Novel miniature microwave quasi-elliptical function bandpass filters with wideband harmonic suppression

Filters are integral components in all wireless communication systems, and their function is to permit predefined band of frequencies into the system and reject all other signals. The ever-growing demand in the use of the radio frequency (RF) spectrum for new applications has resulted in the need for high performance microwave filters with strict requirements on both inband and out-of-band characteristics. High selectivity, high rejection, low loss and extremely wide spurious-free performance are required for both transmitter and receiver channels. In addition, these devices need to be highly compact, easy to integrate within transceivers and should be amenable to low cost manufacturing. High selectivity is essential to enable the guard band between adjacent channels to be reduced thus improving the efficiency of the RF spectrum and hence increasing the capacity of the system. A low insertion-loss, high return-loss and small group-delay in the passband are necessary to minimize signal degradation. A wide stopband is necessary to suppress spurious passbands outside the filter’s bandwidth that may allow spurious emissions from modulation process (harmonic, parasitic, intermodulation and frequency conversion products) and interfere with other systems. The EMC Directive 89/336/EEC mandates that all electronic equipment must comply with the applicable EN specification for EMI. This thesis presents the research work that has resulted in the development of innovative and compact microstrip bandpass filters that fulfil the above stringent requirements for wireless communication systems. In fact, the proposed highly compact planar microstrip filters provide an alternative solution for existing and next generation of wireless communications systems. In particular, the proposed filters exhibit a low-loss and quasi-elliptic function response that is normally only possible with filter designs using waveguides and high temperature superconductors. The selectivity of the filters has been improved by inserting a pair of transmission zeros between the passband edges, and implementing notched rejection bands in the filter’s frequency response to widen its stopband performance. The filter structures have been analysed theoretically and modelled by using Keysight Technologies’ Advanced Design System (ADS™) and Momentum® software. The dissertation is essentially composed of four main sections. In the first section, several compact and quasi-elliptic function bandpass filter structures are proposed and theoretically analysed. Selectivity and stopband performance of these filters is enhanced by loading the input and output feed-lines with inductive stubs that introduce transmission zeros at specified frequencies in the filter’s frequency response. This technique is shown to provide a sharp 3-dB roll-off and steep selectivity skirt with high out-of-band rejection over a wide frequency span. In addition, the 3-dB fractional bandwidth of the filters is shown to be controllable by manipulating the filter’s geometric parameters. Traditional microwave bandpass filters are designed using quarter-wavelength distributed transmission-line resonators that are either end-coupled or side-coupled. The sharpness of the filter response is determined by the number of resonators employed which degrades the filter’s passband loss performance. This results in a filter with a significantly larger footprint which precludes miniaturization. To circumvent these drawbacks the second section describes the development of a novel and compact wideband bandpass filter with the desired characteristics. The quasi-elliptic function filter comprises open-loop resonators that are coupled to each other using a stub loaded resonator. The proposed filter is shown to achieve a wideband 3-dB fractional bandwidth of 23% with much better loss performance, sharp skirt selectivity and very wide rejection bandwidth. The third section describes the investigation of novel ultra-wideband (UWB) microstrip bandpass filter designs. Parametric study enabled the optimization of the filter’s performance which was verified through practical measurements. The proposed filters meet the stringent characteristics required by modern communications systems, i.e. the filters are highly compact and miniature even when fabricated on a low dielectric constant substrate, possess a sharp quasi-elliptic function bandpass response with low passband insertion-loss, and ultra-wide stopband performance. With the rapid development of multi-band operation in modern and next generation wireless communication systems, there is a great demand for single frequency discriminating devices that can operate over multiple frequency bands to facilitate miniaturization. These multi-band bandpass filters need to be physically small, have low insertion-loss, high return-loss, and excellent selectivity. In the fourth section two miniature microstrip dual-band and triple-band bandpass filter designs are explored. A detailed parametric study was conducted to fully understand how the geometric parameters of the filters affected their performance. The optimized filters were fabricated and measured to validate their performance

Investigations on some planar microwave filters

Filters are substantial microwave components. RF/Microwave filters can be implemented using transmission lines. In this thesis microstrip bandpass filters had been designed for RF/microwave applications. Some novel techniques like implementing the open split koch loop resonators, open split square loop resonators, and star shaped multi-mode resonators are implemented in designing the microstrip bandpass filters. Microstrip filters are used in this report to design bandpass filters because of their compact sizes. The goal of this thesis is to investigate on some planar microwave bandpass filters. In this thesis four novel compact bandpass filters has been designed and simulated, specifically three pole koch resonator, seven pole koch resonator, three pole square loop resonator and compact UWB bandpass filter using MMR. The design and simulation of each and every filter is given in detail with including all the required specifications. From the previous research studies it is evident that, to design a good bandpass filter there should be a smooth passband and good stopband with higher insertion loss in the stopband. The four designs which are explained in this thesis has these important factors, which makes these filters useful for the microwave applications

