Design of Differential Absorptive BPFs With Enhanced Differential-Mode Return Loss

In this brief, two differential absorptive bandpass filters (ABPFs) with additional resistively terminated absorptive stubs (RTASs) are presented. The proposed RTAS is constructed of a $\lambda $ /4 microstrip line and a grounded resistor, improving the complementarity between bandpass and bandstop responses effectively. A single-ended ABPF is designed using a coupled-line resonator, two RTASs, and T-shaped bandstop sections. Higher return loss at passband edges is realized with additional RTASs. By employing two microstrip-slotline transition structures, the differential ABPF I is proposed with high common-mode (CM) suppression levels. Differential ABPF II is designed based on two coupled lines loaded by a microstrip stub, two additional RTASs, and lossy bandstop sections. For the differential-mode signals, the out-of-band noise is absorbed and the return loss at two passband edges is improved by additional RTASs. All-stop CM suppression is also realized. The theoretical analysis and explicit equations of two differential ABPFs are detailed and illustrated. For experimental validation, two prototypes are fabricated and measured. The simulated and measured results are in good agreement.