Balanced dual-band BPF with partially coupled bi-section λ/2 and λ/4 SIRs

A new fourth-order balanced dual-band bandpass filter (BPF) designed using partially coupled bi-section λ/2 and λ/4 stepped-impedance resonators (SIRs) is proposed. To obtain the required differential-mode (DM) operation, the BPF is arranged symmetric with respect to the plane of symmetry and all the SIRs are designed to have the same odd-mode resonant frequencies. The common modes of the balanced BPF which are indeed the even modes of the λ/2 SIRs are naturally suppressed since the λ/4 SIRs do not resonate at these frequencies. In addition, the lower-impedance sections of both the λ/2 SIRs and λ/4 SIRs are placed alongside to work as an all-stop coupled-line section in common-mode (CM) operation to achieve a good overall CM response. For the fabricated prototype BPF circuit, the measured minimum insertion losses for the first and the second bands in DM (CM) operation are 2.2 (16.6) and 2.56 (23.75) dB, respectively. The measured CM insertion loss is found greater than 52 dB over the displayed frequency range of 1-7 GHz other than the two DM passbands.