Low loss ladder type SAW filter in the range of 300 to 400 MHz

Abstract

Low loss SAW filters in the range of 300 to 400 MHz for communication systems have been developed, which have ladder circuit structure using 36° rotated Y-cut LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . This type of filter has low loss and high stop band rejection, however, a spurious mode appears, which depends on electrode thickness and a reflector's electrode pitch of a resonator. Low frequency range filters need thick Al electrodes more than 7% of wave length to shift the resonant frequency of spurious mode to a frequency range higher than the pass-band frequency range. To reduce the spurious response, the design of an electrode pattern and electrode material having more mass effect than Al were investigated by simulation and experiment. The spurious response can be suppressed by optimizing a reflector's electrode pitch. By using Au film, thin film of about 1.5% of wave length can be applied to a filter. SAW filters in the range of 300 to 400 MHz with fractional band width from 2 to 4% were manufactured and mounted in SMD type 5×7×1.4 mm ceramic packages, which have 2 dB minimum insertion loss and attenuation characteristics of about 40 dB. Input and output impedance of filter are 50 Ω