Ladder Type of Leaky Surface Acoustic Waves Filters on Substrate of Lithium Niobate

Abstract

High requirements on the electrical parameters of the filters are made in wireless radio communication systems. A number of tasks require bandpass filters with relative bandwidths of 8-12 %. In this case, the filter must have insertion losses of not more than 10 dB, have a rejection is not worse than 40 dB, unevenness in the bandwidth of not more than 1 dB. In addition, the amplitude frequency responses of the filter must have steep slopes due to the closely spaced frequency bands of neighboring communication systems. Due to its small size and other advantages, ladder resonator filters on surface acoustic waves are widely used in communication systems. To realize the high requirements of this type of filters, it is necessary not only to select all the topology parameters of the individual resonators included in the filter very accurately, but also to have a good computational theory and the necessary material parameters for the selected model at the design stage. Purpose: to show on the example of comparison of calculated and experimental frequency responses of ladder filters the validity of the method of extraction of the necessary parameters obtained for an infinite periodic structure by the finite element method for calculating of finite structures of real inter digital transducer and reflective gratings. Results: the method of extraction of parameters necessary for calculation by the method of connected modes on the basis of P-matrices is offered. The technique is based on the analysis of infinite periodic electrodes by the finite element method. Bandpass filters with a relative bandwidth 8-12 % on a piezosubstrate 49° YX LiNbO3 were designed and manufactured based on the proposed theory. It is shown that for this material, the calculation must take into account the direct radiation of bulk acoustic waves, since the design of ladder type filters, the radiation falls into the projected bandwidth of the filters. These calculations are confirmed by the results of experiments.