Novel Varactor-Tuned Coupling Mechanism and Its Applications to High-Order Bandwidth-Agile Bandpass Filters

In this paper, a novel tunable coupling mechanism with bandwidth-tunable capability is presented. Metallic via has been utilized to construct the coupling structure, namely via-coupling. Studies show that the via-coupling mechanism provides more flexibility and owns better tolerance to fabrication errors than the traditional gap-coupling. By employing varactor in parallel with the metallic via, the coupling coefficient between each resonator pairs can be effectively tuned. It is worth noting that only one varactor is required for each resonator pair. The lumped-circuit model of the tunable coupling mechanism is investigated and extracted by classical theory derivation and simple programming. For verification, a fourth-order quasi-elliptic bandpass filter (BPF) and a bandwidth-tunable one are designed, fabricated, and measured. For the bandwidth-tunable BPF, the factional bandwidth varies from 2.52% to 5.04%, meaning a double tuning range. The in-band insertion loss varies from 2.8 to 4.8 dB, and the return loss is better than 17 dB. If needed, BPFs with <inline-formula> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula>th-order (<inline-formula> <tex-math notation="LaTeX">$N > 4$ </tex-math></inline-formula>) could be designed based on the proposed tunable coupling mechanism and only (<inline-formula> <tex-math notation="LaTeX">$N + 1$ </tex-math></inline-formula>) varactors are required.