Design of high selectivity tunable bandpass filter with switchable single-/dual-band responses

This article proposes a switchable filter with four operating states, namely dual-band mode, lower band mode, upper band mode, and all-stop mode. The transition between these states is achieved by using positive intrinsic negative diodes, while the center frequency tuning is done by using varactors. In order to verify the design concept, a prototype has been fabricated on an F4BME substrate with a dielectric constant of 2.25 and a thickness of 1 mm, and the measured results were in agreement with the simulations. Notably, the dual-band mode yields four transmission zeros (TZs), while the single-band mode produces two TZs. The final circuit size of the designed filter is 0.18 × 0.14 λg, making it ideally suited for reconfigurable multi-band communication systems.