A Novel Multi-Functional Coupled Resonator Based Balun Filter in Waveguide Technology

This paper presents a novel multi-functional balun filter designed at 10 GHz in rectangular waveguide technology. The balun filter is realized using the modified coupling matrix of the coupled resonator-based band-pass filter (BPF). It integrates the functionality of balun and BPF 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, imbalance loss arising from the balun. For the proof of the concept, a 4th order waveguide balun filter is designed, fabricated and tested at a center frequency of 10 GHz with 350 MHz bandwidth. The measured peak insertion loss is 0.25 dB and rejection is better than 40 dB at ± 650 MHz offset. The amplitude imbalance is within 0.2 dB and the phase imbalance is within 1.25 degrees over the entire bandwidth. The proposed design methodology is scalable to higher order balun filters.