The design and fabrication of miniature microwave bandpass filters using multilayer liquid crystal polymer technology

This thesis presents the design and fabrication techniques for miniature microwave bandpass filters using multilayer liquid crystal polymer (LCP) technology. As a multilayer technology for microwave devices, LCP is of low cost and light weight. It also has excellent electrical properties across a wide frequency range. These characteristics make it promising for the development of next generation microwave devices for applications across commercial, defence and civil sectors. However, very limited work has been found in the open literature to apply this technology to the design of miniature bandpass filters, especially at low microwave frequencies. In addition, the reported work shows lack of fabrication techniques, which limits the size reduction of multilayer LCP devices. To address these problems, this thesis develops advanced fabrication techniques for sophisticated LCP structures, such as multilayer capacitors, via connections and cavities. These techniques are then used to support the design of novel miniature bandpass filters for wideband and narrowband applications. For the design of miniature wideband bandpass filters, a cascaded approach, which combines highpass and lowpass filters, is presented first to provide a flexible design solution. This is followed by another novel ultra-wideband bandpass filter which produces extra transmission zeroes with minimum number of elements. It does not only have high performance but also a compact structure for high yield fabrication. For narrowband applications, two types of advanced coupled-resonator filters are developed. One type produces a very good selectivity at the upper passband edge, and its spurious-free stopband is extremely wide and of high interference attenuation. The other type, based on novel mixed-couplings approaches developed in this thesis, provides a solution to produce almost the same response as the coupling matrix prototype. This type is used to generate arbitrarily-located transmission zeroes. All designs presented in this thesis are simulated using CAD design tools and then validated by measurements of fabricated samples. Good agreements between simulations and measurements are shown in the thesis

Miniaturised and reconfigurable planar filters for ultra-wideband applications

An increasing demand for electromagnetic spectrum has resulted from the emergence of feature-rich and faster throughputs wireless applications. This necessitates the developments of dynamic reconfigurable or multifunctional systems to better exploit the existing spectrum. Future wireless devices will be expected to communicate over several bands with various other devices in order to fine tune the services they provide to the user. Each band may require a separate RF transceiver and such modern wireless multi-band multi-mode communication systems call for high performance, highly integrated compact modules. Since the Federal Communications Commission (FCC) released the unlicensed frequency band 3.1-10.6 GHz for ultra-wideband (UWB) commercial communications, the development race for commercialising UWB technology has seen a dramatic increase around the world. The aim of this research is to develop reconfigurable planar microwave filters for ultrawideband applications. The project investigates some key design issues of reconfigurable filters, which are being observed constantly in the latest development and realisation of microwave filters. Both analytical and numerical methods are performed to construct a realistic and functional design. Two different types of frequency reconfigurability are investigated in this thesis: discrete (e.g. PIN diode, Optical switch) and continuous (e.g. varactor diode). Using the equivalent circuits and considering the direct coupled filter structure in most cases, several topologies with attractive features are developed for future communication systems. The proposed works may be broadly categorised into three sections as follows. The first section explores a square ring shape close loop resonator along with an opencircuited stub in the symmetry plane. To realise a reconfigurable frequency states within the same spectrum, an innovative approach is developed for this case. An optical or photoconductive switch, comprised of a silicon die activated using near infrared light is investigated as a substitute of PIN diode and performances are evaluated to compare the feasibilities. In addition, a in-band interference rejection technique via externally coupled Tshape resonator is shown. However, it is observed that both structures achieve significant size reductions by utilising the inner part of the resonators. To improve the filter selectivity, a convenient design approach generating a pair of transmission zeros between both passband edges and a single zero in the stop band for harmonic suppression is discussed in the second section. Moreover, the development of notched rejection bands are studied and several novel methods to create a single and multiple notched bands employing the square ring shape structure are proposed. On inspection, it is found that the notch structure can be implemented without deteriorating the filter performances. The discussions are supplemented with detailed design examples which are accompanied by theoretical, simulated and experimental results in order to illustrate the filter development process and showcase practical filter performance. The third section reveals a novel highly compact planar dual-mode resonator with sharp rejections characteristics for UWB applications. A bandwidth reconfiguring technique is demonstrated by splitting its even-mode resonance. Filter structure with the dual-mode resonator is shown to have a relatively wide tuning range, significantly low insertion loss and a constant selectivity along with frequency variations in comparison to similar published works. Finally, the earlier dual-mode structure are modified to realise a dual wideband behaviour. A detail analysis with comprehensive design procedures is outlined and a solution for controlling the frequency bandwidths independently according to the application interest is provided. In line with the previous section, experimental verification is presented to support and supplement the discussions

