Compact high‐selectivity tunable dual‐mode filter with constant bandwidth by adopting frequency‐dependent S‐L coupling

A compact high‐selectivity tunable dual‐mode microstrip bandpass filter (BPF) with constant bandwidth is proposed in this letter. Three self‐adaptive transmission zeros (TZs) are generated due to the frequency‐dependent source‐load (S‐L) coupling and intrinsic characteristic of dual‐mode resonator, which can sharpen the skirt and deepen the upper stopband. The passband performance can be mainly determined by the pole separation Δf of dual‐mode and external quality factor Qe. Finally, a tunable filter with tuning range from 1.15 to 1.65GHz, constant 3 dB absolute bandwidth (ABW3dB) of 120 ± 5 MHz is designed and fabricated. The measurement results show good agreement with simulated ones, which confirms the design feasibility.