Thesis (MEng) -- Stellenbosch University, 2002. / ENGLISH ABSTRACT: The tuning of coupled-resonator filters is an essential part of its production. It is, however, a time consuming and expensive process. The use of model-based parameter extraction algorithms as a semi-automated tuning procedure is examined. This thesis describes the development of such a procedure. Different models for the filter are compared. The extraction algorithm is based on the least squares method. A more robust procedure is presented which makes the parameter extraction possible, without the availability of good starting values. The algorithm is evaluated using a fourth order single mode direct coupled cavity filter with a centre frequency of 10GHz. The algorithm produces good results and makes the tuning of this type of filter possible in an effective way. It also allows for the diagnosis of the filter, which is a great advantage over the more conventional direct tuning methods. / AFRIKAANSE OPSOMMING: Die instelling van gekoppelde-resoneerderfilters is ’n essensiele deel van die vervaardiging daarvan. Dit is egter ’n tydsame en duur proses. Die gebruik van modelgebaseerde parameteronttrekking as ’n semi-geoutomatiseerde instellingsproses is ondersoek. Hierdie tesis beskryf die ontwikkeling van so ’n prosedure. Verskillende modelle vir die filter is vergelyk. Die onttrekkingsalgoritme is gebaseer op die kleinste-kwadraatmetode. ’n Meer robuuste metode word voorgestel wat die parameteronttrekking moontlik maak, sonder dat daar goeie beginwaardes beskikbaar is. Die algoritme word geevalueer aan die hand van ’n vierde orde enkelmodus direkgekoppelderesoneerderfilter met ’n senterfrekwensie van 10GHz. Die algoritme lewer goeie resultate en maak die instelling van hierdie tipe filter moontlik op ’n effektiewe wyse. Die metode kan ook gebruik word vir die diagnose van die filter wat ’n groot voordeel is bo meer konvensionele direkte instellingsmetodes.

This dissertation contains two significant investigations. One is the development of the broadband microwave bandpass filters with high out-of-band performance. The other is the development of low-loss hybrids. These researches are parts of the National Aeronautic and Space Administrator (NASA)s mission to explore the universe. The former is focused on the techniques used in microstrip line bandpass filter design that help achieving both low in-band insertion loss and high out-of-band attenuation level. Moreover, these filters achieve very broadband out-of-band attenuation bandwidth. These techniques are related to the improvement of stepped impedance resonators, coupling between resonators and effective methods to allocate transmission zeros to suppress filters out-of-band spurious responses. The later is focused on the techniques used in planar magic-T designs such that the developed magic-T obtains high isolation between port E (difference port) and port H (sum port). Moreover, it obtains low-loss and broadband characteristics. These techniques are related to the development of the low-loss broadband microstrip-toslotline (MS-to-SL transition and the magic-T with a highly symmetric structure. The theoretical analysis and experimental measurements have been performed. The experimental results of both the filter and magic-T researches show significant improvement over their prior state-of-the-art designs by number of magnitude. The designs also reduce fabrication complexity.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2008. / The design of an aperture-coupled coaxial diplexer for R-band, is presented. To improve the ease of tuning, a tuning procedure for the diplexer with the aid of a MATLAB application with graphical user interface, is developed. Final experimental results show good agreement between the circuit model and the physical structure. Final measurements of the diplexer structure achieved 18.83 dB and 21.52 dB return loss in the lower and upper frequency band respectively and insertion loss of 0.58 dB and 0.61 dB was measured for the two frequency bands. Isolation were measured as 74 dB at 2.01 GHz and 84 dB at 2.17 GHz The accuracy of two techniques for determining coupling coefficients in coaxial and waveguide resonators are investigated. One method is the Eigenmode Method for determining the coupling coefficients in a physical resonator and the other the circuit model representation, utilising inverters to represent the coupling between resonators. Results showed that marked differences occur when using the three different inverter configurations to enable filter dimensioning for a given coupling coefficient. Four waveguide filters, utilising posts and irises respectively, are designed using dimensions obtained from the three inverter configurations as well as the Eigenmode method for a certain coupling coefficient.

