Design of balun filters with high frequency selectivity and low insertion loss

This work designs balun filters with high frequency selectivity and low insertion loss. A symmetrical doubly loaded output structure was constructed to obtain balanced outputs and create a pair of transmission zeros. The stacked cross-coupled structure was further applied for obtaining compactness and creating two more pairs of transmission zeros. A compact fourth-order balun filter was fabricated. Measured results demonstrate an excellent phase difference of 180°± 3.2°, an amplitude imbalance of 0.8 dB, and a low insertion loss of 1.3 dB at 2.45 GHz. High-selectivity responses with three pairs of transmission zeros were observed.