Ladder-lattice bulk acoustic wave filters: Concepts, design, and implementation

Abstract

This article presents a study of ladder-lattice bulk acoustic wave (BAW) filters. First, a review of BAW technology and filters topologies is addressed. Next, a mixed ladderlattice BAW filter for application on W-CDMA reception front-ends (2.11-2.17 GHz) is presented. An improved solidly mounted resonators (SMR) technology was used for the filter implementation. The filter synthesis methodology is briefly described. Layout guidelines are discussed enabling an optimized filter design. The filter on-wafer measurement results are as follows: 23.55 dB of insertion loss, 28.7 dB of return loss, an isolation higher than 247 dB at the transmission band (1.92-1.98 GHz) and an improved selectivity (230 dB at 2.14 GHz 6 60 MHz). Therefore, we can observe that the mixed topology combines the advantages of ladder and lattice networks, having very steep responses and an improved isolation at undesired bands.

Techniques for Tuning BAW-SMR Resonators for the 4th Generation of Mobile Communications

“…The resonance condition is established when the acoustical path (in thickness direction) corresponds to odd integer multiples of the half acoustic wavelength. The bulk acoustic wave resonator is basically constituted by a piezoelectric layer sandwiched between two electrodes ( Fig.1) [2,3].…”

“…Two configurations are proposed: the membrane suspended structure (FBAR -Film Bulk Acoustic wave Resonator) [3], where the resonator is suspended by an air-bridge (Fig.1a); and the solidlymounted structure (SMR -Solidly Mounted Resonator), where the resonator is mounted over a stack of alternating materials (Fig.1b). This stack is built on a Bragg reflector basis and it has an acoustic mirror behavior [2][3]. Both, air and acoustic mirror, present an optimum discontinuity for reflecting the acoustic waves at the interface with the bottom electrode, confining waves into the main resonant structure.…”

“…Ladder filter are characterized by an unbalanced operation mode and very small size able to deliver very high selectivity filtering responses, however presenting low rejection or isolation out-of-band. Typically, ladder bulk acoustic wave filters are quite effective for blocking signals close to the passband, but poor at rejecting undesired bands [3][4].…”

“…They present slower roll off coefficient and higher rejection. Thus, lattice networks are not interesting to block signals close to the passband, but they are more effective for rejecting undesired bands [3].…”

“…Ladder-lattice filters ally advantages of both network types, enabling high isolation at undesired bands and steep responses close to the passband [3]. This topology is able to strongly reduce the linearity constraints of the receiver RF chain.…”