Wideband Bandpass Filters With SAW-Filter-Like Selectivity Using Chip SAW Resonators

Abstract

Bandpass filters with surface acoustic wave (SAW)-filter-like selectivity and much wider bandwidth compared to conventional SAW filters are presented. The proposed topology consists of SAW resonator networks and parallel coupled lines. Each ladder type resonator network has a series resonator and two shunt resonators to define the upper and lower cutoff frequencies and realize the sharp passband selectivity. Analysis of the proposed SAW resonator network and a comparison with a traditional ladder type SAW filter is provided. Two filters are designed, fabricated, and measured using chip SAW resonators and microstrip coupled lines on Rogers RO4003C. As the available SAW resonators are discrete, they are connected together using 1-mil bond wires to form the SAW resonator network. The resonators are also wire bonded to the coupled lines and ground patch. The bandwidth of the two filters is 10.5% and 17%, respectively, which is much wider than conventional SAW filters with a bandwidth of a few percent. An analysis is provided for the bulk wave effects, ripples due to finite transducer, gratings of resonators, and second-order effects from series and parallel resonators, which cause deterioration of the passband performance. The impact of bond wire inductance and parasitic coupling, which cause spurious responses at the stopbands, are also discussed.