Design of Tunable Coaxial Bandpass Filter Based on Embedded Stepped Impedance Resonators

This paper describes a method for designing tunable coaxial bandpass filters based on an inserted coaxial stepped impedance resonator (SIR). The miniaturization of the coaxial resonator is realized by the shape of the stepped impedance structure. In addition, an extended tuning range can be achieved by utilizing the SIR. A third-order tunable bandpass filter with constant absolute bandwidth is designed and fabricated for verification. The fabricated filter exhibits a wide tuning range from 2.13 GHz to 3.54 GHz (52.4%), over which the absolute bandwidth (ABW) varies within 97.5 ± 5.5 MHz (±5.6%).