Over the past decades, microwave/millimetre-wave filtering has been a significant area of research, attracting enormous efforts to achieve a better understanding of filters in order to attain improved performance and increasingly diverse applications. During this period, the state of the art in filter theory and techniques has evolved tremendously. Driven by the advancement in both theory and technology, as well as the mobile communications market, filtering is now not only a technique, but also a prosperous industrial sector.
As key components, filters have almost the same long history as electromagnetic theory and wireless techniques. Whether in radar, wireless communications or any other wireless systems, filters are essential for proper system operation. Conventionally, filters are placed at the input and/or output ports to prevent interference and noise from going into or coming out of a radio frequency (RF) subsystem or functional module. There is a large range of filters available to fulfil these traditional applications. Generally, there are four types of filter: metal cavity filters, planar filters, dielectric filters and surface acoustic wave (SAW) filters.
New applications for filters are recently being found as components in circuits. In these new applications, filters are no longer placed at the input or output ports of a subsystem or functional module; instead, they are used to provide discriminative reactive loads for different frequencies (harmonics) in the circuit. Apart from the traditional passive components (resistor, capacitor, inductor and transmission lines), this extra component is more flexible utilising the building of circuits to achieve a better performance.
I am honoured and delighted to introduce six high-quality papers to the readers of HKIE Transactions in this special issue. The paper by Feng and Che provides a new approach to building a balanced filter. Based on the common coupled lines and open/shorted loaded stubs, two and four transmission zeros near each passband for the differential mode can easily be achieved for two balanced filters. The paper by Guan et al. is an example of using filtering structure/matching network in the design of tuneable active circuits, while Yang et al. make use of a vertical microstrip-to-microstrip transition to achieve a highpass or ultra-wideband (UWB) bandpass filtering structure. The paper by Yao et al. highlights the use of a 3D structure and double-sided parallel-strip line (DSPSL) in designing a balun with frequency-independent phase difference, while Zhang et al. demonstrate several examples of using filters as components in designing circuits. Some power combining and dividing components and power amplifiers are implemented based on implantable filters. Last but not least, Zhao and Wu introduce a novel analytical circuit model extraction scheme for the computer-aided tuning (CAT) of general lossy coupled-resonator bandpass filters. This method is definitely very helpful in manufacturing cavity filters for mobile communications.
I would like to express my sincere gratitude to the authors, reviewers, members of the HKIE Transactions Committee and the HKIE Secretariat for their time and dedication in launching this special issue for the benefit of the engineering profession in Hong Kong.