Miniaturized Monolithic 3-D Printed Bandpass Filters Using Nested Spherical Resonators

This article reports on a new class of highly miniaturized nested spherical bandpass filters (BPFs) alongside a monolithic integration concept. Miniaturization is achieved by a nested spherical resonator approach, i.e., by incorporating a postsupported sphere within the volume of a hollow spherical resonator. In this manner, the resonant frequencies of the first four modes are lowered when compared to conventional unloaded spherical resonators, allowing for smaller BPFs to be materialized. Alternative BPF topologies using the fundamental mode as well as a combination of higher order modes are considered to further miniaturize the filter volume. Scalability to higher order transfer functions through cascaded configurations of multimode resonators is explored. Monolithic integration is enabled by digital additive manufacturing (AM) techniques using stereolithography apparatus (SLA). For proof-of-concept validation purposes three prototypes were designed, manufactured, and tested. They include: 1) a two-pole BPF with a passband centered at 2.72 GHz, fractional bandwidth (FBW) of 33.1%, and minimum in-band insertion loss (IL) of 0.15 dB; 2) a three-pole BPF with a passband centered at 6.94 GHz, FBW of 13.65% and minimum in-band IL of 0.07 dB; and 3) a six-pole BPF with a passband centered at 6.88 GHz, FBW of 12.77% and minimum in-band IL of 0.3 dB.