A W-band air-filled coaxial bandpass filter employing micro metal additive manufacturing technology

This article presents a W-band air-filled coaxial bandpass filter with multiple transmission zeros using micro metal additive manufacturing technology. The coaxial structure was fabricated through the electroforming of thick copper layer by layer and inner conductor of coax is supported by SU-8 photo-resist. The internal cross-section size of the coaxial lines is 0.3 mm x 0.3 mm, which is very compact. The filter is composed of five paralleled dual-behavior resonators (DBRs), four transformers and a pair of coax-to-CPW (Co-plane waveguide) transitions to facilitate the measurement. The relative bandwidth in a wide range can be realized by manipulating the location of the connection points among the resonators and the transformers instead of changing the characteristic impedance of the transformers. Hence, the extreme width of coaxial line and dimensional discontinuities can be avoided to facilitate the fabrication. To validate the filtering design and the manufacturing technology, a fifth order bandpass filter prototype with center frequency 96 GHz, bandwidth of 8 GHz was designed and fabricated. The complete fabrication process was also given in detail. The measured results are in good agreement with simulation: overall insertion loss (IL) of 2.05 dB, 7.95 GHz bandwidth and return loss better than 15 dB in passband. In addition, the influence of processing technology on IL is also discussed, showing that this technology can achieve excellent surface finish and the impact of non-zero surface roughness filtering IL is insignificant.