Methods Of Compensation For Diffraction Effects In Surface Acoustic Wave Filters

Surface acoustic-wave filter (SAWF) is widely used in television intermediate frequency (TV-IF) range, mobile communications and some other fields. With the development of stereophonic television and high-definition television, an improvement in filter performance becomes necessary. In order to design a high-performance SAWF, some second order effects such as diffraction must be compensated strictly. In this thesis, the compensation of diffraction effects has been investigated in detail, and a software package for SAWF design has been developed in this paper.The basic principles of interdigital transducer and SAW filter are introduced at first in the thesis. Then the designing method of a basic SAW filter is discussed. An example of designing a SAW-TV-IF filter is intrduced using the Remez exchange algorithm, with a center frequence at 37.4 MHz. The diffraction effect in SAW filters is described and analysed. The frequency response with diffraction is simulated. The relationship between the inverse discrete fourier transform (IDFT) of sampling frequency response of SAWF and the taps of apodized IDT is analyzed. The corrections to the taps of the apodized transducer are computed iteratively from the deviations of the diffracted frequency response from the desired response, so that the diffracted frequency response is close to the desired response. The simulation results show that this method is successfully applied to remove the deviations caused by diffraction effects.