A Hybrid Film-Bulk-Acoustic-Resonator/Coupled-Line/Transmission-Line High Selectivity Wideband Bandpass FBAR Filter

A hybrid wideband filter composed of two sections with sharp frequency selectivity is proposed in this article. The first section consists of a coupled line (CL), two transmission lines (TL), and a film bulk acoustic resonator (FBAR) with lower frequency. This section mainly controls the lower roll-off. The other section is composed of two CLs and two FBARs with higher frequencies. This section mainly controls the roll-off of the upper band. The insertion loss and rejection outside the passband are well controlled because of the CLs. The analysis of the proposed filter is realized by combining microwave network theory with a modified Mason model for FBAR. Through analytical analysis, conditions for generating transmission zeros and poles are obtained. The methods of enhanced out-of-band rejection and bandwidth control are proposed through FBAR networks and tuning of FBARs, respectively. To verify the proposed theory, a wideband filter with a central frequency of 2.14 GHz is designed, simulated, and assembled. The FBARs, TLs, and CLs are assembled through a 1-mil wire bond technique. The measured results show good consistency with the simulation results; 24.9% of 3-dB relative bandwidth, 1.87-dB in-band insert loss, and an excellent 0.91 shape factor are obtained.