Design of a triple band‐pass balanced filter based on single‐stepped‐impedance ring resonator with ultra‐wide stopband bandwidth

Abstract In this study, a new tri‐band balanced band‐pass filter (BPF) design based on a single‐stepped‐impedance ring resonator (SIRR) with four stepped stubs loaded on the feeding structure was proposed, analyzed, and design equations were derived. Using the SIRR properties in combination with open stepped stubs, multiple transmission zeros (TZs) were generated, and by adjusting the location of the TZs, high selectivity and an ultra‐wide differential‐mode (DM) stopband bandwidth were achieved. A pair of stepped stubs on the symmetry line of the filter was also integrated to enhance the common‐mode (CM) rejection. Compared to other state‐of‐the‐art differential multiband BPFs, superior performance in harmonic suppression and CM rejection was achieved while maintaining high selectivity and a simple planar design. A prototype tri‐band balanced BPF with center frequencies of 1.7, 2.7, and 4.5 GHz was designed and fabricated. Results showed an ultra‐wide DM upper stopband bandwidth of 11.1 (up to 18.9 GHz) and a wideband CM suppression of < −20 dB from 1.3 GHz to 5.9 GHz.