Balanced-Circuit-Based Dual-Band Bandpass Filter With Symmetrical Reflectionless Behavior

A symmetrical dual-band bandpass filter (BPF) with two-port reflectionless characteristics is reported. It is based on a balanced-circuit architecture, which consists of the following parts: (i) two identical input/output wide-band quadrature 3-dB couplers whose isolated ports are loaded by absorptive resistors, and (ii) two similar reflective-type dual-band BPF units connected between the through and coupled ports of the couplers in the two arms. In this manner, the total filtering transfer function is the one of the reflective-type dual-band BPFs, whereas the out-of-band RF signal-power reflections are dissipated by the loading resistors to attain the two-port reflectionless behavior in the overall circuit. Compact three-stage parallel-coupled-line quadrature couplers with via-hole-connected microstrip-to-coplanar-waveguide (CPW) transitions for their crossover junctions are used. Thus, they can be co-integrated in the same planar substrate with the stepped-impedance-resonator-(SIR)-based reflective-type dual-band BPFs within the branches of the overall balanced circuit, while their broad-band nature enables the design of reflectionless dual-band BPFs with widely-spaced passbands. For experimental-validation purposes, a microstrip prototype of reflectionless dual-band BPF, which exhibits 1.1/1.84-GHz four-pole dual passbands with transmission zeros (TZs) at both sides, is manufactured and tested.