Frequency- and Bandwidth-Tunable Absorptive Bandpass Filter

In this paper, an analytic design method for a frequency- and bandwidth-tunable absorptive bandpass filter is introduced. The presented design method relies on the transformation from the normalized absorptive low-pass filter prototype to a bandpass filter structure with parallel resonators and admittance inverters. The inverter values for producing a predefined frequency response are given in closed-form expressions so that a filter design needs no heuristic approach and can be carried out in an analytic manner. This work also presents a new tunable stub resonator structure capable of changing not only its resonant frequency but also the slope parameter. As the slope parameter can be adjusted, it is allowed to use static inverters in designing a filter whose bandwidth can be tuned. For demonstrating our design method for a reconfigurable absorptive bandpass filter, a third-order filter having the target to have the frequency tuning range from 1.35 to 1.65 GHz and the bandwidth tuning range from 5% to 10% has been designed and measured.