Tunable Bandpass Filters With One Switchable Transmission Zero by Only Tuning Resonances

A methodology for tunable bandpass filters (tBPFs) with one switchable transmission zero (TZ) is presented, wherein the frequency tuning and TZ switching can be simultaneously achieved by merely tuning the resonant frequencies of resonators. Theoretically interpreted by coupling matrices with arbitrary filtering orders (n ≥ 2), the TZ can be symmetrically switched between lower and upper sides of the passband while retaining the same return loss level. As a demonstration, a fourth-order waveguide tBPF is synthesized, designed, fabricated, and measured for experimental verification. Two responses, namely TZ on lower and upper sides, are showing the tunable frequency range from 9.5 to 10.8 GHz with insertion loss (IL) from 0.34 to 1.40 and 3-dB bandwidth from 230 to 370 MHz, respectively, which verifies the proposed methodology very well.