A Circulator Integrated With Second-Harmonic Absorptive Filtering Branch for 5G N78 Band

In 5G N78 front-ends, nonlinear harmonics from power amplifiers degrade receiver sensitivity and require compact suppression. This letter presents a miniaturized ferrite circulator integrated with a second-harmonic absorptive filtering (SHAF) branch. Utilizing a WYE resonator, a reflective central junction is constructed to effectively block second-harmonic leakage toward the receiver (RX) port. A filtering branch is shunted to absorb the harmonic component with limited impact on fundamental-band transmission. Measurements show a 21.5% fundamental bandwidth from 3.1 to 3.85 GHz with insertion loss below 1 dB and |S₁₁| below −10 dB over 6.68–7.73 GHz. A continuous subband from 3.185 to 3.59 GHz further achieves IL<0.77dB, RL > 14.6 dB, and isolation > 20 dB. The proposed scheme provides a compact solution for integrating circulation and internal second-harmonic absorption.