A methodology for enhancing bandwidth with keeping steepness of the ladder filters exploiting acoustic‐wave‐lumped‐element resonators

A methodology to acquire a wide bandwidth and steep transition of a hybrid ladder filter with the acoustic‐wave‐lumped‐element‐resonators (AWLRs) structure is proposed. Different from previous works, a unit of the new AWLR structure with two lumped elements respectively in series and shunt is proposed. It can change both the series‐resonant frequency and the parallel‐resonant frequency of the acoustic wave (AW) resonator to enhance the bandwidth of the hybrid ladder filter. For the ladder structure, all the AW resonators possess the same resonant frequency and impedance. This ladder structure is also used to keep the steep transition of the hybrid filter. For the validations, two‐stage and four‐stage AWLRs‐based hybrid filters are designed, manufactured, and tested. They include (1) a two‐stage filter with an insertion loss (IL) of 0.9 dB, a fractional bandwidth (FBW) of 1.90kt2 (kt2 is the electromechanical coupling coefficient), a center frequency of 1249.3 MHz, and (2) a four‐stage filter with the IL of 1.3 dB, the FBW of 1.86kt2, the SF30 dB/3 dB (shape factor defined as 30‐dB bandwidth divided by 3‐dB bandwidth) of 2.52, the figure of merit (defined as FBW divided by SF30 dB/3 dB) is 0.74kt2 which is the best result, a center frequency of 1249.4 MHz.