High-Q Multi-band Filters

Recent development of multifunctional communication systems capable of processing large amount of data has triggered the demand for novel payload configurations with advanced filtering functions. To increase the payload flexibility, a large number of multiplexer and filter networks with different frequency plans are usually employed for the transmitting downlink. Multi-band filters are the required function in many cases for minimizing integration complexity and reducing size and mass of space systems. The multi-band filters combine the frequency spectrums of non-contiguous channels before transmitting through antenna beams, and provide sufficient rejection to the frequency spectrums of the adjacent channels, thus maintaining a high signal-to-interference ratio especially in multi-beam frequency-reuse communication systems. Traditional approaches to realize multi-band filters do not offer advantages in terms of size and mass reduction. Multi-mode resonators have the advantage of size reduction; however they are not often used in multi-band applications due to the challenges of operating the multiple modes in prescribed passbands simultaneously. The main research objective of this thesis is to investigate the feasibility of designing multi-band filters based on high-Q multi-mode resonators. Various multi-mode waveguide and dielectric resonators are explored to realize multi-band filters. The proposed multi-band filters do not require junctions and can achieve an equivalent performance with fewer cavities, thus significantly reducing the footprint when compared to traditional approaches. Furthermore, tunable multi-band filters with a constant absolute bandwidth and minimum degradation during the tuning process is investigated and developed. A systematic design approach of designing multi-band filters based on multi-mode resonators is established in this work starting from the coupling-matrix synthesis of the multi-band network. Following that, dual-band filters based on elliptical and rectangular dual-mode resonators are proposed. The two passbands of the dual-band filter are carried by two independent cavity modes and realized by an inline direct-coupled configuration. The inline dual-band filter design can convert to a diplexer structure by modifying the output ports at the end-resonators. To improve near-band frequency selection of both channels, multiple configurations to realize quasi-elliptic dual-band filter functions are proposed. The first quasi-elliptic design is based on a combination of dual-mode and single-mode rectangular resonators resulting in multiple transmission zeros and improved spurious response. The second structure is a side-coupled design based on dual-quadruplet configuration featuring a pair of transmission zeros on each of the passband and a very compact layout. Limitations of the quasi-elliptic design are investigated and modified structures have been proposed with improved RF performances. Triple-band filters are realized by three types of high-Q cavity resonator structures. Each cavity resonator employs triple-modes with resonant frequencies associated with the three passbands. The first design was an elliptical waveguide triple-band filter with an in-line configuration. Each passband of the filter was controlled by a dedicated polarization and represented by an inline direct-coupled set of resonators. The second design was a rectangular-cavity triple-band filter with a folded configuration. The folded configuration overcomes a number of drawbacks from the elliptical in-line design including an improved tunability and ease of assembly. The last design was a triple-band filter design based on dielectric loaded cavity resonators. The unique dielectric resonator structure results in triple-band filters having a very compact size, high Q, and stable thermal response. Further adding tuning capability to the multi-band filter can provide an additional degree of flexibility for the communication payload. A tunable multi-band filter with a constant absolute bandwidth is developed based on combline resonator and requiring only a single tuning element. The performance is demonstrated with an in-house-developed tuning station. It achieves a constant selectivity over a tuning range of 170 MHz and an unloaded Q better than 3000. The novel filter configurations proposed in this thesis promise to be useful not only for satellite payload applications but also for a wide range of wireless base station applications

UWB Technology

Ultra Wide Band (UWB) technology has attracted increasing interest and there is a growing demand for UWB for several applications and scenarios. The unlicensed use of the UWB spectrum has been regulated by the Federal Communications Commission (FCC) since the early 2000s. The main concern in designing UWB circuits is to consider the assigned bandwidth and the low power permitted for transmission. This makes UWB circuit design a challenging mission in today's community. Various circuit designs and system implementations are published in this book to give the reader a glimpse of the state-of-the-art examples in this field. The book starts at the circuit level design of major UWB elements such as filters, antennas, and amplifiers; and ends with the complete system implementation using such modules

