Multilayered Balanced Wideband Bandpass Filter With High Filtering Selectivity

A type of multilayered balanced wideband bandpass filter (BPF) with high filtering selectivity is presented. It is composed of two back-to-back cascaded microstrip-to-microstrip vertical transitions using slotline stepped-impedance resonators (SIRs). Under differential-mode excitation, a sharp-rejection fifth-order wideband filtering transfer function with two paired close-to-passband transmission zeros can be attained in the proposed balanced BPF. Moreover, high common-mode suppression levels within the differential-mode passband range are achieved. For experimental-demonstration purposes, a microstrip prototype of the devised balanced wideband BPF architecture is developed and tested. It exhibits a measured differential-mode fifth-order high-selectivity wideband filtering response with center frequency of 4.015 GHz, 3-dB fractional bandwidth of 38.06%, and minimum in-band power-insertion-loss level of 0.947 dB. The common-mode suppression levels are above 20 dB from 2.075 to 5.588 GHz.