Multi-Layer Second-Order Balanced Bandpass Filter with Differential-and Common-Mode Two-Port-Quasi-Reflectionless Behavior

Abstract

A second-order balanced bandpass filter (BPF) with both differential-mode (DM) and common-mode (CM) two-port-quasi-reflectionless characteristics is reported. It consists of two multi-layer wideband magic-Ts, which are back-to-back connect-ed by means of a symmetrical-quasi-reflectionless complemen-tary-diplexer-based second-order narrow-band BPF, whose out-put ports are designed as balanced input/output terminals of the overall BPF device. By means of the conceived filter topology, an enlarged DM power-absorption spectral range and an enhanced DM passband flatness are attained. On the other hand, by loading two lossy multi-section microstrip lines at the ∑ ports of the used magic-Ton microstrip-to-slotline transitions, a broadband CM quasi-reflectionless response is featured for the devised BPF. These characteristics make the proposed configuration unique, as most of previously-reported balanced BPFs focus on only CM or DM RF -power absorption. For experimental-validation purposes, a two-layer balanced BPF prototype at 2 GHz with DM and CM two-port-quasi-reflectionless capabilities is built and tested. It exhibits a high-selectivity/enhanced-passband-flatness DM filtering response with 3-dB fractional bandwidth (FBW) of 12.3%, along with minimum DM and CM RF-power-absorption levels of 10 dB from 0.96 to 3.14 GHz (i.e., 3.27:1 spectral ratio) and from 0.96 to 3.33 GHz (i.e., 3.47:1 spectral ratio), respectively.