Balanced dual-band BPF with enhanced DM selectivity and stopband suppression

Abstract In this paper, a balanced dual-band bandpass filter (BPF) with enhanced differential-mode (DM) selectivity and stopband suppression is proposed. The design utilizes four half-wavelength stepped impedance resonators (SIRs) with mixed electromagnetic couplings to generate two DM passbands at 2.47 GHz and 3.53 GHz. By optimizing resonator dimensions ( L r 4 , L r 13 ) and coupling gaps ( g 1 , g 2 ), the center frequencies and fractional bandwidths (FBWs) of both passbands are independently controlled. A U-type transition structure enables intrinsic common-mode (CM) suppression exceeding 42 dB, while source-load coupling introduces four transmission zeros (TZs) to sharpen DM selectivity. Measured results show insertion losses (ILs) of 1.72 dB and 1.70 dB, return losses >17 dB, and a wide out-of-band suppression up to 6.7 GHz. The design outperforms the existing works in TZ count and flexibility, validated through rigorous simulation and measurement.