Synthesis and new applications of multi-port filtering networks

Microwave filters are essential components in many wireless systems from mobile base stations to satellites. They are used to select useful signals while rejecting unwanted interferences or spurious signals. The most widely used microwave filters are formed of resonators that are electromagnetically coupled together to generate the required transmission responses between the two ports - input and output. The properties of the resonators and the couplings between them can be mathematically represented by a so-called 'coupling matrix'. Such a matrix may be found - synthesised - from the required frequency response. The synthesis of two-port filters is an established art. Recently this coupling matrix approach has been extended from two-port filters to multi-port filtering networks (MPFNs).
The fundamental difference between a filter and a MPFN is the 'junction resonators', introduced to route the signal to different ports. Such resonators serve not only as resonant poles as in a filter, but also as splitters of the signal which are traditionally achieved by non-resonant transmission lines. One of the microwave circuits that benefit most from the MPFN concept is a multiplexer, also known as a combiner or a filter bank. It basically contains multiple interconnected filters, used to combine multiple channels and feed to one antenna for transmission or reception. It is one of the most complex passive circuits in wireless base stations and satellite payloads. Conventionally all the channel filters are connected to the common port through a signal distribution network based on transmission lines. Using the MPFN concept, the transmission line network can be replaced with resonators. This significantly increases the selectivity of the multiplexer without sacrificing the circuit size, which is highly desired by industrial applications. This means the multiplexer, usually a large component, can be reduced in size and mass for a more contact system. In the case of satellites, this can be translated to a significant cost reduction. The exclusive use of resonators in a microwave circuit also enables integrating filtering function into traditional non-filtering circuit. For instance, common microwave power dividers and couplers are transmission-line based with very limited selectivity. By using the MPFN concept, all-resonator-based power dividers and couplers can be realised with embedding filtering functions. This means two circuit functions are merged into one circuit. This approach is known as 'co-design'.
Despite the significant increase in the usage of the MPFN concept and co-design approach in microwave circuit design, there are still significant challenges associated with the technique. The synthesis of the MPFNs is much more demanding than the filters. It requires a new understanding of the coupling characteristics around the junction resonators. The currently inaccessible synthesis technique impedes the take-up of the MPFN concept by microwave engineers. Also there are concerns with the bandwidth and power handling capability of the MPFN-enable devices, as the junction resonator is narrowband in nature and may be a concentration of power. This project aims to develop a robust, more accessible and applicable synthesis technique for MPFNs and to address the practical challenges in bandwidth and power handling by proposing novel junction resonators. The research will help to release the full potentials of MPFNs for industrial applications. There is no doubt the MPFN concept will lead to more innovations in microwave circuits. Built on from the synthesis technique, the project will investigate two new circuit concepts. It is expected new research directions on novel microwave circuits, opportunities for further development and commercial exploration will be generated from this project.

This research has been designed to have an impact beyond the academic domain. The project aims to raise the technology readiness level of the multi-port filtering network (MPFN) techniques and demonstrate their capability. This will bring the technology closer to industrial requirements. Companies have shown a lot of interests in various aspects of the research. This is evident from the support of this project by industrial partners from different sectors - space (ESA), telecommunications (Huawei) and defence (BAE Systems). The success of the research will also lead to new and enabling device technologies which will enhance the innovative capacity and competitiveness of the industries. The application of the MPFN concept to multiplexers has attracted lots of interests. Multiplexer is one of the most complex passive circuits in wireless systems such as satellite payloads and mobile base stations. The potential reduction in size and mass offered by the MPFNs is a huge benefit to satellites, which will result in a significant cost reduction. This is highly desired for the space industry. The multiplexers used in mobile base stations present a different challenge. This project will tackle the practical challenge and demonstrate the capability to the industry. Both communications satellites and mobile base stations are becoming an important part of our daily lives, to support the ever expanding need from personal communications, data transfer to TV. The improved device technologies will not only have an economic impact but also indirectly contribute to the better quality of life. For this research work to be translated into suc h impacts, commercial development may be required. The Pathway to Impact outlines the routes for commercial exploration.
The proposed research will have a broad impact on many aspects of microwave engineering. The synthesis technique will equip engineers with a new and more accessible tool for microwave circuit design. It underpins a range of passive microwave circuits such multiplexers, power dividers, couplers and coupler networks. The users of the research will further benefit from the information and the GUI interface to the synthesis code, provided through the website of the project.
The training of people is an integral part of this project. The close cooperation with industrial partners will provide the researchers and students the needed skills. The research will raise their awareness of both the intellectual challenges and the potential for industrial applications. This has far-reaching impact on the health and competitiveness of the industry.
This research will attract R&D investment from global business. This is evident by the support of Huawei Technologies, the world largest telecommunications equipment maker with a very strong international presence. The research also has the potential to attract funding from European Space Agency, who is another project partner of this proposal.