Low loss recursive filters for basestation applications without spurious modes

Abstract

In SAW filters spurious waveguide modes may seriously affect the performance: Undesired lobes in the upper pass- or stopband and ripples in the group delay are typical for devices suffering from waveguiding effects. In order to suppress spurious modes and to excite only the fundamental symmetric mode various approaches as overlap weighting or sawtooth shaping of the busbars in order to destroy the waveguide have been applied. Alternatively recently it was suggested to employ the first antisymmetric mode by splitting the acoustic track into two subtracks, which are longitudinally shifted by half a wavelength with respect to each other. The very reason of the excitation of higher modes is, however, the fact that the excitation profile is of different shape than the fundamental symmetric or antisymmetric mode. In most cases the excitation profile is of rectangular shape while the waveguide modes usually are of sinusoidal shape. While the approaches of prior arts aimed at changing the excitation profile to maximize excitation of the fundamental symmetric or antisymmetric mode we suggest an approach where the waveguide is changed in order to shape the fundamental symmetric mode rectangularly. The approach is applied to low loss recursive inline filters which are employed as intermediate frequency filters in GSM infrastructure systems. The method proves to be particulary successful in reducing group delay ripple and sidelobes. I. INTRODUCTION Spurious modes are a serious problem in SAW filters. They may lead to ripples in the passband, distortions of the group delay time and reduced out of band suppression.