Spurious Suppression Techniques for 3-D Printed Coaxial Resonator Bandpass Filters

Design methods to enhance the passband-to-stopband bandwidth of coaxial bandpass filters (BPFs) are reported. Stopband enhancement is achieved by: 1) introducing a transmission zero (TZ) through mixed electromagnetic (EM) coupling and 2) by reducing the RF signal coupling to one of the spurious modes by varying the relative orientation of the RF posts. A low-cost additive manufacturing (AM) concept is proposed as the key-enabling integration scheme for these types of filters due to 1) easing the manufacturing of otherwise complex geometries and 2) allowing for monolithic integration. For proof-of-concept demonstration purposes a two-pole prototype and a four-pole prototype were designed at 3.5 and 3.6 GHz. They were manufactured using a low-cost stereolithography apparatus (SLA)-based 3-D printer. The BPFs exhibited low insertion loss (IL < 0.5 dB) and a 4.5 stopband-to-passband ratio with 35 dB rejection, which is the highest among the existing spurious-free coaxial cavity-based BPFs.