Design Methodology of Multifunctional Balun Filters Based on Modified Coupling Matrix

This article presents a systematic design methodology of novel multifunctional balun filters at 3.5 GHz for 5G and MIMO transceiver applications. The balun filters are realized using the modified coupling matrix of the coupled resonator-based bandpass filter. The balun filter integrates the functionality of balun and bandpass filter within a single filtering component, thus enabling system miniaturization (due to functional integration). In addition, it enhances the system performance by eliminating additional insertion loss and imbalance loss arising from the balun. For the proof of the concept, a fourth-order balun filter is designed, fabricated, and tested at a center frequency of 3.5 GHz with 150-MHz bandwidth (BW) in microstrip and coaxial technologies. The measured insertion loss is 1.5 and 0.5 dB, respectively. The rejection is better than 40 dB at +/- 300-MHz offset. The amplitude imbalance is within 0.2 and 0.4 dB, respectively. Finally, the phase imbalance is within 0.5 degrees and 2.5 degrees, respectively, over the entire BW. For the sake of completeness, a balun filter is also realized in rectangular waveguide technology at 10 GHz with 350-MHz BW. The proposed design methodology is scalable to realize higher order balun filters. The realized microstrip balun filter is tested in one of the channels of ZCU216 RF system on chip (RF SoC) for 5G and MIMO transceiver applications for validation.