Synthesis, design and fabrication of microwave filters and multiplexers

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Mostaani, Abolfazl (2024). Synthesis, design and fabrication of microwave filters and multiplexers. University of Birmingham. Ph.D.

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Abstract

This thesis presents new synthesis processes for filters and multiplexers. It also demonstrates novel implementation of microwave filters using Selective Laser Melting (SLM) and Micro Laser-Sintering (MLS) based 3-D printing technologies. The thesis contains six projects.
The first project proposes a synthesis and design approach for filters with multiple transmission zeros generated using dual/multiple dangling nodes. It starts with the synthesis of the general characteristic polynomials. An analytical method is proposed to derive the coupling matrix from the polynomials. Two third-order-two-zero bandpass filters (BPFs) are used to demonstrate the synthesis and design procedure. Two third-order-four-zero BPFs are then designed and prototyped using two pairs of mixed E/H-plane dual dangling nodes. The fabrication is enabled by 3D printing technology. Excellent agreement between the measurement and the design substantially verifies the synthesis and design approach and demonstrates the capability of the additive manufacture technology.
The second project presents an iterative method to achieve a strong initial coupling matrix for triggering the subsequent matrix optimization of diplexers/multiplexers with all-resonator topologies. The method is based on the synthesis of a single channel filter in consideration of the estimated loading effect of other channels. The synthesis starts with the characteristic polynomials of each channel filter, obtaining corresponding input admittance, and extracting the equivalent circuit model. Seven examples are used to demonstrate the synthesis process and validate the proposed technique. For most topologies, the iterative method converges after less than 20 iterations. The proposed method provides good initial values of the coupling matrix which can substantially reduce the optimization time to seconds, even for complicated topologies.
The third project demonstrates a narrowband third-order filter at 9 GHz with two transmission zeros generated using the non-resonating node (NRN) method. The filter is fabricated out of aluminium-copper alloy (A20X) using selective laser melting (SLM) technology. Without tuning, the measurements show promising results, but with a resonator detuning. Micro X-Ray CT imaging is performed to investigate the imperfection in the manufacture. Dimension measurement of the structure show the level of dimension offset and some evidence of excessive shrinkage from the small iris structures.
The fourth project presents a fifth-order BPF for communication satellite feed systems, fabricated using 3D-printing with Titanium alloy Ti-6Al-4V or Ti64. Taking advantage of the alloy's high strength-to-weight ratio, the filter is printed with a wall thickness of only 1 mm and a weight of 40 g. Ti64 also has a much lower coefficient of thermal expansion (CTE) than aluminium. The filter was designed with modified rectangular cavity resonators with rounded corners, enabling monolithic printing of the structure in a lying-down position without any internal supports. The coupling irises are moved to one side of the filter to facilitate polishing and silver coating. The measured response of the as-printed filter showed a minimum insertion loss of 2.7 dB and a frequency shift of about 140 MHz. After silver plating, the insertion loss was significantly reduced to a minimum of 0.55 dB. The challenge with plating on the Ti64 filter is discussed.
The fifth project presents an irregular-hexagonal resonator geometry for additively manufactured microwave filters. This geometry allows closely packed resonators to form very compact filters of various topologies fulfilling transfer functions with transmission zeros. It also minimizes the overhangs and allows vertical printing along the direction of the input/output ports, without any internal support structure and with a much-reduced profile. The characteristics of the resonator and the trade-off between compactness and quality factor is analysed. Three BPFs are demonstrated. Two of them are designed with NRN topologies with fractional bandwidths of 5% and 1%, respectively, using the step-tuning technique. All three filters are fabricated using SLM technique. The performance of the fabricated filters is compared with the EM simulation. Very good agreement is achieved for all three filters. The effectiveness of tuning is also demonstrated. The resonator structure offers high printability and high level of modularity and flexibility in the design.
Finally, in the sixth project, an E-band bandpass (81 – 86 GHz) filter has been demonstrated to show the effectiveness of surface treatments and a new electrochemical polishing technique, in particular, on the metal 3D-printed millimeter-wave waveguide device. The third-order filter is designed using a modified hexagonal resonator, a compact and self-supportive structure, and two samples have been printed monolithically using a high-precision micro-laser-sintering (MLS) technology out of stainless-steel. The effects of polishing and gold coating have been investigated. The coating improves the insertion loss by ~1 dB whereas the polishing step further reduces the loss by ~0.3 dB. This results in a very low insertion loss of only 0.14 dB for one of the two samples. This correlates with the 50% reduction in the surface roughness after polishing. The demonstrated end-to-end manufacture and surface treatment process shows great promise in addressing the surface quality issues with metal 3D printing for millimeter-wave devices.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):