A Hybrid Low-Cost Bandpass Filter With SAW Resonators and External Lumped Inductors Using a Dual-Coupling Scheme

A class of hybrid low-cost bandpass filter (BPF) with only two kinds of circuit components, straightforward design procedure, and miniaturized circuit footprint is presented. The proposed BPF is based on a hybrid module composed of one type of surface-acoustic-wave (SAW) resonator and one external lumped inductor. Compared with traditional SAW-resonator-based BPF devices, the use of a single-SAW-resonator model for its design reduces the cost and complexity by avoiding the need for well-matched SAW-resonator pairs. The engineered filter topology consists of a dual-path coupling scheme, in which one coupling path is simply a series-type SAW resonator and another path is made up of two SAW resonators that are intercoupled by means of an external inductor in the middle. Such coupling architecture generates two additional transmission zeros (TZs) that allow to increase the out-of-band power-rejection levels and enlarge the stopband bandwidth. A circuit analysis based on the even-/odd-mode method is applied to derive its equivalent circuit that can be directly matched to a conventional transversal filter topology. The obtained equivalent circuit readily illustrates the operational mechanisms of the proposed filter principle in terms of reflection zeros and TZs. Subsequently, the quality factors of its even- and odd-mode-subnetwork resonators are analytically derived, and the obtained numerical results determine the tradeoff existing when choosing the external-inductor value among various performance metrics. For experimental validation purposes, two 418-MHz BPF prototypes formed by one and two in-series-cascade-connected second-order filtering units, respectively, are manufactured and tested. The circuit footprints of these filters are largely reduced by using a three-layer lamination technique for their physical implementations. The measured results show sharp-rejection filtering responses and are in close agreement with the theoretical predictions.