Compact, Multilayer 5G Filter Based on Extracted-Pole Shielded Lumped Resonators

This paper investigates two second-order stacked filtering units featured by cross-coupling based on extracted-pole lumped resonators. It can generate one or two transmission zeros (TZs) on the left side of the passband. The size of the unit is miniaturized by the vertical layout. And the filtering unit with two TZs is applied to develop a fourth-order N79 bandpass filter (BPF) based on multilayer substrates. With the two TZs brought by the extracted-pole sections and an additional TZ introduced by mixed electromagnetic coupling of extra resonator nodes, a filter with high selectivity is implemented. The transmission characteristics of the proposed designs are analyzed and validated from both equivalent circuit and coupling routing diagram. The proposed filter was fabricated and measured for verification. It exhibits a 3-dB fractional bandwidth of 9% and a minimum insertion loss of 1.5 dB, with a size of only 0.31 λg × 0.14 λg. And it is also accompanied with good suppression level: upper stopband suppression over -20 dB beyond 10.94 GHz (> 2.27 f₀, f₀ = 4.82 GHz), left stopband suppression exceeds -48 dB from DC to 0.9 f₀.