Compact Tunable Microstrip Bandpass Filters with Asymmetrical Frequency Response

A new type of electronically tunable asymmetric frequency response bandpass filter based on dual-mode open-loop resonator is investigated. 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 to each other, tuning the passband frequency becomes simple with single DC bias circuit. Two designed tunable bandpass filters of this type are demonstrated experimentally, which show more than 10 dB of return loss and less than 3 dB insertion loss over a tuning range of 12% from 0.83 to 0.93 GHz.