution to the desired overall gain versus frequency characteristic of the filter, is derived in terms of the z-transform and the roots or the reciprocals of the roots of the resulting polynomial in z.sup.-1 are found. These roots or reciprocals are then divided into two groups so that at least one pair of non-identical roots or reciprocals which have equal ratios between their real and imaginary parts and the product of the moduli of which is equal to unity are included one in each group. Two further polynomials are then found which have as their roots or the reciprocals of their roots those of the first group and those of the second group. The two transducers include successive active electrode pairs which have relative weights proportional to the coefficients of the successively increasing powers of the variable in one further polynomial and the variable in the other further polynomial. The active electrode pairs are connected with polarities in accordance with the signs of the corresponding coefficients.