Input-absorptive high-order wideband balun bandpass filters with quasi-elliptic-type response

Abstract

A class of input-reflectionless high-order wideband balun bandpass filters (BPFs) with quasi-elliptic-type response are reported. They consist of two main circuit parts: 1) a three-port reflective-type high-order balun BPF network and 2) a resistivelyterminated microstrip T-junction that is shunt connected at the unbalanced port. The reflective-type balun BPFs developed with 5th- and 6th-order Chebyshev equal-ripple responses are firstly described. By replacing the last two virtually-short-circuit-ended quarter-wavelength transmission-line (TL) stubs with opencircuit-ended half-wavelength TL stubs, their balun counterparts with two close-to-passband transmission zeros (TZs) are then presented. As design example, a 5th-order input-reflectionless quasi-elliptic-type wideband balun BPF is theoretically discussed. Furthermore, for experimental-validation purposes, a proof-ofconcept two-layer microstrip prototype of 5th-order balun BPF is simulated and tested. Its measured metrics are 2.05-GHz center frequency, 3-dB fractional bandwidth of 63.61% for the two output ports, input-power-matching levels higher than 10 dB, and stopband-power-rejection levels above 26.1 dB from DC to 5 GHz.