Monolithic integrated ceramic waveguide filters

Design techniques for a new class of monolithic integrated high permittivity ceramic rectangular waveguide microwave filters are presented in this thesis. These filters enable a size reduction of 50 % as compared to air filled coaxial resonator filters with the same unloaded Q-factor. Initially, an integrated ceramic rectangular waveguide resonator structure is investigated. It consists of a metal plated high permittivity ceramic rectangular block with Q-factor comparable to transverse electromagnetic (TEM) coaxial resonator but in a much miniaturised volume with good out of band spurious performance. The three dimensional finite element method (FEM) electromagnetic solver HFSSTM is used to analyse the resonant modes, Q-factor and field patterns of the ceramic waveguide resonator. High performance Chebyshev and generalised Chebyshev monolithic integrated ceramic rectangular waveguide filters are designed to meet the stringent electrical requirement for cellular radio base station. Inter-resonator couplings are achieved by placing various through and blind holes in the broad dimension of the waveguide. In the generalised chebyshev filter, both negative and positive cross couplings are introduced to achieve transmission zeros on both sides of the passband. Metal tuning screws are added to the generalised chebyshev design to correct any practical imperfections. The ceramic waveguide filters are excited through coaxial probes placed at the centre of the broad wall of the external resonators. An integrated ceramic rectangular waveguide diplexer design is also presented to be used at mobile base station front end to replace an existing TEM diplexer without degrading electrical performance in a much miniaturised volume. The both filters and the common junction of the diplexer consist of single metal coated ceramic block with various blind and through holes to realize a complex coupling scheme. Finally a low pass ceramic corrugated waveguide filter design is presented to be used along with diplexer at cellular base station to achieve very wide spurious free out of band bandwidth. The miniaturisation techniques discussed in this thesis will provide overall cost reduction for cellular communication systems requiring low loss narrowband bandpass filters

Design and analysis of miniaturized substrate integrated waveguide reconfigurable filters for mm-wave applications.

Doctoral Degree. University of KwaZulu-Natal, Durban.Microwave filters are an integral part of communication systems. With the advent of new technologies, microwave devices, such as filters, need to have superior performance in terms of power handling, selectivity, size, insertion loss etc. During the past decade, many applications have been added to the communication networks, resulting in communication systems having to operate at high frequencies in the region of THz to achieve the stringent bandwidth requirements. To achieve the requirements of the modern communication system, tunability and reconfigurability have become fundamental requirements to reduce the footprint of communication devices. However, the communication systems that are more prevalent such as planar circuits have either a large footprint or are not able to handle large amounts of power due to radiation leakage. In this thesis, Substrate Integrated Waveguide (SIW) technology has been employed. The SIW has the same properties as the conventional rectangular waveguide; hence it benefits from the high quality (Q) factor and can handle large powers with small radiation loss. The Half-mode (HMSIW), Quarter-mode (QMSIW), and Eighth-mode (EMSIW) cavity resonators have been designed and used for the miniaturization of the microwave filters. The coupling matrix method was used to implement a filter that uses cross-coupled EMSIW and HMSIW cavity resonators to improve the selectivity of the filter. Balanced circuit techniques have been used to design the circuits that preserve communication systems integrity whereby the Common Mode (CM) signal was suppressed using Deformed Ground Structure (DGS) and a center conductor patch with meandered line. For the designed dual-band filter, the common mode signal was suppressed to -90 dB and - 40 dB for the first and second passband, respectively. The insertion loss observed is 2.8 dB and 1.6 dB for the first and second passband, respectively. For tunability of the filter, a dual-band filter utilizing triangular HMSIW resonators has been designed and reconfigurability is achieved by perturbing the substrate permittivity by dielectric rods. The dielectric rods’ permittivity was changed to achieve tunability in the first instance, and then the rods’ diameter changed in the second instance. For the lowerband, frequency is tunable from 8.1 GHz to 9.15 GHz, while the upper band is tuned from 14.61 GHz to 16.10 GHz. A second order SIW filter with a two layer substrate was then designed to operate in the THz region. For reconfigurability, Graphene was sandwiched between the Silicon Di-Oxide substrate and the top gold plate of the filter, and the chemical potential of Graphene was then varied by applying a dc bias voltage. With a change in dc voltage the chemical potential of Graphene changes accordingly. From the results, a chemical potential change of 0.1 eV to 0.6 eV brings about a frequency change from 1.289 THz to 1.297 THz

