Independently Tunable/Controllable Differential Dual-Band Bandpass Filters Using Element-Loaded Stepped-Impedance Resonators

This paper presents a new approach for designing differential bandpass filter (BPF) with independently tunable/controllable dual passbands using the element-loaded stepped-impedance resonators (SIRs). The frequency-agile ability of the fundamental and third-harmonic resonant frequencies (f(1) and f(3)) of the element-loaded SIR against the value of the loaded element under different loading positions on the SIR is theoretically analyzed. It can be found that there must be a position that could be utilized to load element to realize the independent control of f(1) without affecting f(3). This feature is successfully applicable to design independently tunable differential dual-band BPF when the loaded elements are tunable capacitors. Furthermore, by making the loaded elements to be shunt stubs, a new coupling path can be constructed for f(1), and then the coupling coefficients for the two bands can be separately varied. Both the operational frequencies and bandwidths of the two bands can be separately adjusted. As a result, the traditional design procedure for the differential dual-band BPF can be simplified significantly. For demonstration, two examples are designed, implemented, and measured, and their simulation and measured results are presented, showing good accordance.