Constant In-Band Group-Delay Acoustic-Wave-Lumped-Element-Resonator-Based Bandpass Filters and Diplexers

The RF design of acoustic-wave (AW)-resonator-based bandpass filters (BPFs) with constant in-band group delay ( \tau -{g} ) and analog transfer-function reconfigurability is presented. The proposed filter concept is based on N identical acoustic-wave-lumped-element resonators (AWLRs) that are electromagnetically coupled through impedance inverters and result in quasi-elliptic-type transfer functions shaped by N poles and 2N transmission zeros (TZs). Unlike conventional ladder- or lattice-type AW-resonator filter configurations, they facilitate: 1) flat in-band \tau -{g} that does not depend on the electromechanical coupling coefficient ( k-{t}^{2} ) of its constituent AW resonators and does not require the incorporation of lossy elements; 2) passbands with fractional bandwidths (FBWs) that can exceed k-{t}^{2} ; and 3) continuous analog-type FBW tuning. The operating principles of the devised concept are presented through two different filter architectures that, respectively, present advantages in terms of size and FBW tuning and are subsequently extended to the design of RF diplexers. They are experimentally verified through the following prototypes: 1) a three-pole/six-TZ BPF that is centered at 418 MHz and exhibits bandwidth (BW) of 0.3 MHz, minimum in-band insertion loss of 2.1 dB (i.e., effective quality factor Q-{\mathrm {eff}} of 9000), and in-band \tau -{g} between 1.78 \pm 0.02~ \mu \text{s} ; 2) a two-pole/four-TZ BPF with 2.4:1 BW tuning ratio, flat \tau -{g} , and Q-{\mathrm {eff}}> 9000 ; and 3) a flat \tau -{g} diplexer with two transmission bands centered at 418 and 433.9 MHz.