Design of Third-Order Dual-Band BPF With a Tunable Lower Band Using Third-Harmonic Dual-Null-Points Loading Scheme on λ/4 UIR

This letter presents a reconfigurable third-order dual-band bandpass filter (BPF) that features independent frequency tuning of a lower passband with constant absolute bandwidth (CABW) and a fixed upper passband. A combline structure using a third-harmonic ( ${f} _{3}$ ) dual-null-point semi-lumped loading scheme on quarter-wavelength ( $\lambda $ /4) uniform impedance resonator (UIR) is proposed. This configuration enables frequency tuning of the lower band at fundamental frequency ( ${f} _{1}$ ) and the fixing of the upper band at ${f} _{3}$ by series-loading varactors at the current null point (CNP) at ${f} _{3}$ . Meanwhile, the coupling coefficient of the lower band can be controlled to meet the requirements of CABW by parallel-loading lumped inductors between each two resonators and located at their voltage null points (VNPs) at ${f} _{3}$ . The mutual independence of parallel- and series-loading at the VNP and CNP is advantageous, as both lumped loadings do not affect the upper band entirely, including frequency and ABW. Based on these, the fabricated BPF exhibits a frequency-tuning range (TR) of 52.1% with a CABW of $150~\pm ~6$ MHz in the lower band, while the upper band maintains a fixed frequency and an ABW of 150 MHz, which shows good agreements between simulated and measured results.