Distributed-Element Absorptive Bandpass Filter with A Broadband Impedance Matching

In this paper, we present a new method for designing absorptive bandpass filters using coupled lines. The new design method and filter structure allow an absorptive bandpass filter to have good impedance matching performance over a wider frequency range in comparison with distributed-element absorptive filters reported in the literature. Closed form design equations are provided so that a filter can be designed in a way to produce a target frequency response. For verifying the presented design approach, a second-order absorptive bandpass filter centered at 2 GHz having 10% fractional bandwidth has been designed, fabricated, and measured. The measurement shows that the return losses (S11 and S22) are larger than 10 dB from DC to 2f0, which is the largest impedance-matched frequency range obtained from a distributed-element bandpass filter.