A Balanced Quad-Band BPF With Independently Controllable Frequencies and High Selectivity

A balanced microstrip quad-band bandpass filter (BPF) with independently controllable frequencies, controllable fractional bandwidths (FBWs) and high selectivity is designed in this paper. The proposed BPF is achieved by employing two pairs of folded asymmetric stub-loaded resonators (FASLRs) and balanced microstrip/slotline transition structures (MSTSs). The center frequencies of the four differential-mode (DM) passbands can be controlled independently by changing the electrical lengths of each FASLR. A novel interdigital coupled line is firstly proposed to enhance source-load coupling further, which contributes to a higher selectivity. Meanwhile, the DM passbands are independent of the CM responses, which significantly simplifies the design procedure. In addition, eight transmission zeros (TZs) are generated to improve the selectivity of the four passbands obviously. The measured results of the fabricated balanced BPF centered at 2.48/3.45/5.17/5.78 GHz agree well with the simulated ones, which validates the proposed design method well.