3-D Printed Slotted Spherical Resonator Bandpass Filters With Spurious Suppression

Abstract

In this paper, a third-order waveguide bandpass filter (BPF) based on slotted spherical resonators with a wide spurious-free stopband is presented. The resonator consists of a spherical cavity with slots opened at the top and bottom. Compared with a non-slotted spherical resonator, the slotted resonator suppresses the two spurious modes (TM 211 and TE 101 ) whilst maintaining the fundamental TM 101 mode. The unloaded quality factor of the TM 101 mode is not significantly degraded. This is achieved by interrupting surface current and radiating the unwanted spurious modes with the slots. The BPF is designed at a center frequency of 10 GHz with a fractional bandwidth of 1%. Two filter prototypes are fabricated, one using metal-based selective laser melting (SLM) and the other by polymer-based stereolithography apparatus (SLA) techniques. The slots also facilitate the copper electroplating process for the SLA-printed filter. The measured results show that the average passband insertion losses of the SLM-and SLA-printed filters are 0.33 and 0.2 dB, respectively. The corresponding passband return losses are better than 22 and 20 dB. The filters demonstrate excellent passband performance and wide spurious-free stopbands up to 16 GHz with stopband rejections of over 20 dB.

A 3-D Printed Bandpass Filter Using TM₂₁₁-Mode Slotted Spherical Resonators With Enhanced Spurious Suppression

“…In [6], quasi-elliptic filters were realized using mushroom-shaped resonators. Recently, spherical resonators have been shown to be suitable for 3-D printing [14]- [21], with ultra-low insertion loss due to the high Q u of the printed resonators. However, they suffer from higher order modes that are close in frequency to the fundamental TM 101 mode, and this degrades the out of band performance of the filter.…”

“…However, they suffer from higher order modes that are close in frequency to the fundamental TM 101 mode, and this degrades the out of band performance of the filter. This problem has been investigated by using the rotated topology [14], the depressed super-ellipsoid cavity [3], and the slotted spherical resonators [20,21]. Electrical conductivity of the cavity walls is assumed to be 3.56 × 10 7 S/m in the CST simulation.…”

“…To overcome this problem, the geometrical configuration of the filter is arranged in a "Z" topology to achieve better spurious suppression by decreasing the coupling strength of the spurious TM 311 mode. Compared with previous work on spherical resonators [14]- [21], the proposed new design concept based on high-order mode offers higher unload quality factor, and allows higher tolerance to fabrication, which is critically important for practical applications. Moreover, higher lower and upper spurious suppression can be achieved.…”

“…It should be mentioned that the proposed slotted spherical resonator can also be utilized in other topologies, such as cascaded trisection (CT) filters, extracted pole filters, and canonical filters [22]. The first four resonant modes of a spherical resonator are the TM 101 , TM 211 , TE 101 , and TM 311 modes [14,21], their simulated surface current distributions (side views) in a weakly coupled spherical resonator are shown in Fig. 1, while the corresponding top views of these modes can be found in our previous work [21].…”

3D printed waveguide step‐twist with bandpass filtering functionality

“…Stereolithography apparatus (SLA) can offer high precision and low surface roughness, and therefore is employed in this work. The SLA 3D printing has been successfully utilised to demonstrate a wide range of microwave devices [4][5][6][7][8][9][10][11]. Additionally, different to [3], the filtering twist reported here have slots on the waveguide walls, yielding a further reduction in device weight and improved metal plating at inner walls.…”