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.