Integration of more communication standards in one microwave wireless device created a demand on developing compact, low-cost, and robust multi-band microwave components. This dissertation presents three studies for designing miniaturized and multi-band circuits that can be used for multi-band radio frequency (RF) front-ends. These three studies are the design of dual-band and tunable bandpass filters as well as dual- and triple-band equal-split power dividers/combiners. The dual-band filter is based on split ring resonators and double slit complemantary split ring resonators. A dual-band prototype three-stage Chebyshev filter, with a fractional bandwidth of 2% at 0.9 GHz and a fractional bandwidth of 3% at 1.3 GHz with equal-ripple of 0.4 dB at both passbands, is presented. The overall size of the dual-band filter is three times smaller compared to edge-coupled microstrip filters. Good out-of-band signal rejection (< 38 dB) and insertion losses (< 4.9 dB for the lower passband and <2.7 dB for the upper passband) are achieved. The proposed tunable filter is designed from varactor loaded split ring resonators. The size of the tunable filter is reduced by a factor of 3.5 compared to quarter wavelength-based coupled line filters.The power divider is based on composite right- and left-handed transmission lines. Dual-band and triple-band power divider prototypes are designed, fabricated, and tested. The passbands of the triple-band Wilkinson power divider are centered at 0.8 GHz, 1.3 GHz, and 1.85 GHz, and the passbands of the dual-band Wilkinson power divider are centered at 0.7 GHz, 1.5 GHz. The triple-band divider has a length of 0.66 wavelength in the substrate and its size is reduced to 3/4 of right-handed transmission line-based Wilkinson power dividers. The dual-band power divider has wide fractional bandwidths ( 20% at the lower passband and 41% at the upper passband). Excellent input matchings (input return losses < 29 dB), output matchings (output return losses < 23 dB), and output port isolations (< 24 dB) are achieved at all passbands of the power dividers. The proposed filters and power dividers are compact and low-cost, and are promising candidates for the miniaturization and cost-reduction of multi-band microwave wireless system.

Superconducting microelectronics (SME) technology has the potential of realizing very high speed digital receivers capable of performing direct digitization of radio frequency signals with very low power consumption. The SME receiver is implemented on a single chip using Niobium based low temperature superconductive (LTS) Josephson Junction (JJ) technology by HYPRES. Analogue RF filters are still required at the receiver front end and are key components of the overall superconductor digital receiver. SME receivers usually require two types of RF filters; a wideband bandpass filter and a bandstop filter (a notch filter). The notch filter is required to eliminate interference and unwanted signals in the passband. In this thesis, design of highly miniaturized lumped element wideband and bandstop filters is investigated and some challenges are addressed. The filters are fabricated by the HYPRES process and therefore can be integrated with the SME receiver on the same chip. In a wideband filter, the coupling between the adjacent resonators is high. Achieving such a strong coupling is one of the challenges of designing wideband filters. The wideband filters realized with distributed elements usually suffer from very low spurious frequency. As the bandwidth of the filter becomes wider, the spurious peak of the second harmonic gets closer to the passband of the filter. In the first part of this work, the possibility of realizing lumped element superconducting bandpass filters (BPF) with a relative bandwidth of 80% is investigated. In the second part of the thesis, design and realization of lumped element superconducting bandstop filters (BSF) is discussed. The challenge for designing a bandstop filter is providing a good match over a wide frequency range. So narrowband inverters cannot be used. Instead, usually λ/4 matched transmission lines provide 90° phase shift between the resonators of a notch filter. The possibility of replacing the long transmission line with other means or eliminating the inverters and using both shunt and series resonators are investigated. Having both series and shunt resonators introduces some new challenges that are addressed in the thesis and discussed thoroughly. A tunable notch resonator is presented. The tunability is provided by a superconducting MEMS varactor that is realized in our group by doing some post processing on the device fabricated by HYPRES. The tunability range of the device at cryogenic temperatures is investigated. A 3-pole tunable BSF is also designed that uses the same tunable resonators. The tunability of the filter is investigated through simulation.

This thesis presents the first fully tunable substrate integrated waveguide (SIW) filter implemented with PIN diodes and RF MEMS switches. The methodology for tuning SIW filters is explained in detail and is used to create three separate designs. Each SIW cavity is tuned by perturbing via posts connecting or disconnecting to/from the cavity's top metal layer. In order to separate the biasing network from the SIW filter, a three-layer PCB is fabricated using Rogers RT/duroid substrates. The first tunable design utilizes the Philips BAP55L PIN diode. This two-pole filter provides six frequency states ranging from 1.55 GHz to 2.0 GHz. Fractional bandwidth ranges from 2.3 percent – 3.0 percent with insertion loss and return loss better than 5.4 dB and 14 dB respectively for all frequency tuning states. The second tunable design utilizes the Radant RMSW-100 MEMS switch, providing six states ranging from 1.65 GHz to 2.1 GHz. Fractional bandwidth for this filter varies from 2.5 percent - 3.0 percent with insertion loss and return loss better than 12.4 dB and 16 dB respectively for all states. The third design utilizes the OMRON 2SMES-01 RF MEMS relay, providing fourteen states ranging from 1.19 GHz to 1.58 GHz. Fractional bandwidth ranges from 3.6 percent - 4.4 percent with insertion loss and return loss better than 4.1 dB and 15 dB respectively for all frequency states. Two of the three designs (Philips PIN diode and OMRON MEMS) produced good results validating the new SIW filter tuning methodology. Finally, to illustrate the advantage of microstrip planar structures integrated with SIW structures, low pass filters (LPFs) are implemented along the input and output microstrip-to-SIW transition regions of the tunable SIW filter. With minimal change to the overall filter size, this provides spurious suppression for the additional resonant modes inherently present in waveguide structures. The implemented design utilizes the same OMRON MEMS tunable SIW filter specifications. This two-pole tunable filter provides the same performance as the previous OMRON MEMS design with exception to an added 0.7 dB insertion loss and spurious suppression of -28 dB up to 4.0 GHz for all frequency tuning states.

