High‐Selectivity n‐Route Bandpass Filtering Crossover for 5G Massive MIMO Communication Systems

ABSTRACT A compact, high‐selectivity third‐order multichannel bandpass filter is proposed in this letter based on creased microstrip resonators (CMRs), which employs the complementary coupling technique using stubs. A six‐channel filtering response with equal bandwidths that operate at the same frequency has been demonstrated. To achieve transmission zeroes on both sides of the passband, stepped‐impedance open stubs as the coupling mechanism are used, resulting in high frequency selectivity in the passbands. The transmission zero positions are adjustable by a change in the coupling gap between the stubs and the resonators, and a combination of its electrical parameters. The circuit has been optimized and fabricated for measurement, at a 3.5 GHz center frequency. An overall size of 0.39 λg × 0.39 λg is achieved. The results exhibited high selectivity with two transmission zeroes located at 3.25 and 3.75 GHz, isolation levels below 21 dB, insertion loss lower than 1 dB and return loss higher than 20 dB in all passbands. A stopband range of 0.75 GHz each is achieved on both sides, and the rejection level is below 25 dB in both stopbands. The measured responses conform to the simulated responses.