Switched-bandwidth SAW-based bandpass filters with flat group delay

Tunable surface-acoustic-wave-(SAW)-based bandpass filters (BPFs) with flat in-band group delay (τ_g) and variable passband bandwidth (BW) are reported. They are based on N hybrid acoustic-wave-lumped-element-resonator (AWLR) modules shaped by one lumped-element resonator and K RF-switched SAW resonators that are arranged in a Gaussian-type source-to-load impedance-inverter network. Fractional bandwidth (FBW) tuning is achieved by reconfiguring the number of SAW resonators within the AWLR modules. Major advantages of the proposed filter concept when compared to conventional SAW BPFs are as follows: (i) they do not depend on the electromechanical coupling coefficient (k_t²) of the SAW resonators – FBW>k_t² can be realised –, (ii) they do not require lossy elements or SAW resonators with different frequencies, and (iii) they can be tuned. For experimental-validation purposes, a 433.9 MHz two-pole/four-transmission-zero prototype was built and measured. It exhibited flat-in-band-τ_g passbands with discretely-tunable BW between 0.18 and 0.45 MHz (i.e. 2.5:1 tuning ratio).