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 (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f} _{3}$ </tex-math></inline-formula>) dual-null-point semi-lumped loading scheme on quarter-wavelength (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula>/4) uniform impedance resonator (UIR) is proposed. This configuration enables frequency tuning of the lower band at fundamental frequency (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f} _{1}$ </tex-math></inline-formula>) and the fixing of the upper band at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f} _{3}$ </tex-math></inline-formula> by series-loading varactors at the current null point (CNP) at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f} _{3}$ </tex-math></inline-formula>. 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f} _{3}$ </tex-math></inline-formula>. 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$150~\pm ~6$ </tex-math></inline-formula> 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.