Improvement of ladder-type SAW filter characteristics by reduction of inter-stage mismatching loss

Since the ladder-type SAW filter has become a very important key device in the mobile phone system, a lot of new design techniques have been actively proposed and developed, and progress in the amelioration of the filter characteristics has been made. The purpose of this paper is to clarify the cause of the characteristic degradation in the lower side of the filter passband and to solve the problem. By a careful impedance analysis of each stage of the filter, it is found that the degradation is due to inter-stage impedance mismatching which is usually neglected in design. Therefore, the authors propose a practical design of the filter circuit to reduce this impedance mismatch which contributes greatly to the improvement of the filter characteristics. In the analysis, the authors used the 800 MHz band ladder-type SAW filter fabricated on 36° YX-LiTaO₃ piezoelectric substrate to show the existence of inter-stage impedance mismatching and the reason for the new filter circuit proposition. In experiments, the application of this new theory reduced effectively the insertion loss of the filter from 1.9 dB to 1.6 dB and increased the attenuation level of the upper stopband from 30.0 dB to 35.0 dB