Balanced BPF With Dual-Port Quasi-Reflectionless Characteristic and Selectivity Enhancement

A balanced microstrip bandpass filter (BPF) with quasi-reflectionless (QR) characteristics on both input and output ports and good common-mode (CM) rejection is proposed in this brief. The presented QR BPF consists of a Pi-shaped BPF and two absorptive bandstop filters (ABSFs). Good QR performance is achieved by the complementary duplexer architecture formed with these two parts. The balanced microstrip-to-slotline (MSTS) transition structures are used to achieve exceptional CM rejection without deteriorating differential-mode (DM) transmission. The in-band energy is transmitted by the BPF section, where the ABSFs appear as open circuits. The out-of-band energy is absorbed by a pair of loaded resistors in the bandstop section. Through detailed analysis, it shows that by loading the ABSF structures at both ends of the BPF section, the DM passband selectivity is improved and six quasi-transmission poles (Quasi-TPs) are generated on both sides of the DM passband to eliminate out-of-band reflections. To verify the feasibility of the proposed model, a prototype was designed and fabricated, whose simulations and measurements are in good agreement.