Microstrip quasi-elliptic function bandpass filter with improved tuning range

This paper proposes a new type of varactor-tuned microstrip quasi-elliptic bandpass filter with improved tuning range. By employing the unique characteristic of the dual-mode open-loop resonator, that is, whose two operating modes (odd-and even-modes) do not couple each other, tuning the passband frequency becomes simple with single DC bias circuit. In addition, by using a wideband transformer, the tuning range of the proposed filter is doubled the previous work [1] while keeping nearly constant absolute-bandwidth. A designed four-pole tunable bandpass filter of this type is demonstrated experimentally.