Compact 3-D Printed Bandpass Filters Using Folded Mixed Hemispherical Resonators

A new class of high quality factor and highly miniaturized hemispherical resonator-based bandpass filters (BPFs) with asymmetric transfer functions is presented. The proposed BPF concept is based on a monolithic integration scheme using 3-D printed hemispherical resonators. Size compactness is achieved by: 1) effectively using the available 3-D volume through resonator folding and capacitive loading; 2) introducing transmission zeros (TZs) by cross coupling; and 3) by monolithic integration enabled by a stereolithography apparatus (SLA) 3-D printing manufacturing method. A design methodology starting from coupled-resonator-based synthesized examples and translating them to a physical geometry is presented. The concept has been validated at X-band through the manufacturing and testing of a three-pole/one-TZ BPF prototype with: center frequency: 8 GHz, 3-dB BW of 0.37 GHz fractional bandwidth (FBW): 4.6%, and minimum insertion loss (IL): 0.287 dB (effective Q of 1400).