Balanced reflectionless BPF with wide-frequency differential- and common-mode quasi-reflectionless behavior and high selectivity

In this paper, a balanced reflectionless bandpass filter (BPF) with wide-frequency differential-mode (DM) and common-mode (CM) quasi-reflectionless behavior is proposed for the first time. A novel out-of-phase fed double-sided parallel-strip line (DSPSL) structure is designed, which provides intrinsic all-frequency and high-level CM suppression, significantly simplifying the design procedure of balanced filters. Two absorptive stubs are employed to absorb the out-of-band energy of both DM and CM. While in-band CM noise is absorbed by the resistors loaded between two transmission lines. As a result, wide-frequency quasi-reflectionless performance can be attained for both DM and CM. Two transmission zeros are introduced by the ring resonator, which effectively improves the DM selectivity. For experimental demonstration purposes, the proposed balanced BPF is designed and fabricated. The measured results are in good agreement with the simulated ones.