This thesis presents new procedures and guidelines for the design of advanced microwave filters. Practical implementations include dual-band, multi-band and ultra-wideband filters for applications in wireless and telecommunication systems. The traditional narrow-band coupling-matrix approach is extended to include multiple and out-of-band resonances, which allows the model to accurately predict and design the filter performance over a wide frequency range. A variety of coupling scenarios are presented and their advantages over current designs demonstrated. The approach includes the placement of transmission zeros in order to obtain quasi-elliptic responses, improve stopband performance and generate sharp transitions from the passbands to their adjacent stopbands. In order to contribute to filter miniaturization, bandwidth enhancement and manufacturing simplicity, a new class of step-impedance resonators is proposed and their harmonic tuning properties utilized. Advanced coupling configurations are realized, which are applicable to single- or multi-band filter operations with bandwidths ranging from a few percent to fifty percent. Moreover, the developed design procedures are applied to ultra-wideband filters covering bandwidths of up to 150 percent. All filters designed with the procedures developed in this thesis are validated by several commercially available electromagnetic field-solver packages, in-house numerical codes and/or measurements. The excellent overall agreement between computational and experimental results verifies the advanced filter design procedures presented in this thesis.

It is often necessary to operate a number of radio communication channels from a single control room without time-sharing between the various channels. Here it is necessary to operate a number of transmitters and receivers simultaneously from the same base station or mobile unit without interference. The best method to achieve this has been found in the use of filters inserted in the transmission line between the antenna and the transmitter(s) on one hand and the receiver(s) on the other hand. The basic unit employed in the design of microwave filters is usually a cavity resonator of which the most important factors are the Q, insertion loss and resonant frequency. However, a problem which frequently arises with cavity resonators is the accurate determination of these resonant characteristics complicated by the presence of coupling port, materials and various design and geometrical deviations. Such cavities have been investigated in several cases and the results have been generalised, but this investigation has been conducted to examine thoroughly most of the problems being met in present practice. Design and development of some common resonant structures are considered. Emphasis is placed on solutions found to special problems especially regarding complicated boundary conditions. Furthermore, investigation includes methods for optimising resonant parameters such as insertion loss and Q, trading of insertion loss with coupled cavity selectivity, frequency tuning and compensation for frequency variations due to wide ranges of operating temperatures. By comparing Q values obtained in practice with theoretical values, it has been possible to establish an appropriate Q loss budget to as to facilitate accurate prediction of coupled cavity unloaded Q. A satisfactory agreement between theory and practice has been obtained. By application of the results of theoretical analysis and experiment, it is shown that microwave filters can be designed to have a desired insertion loss and off-band attenuation slope. Steps leading to designs of any number of cascaded cavities in a two-port network and, subsequently, multi-port networks are discussed in detail.

Superconducting microelectronics (SME) technology has the potential of realizing very high speed digital receivers capable of performing direct digitization of radio frequency signals with very low power consumption. The SME receiver is implemented on a single chip using Niobium based low temperature superconductive (LTS) Josephson Junction (JJ) technology by HYPRES. Analogue RF filters are still required at the receiver front end and are key components of the overall superconductor digital receiver. SME receivers usually require two types of RF filters; a wideband bandpass filter and a bandstop filter (a notch filter). The notch filter is required to eliminate interference and unwanted signals in the passband. In this thesis, design of highly miniaturized lumped element wideband and bandstop filters is investigated and some challenges are addressed. The filters are fabricated by the HYPRES process and therefore can be integrated with the SME receiver on the same chip. In a wideband filter, the coupling between the adjacent resonators is high. Achieving such a strong coupling is one of the challenges of designing wideband filters. The wideband filters realized with distributed elements usually suffer from very low spurious frequency. As the bandwidth of the filter becomes wider, the spurious peak of the second harmonic gets closer to the passband of the filter. In the first part of this work, the possibility of realizing lumped element superconducting bandpass filters (BPF) with a relative bandwidth of 80% is investigated. In the second part of the thesis, design and realization of lumped element superconducting bandstop filters (BSF) is discussed. The challenge for designing a bandstop filter is providing a good match over a wide frequency range. So narrowband inverters cannot be used. Instead, usually λ/4 matched transmission lines provide 90° phase shift between the resonators of a notch filter. The possibility of replacing the long transmission line with other means or eliminating the inverters and using both shunt and series resonators are investigated. Having both series and shunt resonators introduces some new challenges that are addressed in the thesis and discussed thoroughly. A tunable notch resonator is presented. The tunability is provided by a superconducting MEMS varactor that is realized in our group by doing some post processing on the device fabricated by HYPRES. The tunability range of the device at cryogenic temperatures is investigated. A 3-pole tunable BSF is also designed that uses the same tunable resonators. The tunability of the filter is investigated through simulation.

Thesis (Ph.D.). / Written for the Dept. of Electrical & Computer Engineering. Title from title page of PDF (viewed 2009/06/10). Includes bibliographical references.