Design Of Ladder Type Saw Filters

With the development of communication, navigation, radar, mobile communi -cation, programmed telephone and measured instrument devices, as the key devices of these communication systems, the higher performance of surface acoustic wave filters(SAWF) are needed, including low insert loss, high stopband attenuation, low passband fluctuation, low shape factor and so on.In this thesis, the basic concept of SAW, the working principles and design methods of SAWF as well as typical LCR equivalent circuit models are introduced first. Then an improved model is presented. Compared with the typical one, new equivalent circuit model focuses on the importance of quality factor and ratio of capacitance and is easier for circuit parameter optimizing. One modeling method based on ADS linear array model is proposed and the correctness is validated corresponds to the typical LCR modeling method.All ladder type SAW filters satisfy the specifications after optimization, but for bandpass filter, the phenomenon of degradation in the lower passband edge of the filter appears. When unity filters are cascaded into a ladder-type filter, two kinds of circuits (T-type andΠ-type) are obtained. By investigating reactance characteristic, the problem is solved by the method of using one resonator or more as matching circuit to reduce the inter-stage impedance mismatching.