Miniaturized Ring Resonator Wideband Bandpass Filter with Wide Stop Band

In this paper miniaturized quarter wavelength rectangular shaped multimode ring resonator bandpass filter with extended diagonal corners and internally located high impedance perturbation stubs, is proposed. Input/output open stubs are tightly coupled to the extended diagonal corners running parallel to the two sides of the ring resonator, implemented to generate wide passband and wide stop-band. Cut-off frequencies can be shifted to the higher side by increasing the length of the sides of resonator. By inserting the perturbation stubs, rectangular ring resonator produces three degenerate modes out of which first two form a wide passband. Small square patch is attached to the opposite interior corners of the ring resonator and T-shaped stub attached to the opposite longer side of the resonator are tightly coupled with feeder line to improve the return loss, insertion loss and skirt-characteristics. Shorter sides of the rectangular ring resonator are bent in U-shaped to increase the effective length of the resonator eventually the bandwidth is widened. Filter is designed and simulated for the center frequency of            3.2 GHz, bandwidth from 2.0 GHz to 4.0 GHz, on dielectric constant 3.38 and thickness 0.508 mm. Electromagnetic simulator Ansoft HFSS is used to optimize the filter dimensions

The Design of an Anti-Aliasing Filter for the Next Generation Digitiser

MeerKAT, is a 64-element radio astronomy antenna array which has been recently constructed in the Northern Cape Province of South Africa. It serves as South Africa's contribution towards the international Square Kilometre Array (SKA) project. The MeerKAT array has been designed to observe radio signals produced by celestial sources at UHF-Band, L-Band, S-Band and X-Band frequencies. The first phase of the construction included the design, development and integration of the UHF-Band, L-Band and S-band Receivers, whilst the X-band design has been superseded by the incorporation of the next phase of the SKA international project. In preparation of the next the roll-out, research is required to determine optimal wideband filter topologies suitable for direct digitisation of signal frequencies over the frequency range of 3-6 GHz. In this thesis, exploration of suitable wideband planar filters is performed, noting those with an improved out-of-band rejection. The outcome of the investigation leads into the design and development of the suitable wideband planar filter based on key performance specifications. The performance of the manufactured wideband planar filter is then compared to the theoretical design, and validated against the key performance requirements

Development of tunable and miniature microwave filters for modern wireless communications

Due to the increasing demand for new wireless services and applications, the high level of integration and the coexistence of multi-standard (MS) or multi-band operations into a single device are becoming defining trends in designing microwave filters. This has driven considerable technological advances in reconfigurable/tunable and miniaturized filters. More specifically, reconfigurable/tunable filters that tune to different frequency bands instead of classical filter banks have great potential to significantly reduce the system size and complexity; while reducing the filter size becomes essential to achieve the highest degree of integration density in compact and portable wireless devices. In the light of this scenario, the objective of this dissertation is to develop the new design technologies, concepts and filtering configurations for tunable microstrip filters and compact passive microwave filters. To this aim, this dissertation is divided into two main parts. The first part (Part I) focuses on the designs of novel varactor-tuned microstrip filters with advanced performances. In this aspect, new topologies for realizing tunable lowpass and highpass filters are firstly developed. State-of-the-art performances, including wide tuning range, high selectivity with multiple transmission zeros, low insertion loss and compact size for all the tuning states are obtained in both of these filters. Secondly, two novel classes of tunable bandpass filters are presented. One of them is designed based on varactor-loaded parallel-coupled microstrip lines (PCML) and short-circuited stubs, which allows the lower passband edge together with two transmission zeros located around the lower passband skirt to be reconfigured separately. While the other tunable bandpass filter is constructed by the combination of tunable bandpass and lowpass filters, featuring both centre frequency and bandwidth tunabilities, as well as high selectivity with abundant transmission zeros. Furthermore, a new concept of tunable lossy filter is demonstrated, which attempts to achieve an equivalent high-Q tunable performance by using low-Q resonators. This concept makes the presented tunable combline filter interesting for some frequency-agile applications in which the low in-band loss variation and high selectivity are much desired while the absolute insertion loss can be a tradeoff. The second part (Part II) is devoted to the design of miniaturized passive microwave filters with improved characteristics. For this, the concept of artificial right-handed and left-handed transmission lines are applied to the signal interference filtering topology, which results in a compact circuit size and good out-of-band performance. In particular, for a further size reduction, such filter is implemented in the forms of multilayered structure by using liquid crystal polymer (LCP) technology. Additionally, another two types of miniaturized bandpass filters using stepped impedance resonators are demonstrated, which are implemented based on different fabrication processes (i.e. LCP bonded multilayer PCB technology and a standard planar PCB technology). Among their main features, the compact size, wide passband, broad stopband with multiple transmission zeros and circuit simplicity are highlighted. For all the proposed design techniques and filtering structures, exhaustive theoretical analyses are done, and design equations and guide rules are provided. Furthermore, all the proposed schemes and/or ideas have been experimentally validated through the design, implementation and measurement of different filters. The fabrication processes of multilayer technology utilized: liquid crystal polymer (LCP) technology and liquid crystal polymer (LCP) bonded multilayer printed circuit board (PCB) technology, are also demonstrated for reference. All of the results achieved in this dissertation make the proposed filters very attractive for their use in modern wireless communication systems