ELECTRICAL FILTER COMPRISING AN ACOUSTIC SURFACE WAVE DEVICE, AND METHOD OF MANUFACTURING SUCH A FILTER

摘要

ELECTRICAL FILTER COMPRISING AN ACOUSTIC SURFACE WAVE DEVICE AND METHOD OF MANUFACTURING SUCH A FILTER. In order to obtain greater flexibility in the design of a linear-phase electrical filter which is in the form of an acoustic surface wave device comprising 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 designed to be apodized differently but to have mutually complementary non-linear phase characteristics. In order to achieve this, first a transfer characteristic corresponding to the collective contribution to the overall gain versus frequency characteristic of the filter desired of the two transducers, is derived in terms of the z-transform and the roots or the reciprocals of the roots of the resulting polynomial in z-1 are found. These roots or reciprocals are then divided into two groups in such manner 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 respectively, and the two transducers are constructed with successive active electrode pairs the electrodes constituting 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 respectively. The active electrode pairs are connected with polarities which accord with the signs of the corresponding coefficients.