Input-Reflectionless Balanced Wideband Bandpass Filter Using Multilayered Vertical Transitions

A type of input-reflectionless balanced wideband bandpass filter (BPF) using multilayered vertical transitions is presented. By means of two back-to-back cascaded vertical transitions, which are designed with open-circuited microstrip lines and short-circuited two-section microstrip stubs printed on the top and bottom layers, along with two shunt slotline-coupled microstrip sections, a high-order balanced wideband BPF with improved selectivity and intrinsic ideally-perfect common-mode suppression is firstly conceived. Subsequently, by inserting in this circuit two resistively-terminated microstrip T-junctions that are shunt connected with the open-circuited microstrip lines of the vertical transitions, an input-reflectionless behavior for the proposed balanced wideband BPF is then obtained. The RF operational principles of this engineered input-reflectionless balanced wideband BPF are detailed. For practical-validation purposes, a two-layered balanced BPF prototype is fabricated and characterized. It shows a measured differential-mode 3-dB fractional bandwidth of 65.5%, minimum power-matching level of 10.6 dB from DC to 5.2 GHz, and common-mode suppression levels higher than 29.1 dB within the whole passband of interest.