Novel substrate integrated waveguide filters and circuits

The main work in this thesis is to explore novel microwave filters with more compact size and improved performance by taking advantage of new substrate integrated waveguide (SIW) structures, such as the ridge substrate integrated waveguide, half mode substrate integrated waveguide (HMSIW) and SIW with complementary split ring resonators (CSRRs). This thesis therefore presents the following topics: 1. Development of a design strategy to convert from a conventional ridge waveguide configuration with solid walls to the SIW counterpart, and the design of a bandpass filter based on the ridge SIW with the proposed design method. 2. Development of a ridged HMSIW to reduce the physical size of the HMSIW by loading the HMSIW with a ridge, and application of the ridged HMSIW to the design of compact bandpass filters. 3. Development of a broadside-coupled complementary split ring resonator and a capacitively-loaded complementary single split ring resonator to reduce the size of SIW with conventional CSRRs, and application of the proposed modified structures in the design of SIW and HMSIW filters with improved compactness and performance. 4. Investigation of the application of the complementary electric-LC (CELC) resonator in SIW filters with improved stopband performance, and development of a cascaded CELC resonator to further enhance the out-of-band performance

Filtenna design with selectivity enhancement for modern communication system

This thesis presents filtenna design with selectivity enhancement for modern communication systems. Filtennas are designed to simplify the radio frequency (RF) front-end, reduce cost and eliminate signal losses. Two filters, (Filter A and Filter B) and two filtennas, (Filtenna A and Filtenna B) have been designed to overcome a few drawback such as decrease in the peak gain, poor selectivity, increase in feeding area and structure complexity of the existing filtennas. Filtenna A and Filtenna B are designed based on Filter A and Filter B, respectively, by using filter synthesis technique. All designed structures were simulated using Computer Simulation Technology (CST) Microwave Studio and validated through fabrication and measurement of the prototypes. Firstly, an improved technique for creating sharp selectivity of a T-shaped stub bandpass filter (Filter A) is designed. The T-shaped stub is loaded with vertical resonators to produce good selectivity at both edges of the passband at 3.6 GHz. The advantage of this filter is the potential ability to adjust the center frequency and the bandwidth to suit the system demands. Secondly, a novel and compact second order Chebyshev bandpass filter (Filter B) with sharp selectivity is designed to operate at 5.8 GHz. The sharp selectivity is obtained by using U-shaped resonators and Defected Ground Structure which are responsible for the rejection at the higher and lower band edge, respectively. The advantage of this design is size compactness. About 56% area reduction is achieved over the second order Hairpin bandpass filter. Thirdly, the T-shaped stub bandpass filter is synthesized with a microstrip patch antenna to form a T-shaped stub fed filtenna (Filtenna A) with enhanced selectivity. The advantage of this design is that it maintains the same bandwidth as the conventional patch antenna with enhanced gain and good out-of-band suppression. The fourth design involves the synthesis of the second order Chebyshev designed filter with a U-shaped patch (Filtenna B). The designed filtenna operates at 5.8 GHz and has sharp selectivity as no degradation of the peak gain. The superiority of the proposed design over the conventional patch antenna is verified by a 99 % decrease in the out-of-band suppression and a 11.86 % increase in the gain performance. The designed filtennas address the limitations faced by existing filtennas and can be used in Wireless Local Area Network (WLAN) application

Analysis and Design of a Substrate Integrated Waveguide Multi-Coupled Resonator Diplexer

A microwave diplexer achieved by coupling a section of a dual-band bandpass filter onto a section of two single-bands (i.e. transmit and receive) bandpass filters is presented. This design eliminates the need for employing external non-resonant junctions in diplexer design, as opposed to the conventional design approach which requires separate non-resonant junctions for energy distribution. The use of separate non-resonant junctions in diplexer design increases the design complexity, as well as gives rise to bulky diplexer devices. The proposed design also removes the too much reliance on the evaluation of suitable characteristic polynomials to achieve a diplexer. Though the evaluation of complex polynomials to achieve a diplexer is seen as a viable option, the technique is hugely dependent on optimisations which come with loads of uncertainties. This thesis relies on well-established design formulations to increase design reliability, as well as simplicity. A 10-pole (10ᵗʰ order) microwave diplexer circuit has been successfully designed, simulated, manufactured and measured. The measured results have been used to validate the circuit model and the electromagnetic (EM) simulated results. The diplexer is composed of 2 poles from a dual-band bandpass filter, 4 poles from a transmit bandpass filter and the remaining 4 poles from a receive bandpass filter. The design was initially implemented using asynchronously tuned microstrip square open-loop resonators. The EM simulation and the measurement results of the microstrip diplexer were presented and show good agreement with the proposed design theory. The design was also implemented using the substrate integrated waveguide (SIW) technique and results presented and discussed. In comparison to the results achieved with the microstrip diplexer, the EM simulation and the measurement results realised with the SIW diplexer, show that a slightly better insertion loss was attained across both the transmit and the receive channels, respectively