Compact filtering balun based on coupled‐line‐loaded hairpin ring resonators with extended stopband

Abstract

A filtering balun with compact size and extended stopband is designed in this paper. This design is based on a novel coupled-line loaded hairpin ring resonator (C-HRR). First, the odd- and even-mode transmission characteristics of the C-HRR are carefully analyzed. Then, the miniaturization and large-frequency-ratio features of the C-HRR are discussed and summarized. The out-of-phase current distribution at the odd-mode resonant frequency is noticed. Finally, to validate the features of the C-HRR, a filtering balun with wide upper stopband is designed, fabricated, and measured. Measured results show that the filtering balun can operate at 2.46 GHz with 0.5 dB magnitude imbalance and 4° phase imbalance. The circuit size of the filtering balun is only 0.24λg × 0.068λg, where λg is the guide wavelength at the center frequency fc. A wide stopband with 19.3 dB rejection from 1.05fc to 3.25fc is obtained. Both simulated and experimental results are in good agreement with each other.