Bandwidth enlargement in acoustic-wave RF bandpass filters with planar transversal circuits

A straightforward technique to increase the bandwidth of narrow-band frequency-selective circuits for its exploitation in surface-acoustic-wave (SAW) RF bandpass filters (BPFs) is presented. It is based on inserting the narrow-band circuit in a two-path transversal filtering section (TFS) so that its transmission band is enlarged by means of constructive signal-energy interactions. Out-of-band transmission zeros (TZs) can also be produced through destructive signal-interference phenomena for high-selectivity BPF development with multi-TFS-cascaded design strategies. The proposed bandwidth-enlargement procedure can be either applied to complete BPFs or single resonators as it is theoretically proven through two ideally synthesized examples. Furthermore, its experimental usefulness is verified with the manufacturing and characterization of a two-stage mixed-technology SAW/microstrip BPF prototype at 890 MHz. This circuit shows a passband-width of ≈ 5× broader than that of its embedded on-chip SAW BPF (i.e., ≈ 3× wider than the kt2 of the constituent SAW resonator), while offering physical-size-reduction, passband-insertion-loss, and stopband-bandwidth advantages in relation to its conventional signal-interference planar filter counterpart.