Tunable microstrip quasi-elliptic function bandpass filters

In this paper, a new type of varactor-tuned microstrip quasi-elliptic function bandpass filter based on dual-mode open-loop resonator is investigated. By employing the unique characteristic of the dual-mode open-loop resonator, i.e. whose two operating modes (odd- and even-modes) do not couple to each other, tuning the passband frequency becomes simple with a single DC bias circuit. Two finite-frequency transmission zeros on each side of the passband are tunable along with the center frequency, resulting in high selective frequency response. A designed four-pole tunable bandpass filter of this type is demonstrated experimentally.