SAW-based bandpass filters with flat in-band group delay and enhanced fractional bandwidth

This paper focuses on the design of surface-acoustic-wave-(SAW)-based bandpass filters (BPFs) with enhanced fractional bandwidth (FBW) and constant in-band group delay (τ_g). They are based on a hybrid RF-design approach in which high-quality-factor (Q) SAW resonators are effectively combined with low-Q lumped elements (LEs). Contrary to conventional ladder-or lattice-type filter architectures, the devised configuration enables passbands that can simultaneously achieve: i) flat in-band τ_g, ii) FBWs that are larger than the electromechanical coupling coefficient (kt²) of its constituent substrate, and iii) Qs of the order of a thousand. In order to validate the proposed filter design approach, a two-pole/four-transmission-zero (TZ) prototype has been designed, manufactured, and measured. It exhibited passband bandwidth of 0.45 MHz, FBW of 1.4kt², minimum in-band insertion loss of 1.5 dB, effective Q of 8,000, in-band return loss of 23 dB, and an average τ_g of 0.86 ps.