Compact filtering balun diplexer with independently tuned channels and constant fractional bandwidth

Abstract

A highly-integrated frequency tunable filtering balun-diplexer (FBD) is introduced in this article. Firstly, the tunable coupled-line-loaded hairpin ring resonator (C-HRR) with two independently tunable resonant modes is analyzed. Then two tunable filtering baluns (FBs) for the two frequency tunable channels of the FBD are designed based on the tunable C-HRR. The constant fractional bandwidth (FBW) performance of two tunable FBs is discovered and discussed. Finally, a tunable FBD is designed by simply uniting the two tunable FBs and sharing the common input port and one C-HRR which reduces the circuit size effectively. The low loading effect ensures the two frequency channels of the FBD being tuned independently. For validation, a prototype of the tunable FDB is fabricated and measured. The lower and higher channels can achieve a constant 4 % FBW in a tuning range of 9 % and 14 %, respectively. The CM noise rejection is higher than 36 dB with in-band channel isolation greater than 35 dB. The circuit size is 0.27λg × 0.15λg, where λg is guide wavelength at the highest operation frequency of the lower channel.