Synthesis and new applications of multi-port filtering networks

微波滤波器是从移动基站到卫星的许多无线系统中必不可少的部件。它们用于选择有用的信号，同时抑制不想要的干扰或杂散信号。最广泛使用的微波滤波器由谐振器组成，这些谐振器通过电磁耦合在输入和输出两个端口之间产生所需的传输响应。谐振器的特性和它们之间的耦合可以用一个所谓的耦合矩阵来数学表示。这样的矩阵可以从所需的频率响应中找到-合成-。二端口滤波器的综合是一门公认的技术。最近，这种耦合矩阵方法已经从二端口滤波器扩展到多端口滤波网络(MPFN)。滤波器和MPFN之间的根本区别是“结谐振器”，它的引入是为了将信号路由到不同的端口。这种谐振器不仅用作滤波器中的谐振极，还用作传统上由非谐振传输线实现的信号的分离器。从MPFN概念中获益最多的微波电路之一是多路复用器，也称为组合器或滤波器组。它基本上包含多个相互连接的滤波器，用于组合多个通道并馈送到一个天线以进行发送或接收。它是无线基站和卫星有效载荷中最复杂的无源电路之一。传统上，所有通道滤波器通过基于传输线的信号分配网络连接到公共端口。利用MPFN的概念，可以用谐振器取代传输线网络。这大大提高了多路复用器的选择性，而不牺牲电路尺寸，这是工业应用非常需要的。这意味着多路复用器通常是一个很大的组件，可以减小尺寸和质量，以实现更多的接触系统。就卫星而言，这可以转化为显著的成本降低。在微波电路中独家使用谐振器还可以将滤波功能集成到传统的非滤波电路中。例如，常见的微波功分器和耦合器是基于传输线的，选择性非常有限。通过使用MPFN的概念，可以通过嵌入滤波功能来实现基于全谐振器的功率分配器和耦合器。这意味着两个电路功能合并到一个电路中。这种方法被称为“联合设计”。尽管MPFN概念和协同设计方法在微波电路设计中的使用显著增加，但与该技术相关的挑战仍然很大。MPFN的合成比过滤器的要求高得多。这就要求对结谐振腔周围的耦合特性有一个新的认识。目前无法获得的合成技术阻碍了微波工程师对MPFN概念的接受。此外，还存在对支持MPFN的设备的带宽和功率处理能力的担忧，因为结谐振器本质上是窄带的，并且可能是功率集中。该项目旨在开发一种健壮的、更易获得和适用的MPFN合成技术，并通过提出新的结谐振器来解决带宽和功率处理方面的实际挑战。这项研究将有助于充分发挥MPFN的工业应用潜力。毫无疑问，MPFN的概念将导致微波电路的更多创新。在综合技术的基础上，该项目将研究两个新的电路概念。预计该项目将为新型微波电路的研究提供新的方向、进一步发展的机会和商业探索。

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.