Tunable Superconducting Microwave Filters

Adaptive microwave systems can benefit from the use of low loss tunable microwave filters. Realizing these tunable filters that show low loss characteristics can be very challenging. The proper materials, tuning elements, and filter designs need to be considered when creating a low loss tunable filter. The integration of low loss microelectromechanical systems (MEMS) and superconducting circuits is one method of achieving these types of tunable filters. The thesis introduces new multi-layer low temperature superconducting (LTS) filters and diplexers and novel topologies for tunable filters and switched multiplexers. An efficient method of designing such filters is proposed. A fabrication process to monolithically integrate MEMS devices with high temperature superconducting (HTS) circuits is also investigated in this thesis. The reflected group delay method, usually used for filter tuning, is further developed for use in designing microwave filters. It is advantageous in the design of filters to have electromagnetic simulation results that will correlate well to the fabricated microwave filters. A correction factor is presented for use with the reflected group delay method so the group delay needs to be matched to the appropriate value at the center frequency of the filter and be symmetric about the center frequency of the filter. As demonstrated with an ideal lumped element filter, the group delay method can be implemented when a closed form expression for the circuit is not known. An 8-pole HTS filter design and an 8-pole multi-layer LTS filter design demonstrate the use of the reflected group delay method. Low temperature superconducting filters, couplers and diplexers are designed and fabricated using a multilayer niobium fabrication process traditionally used for superconducting digital microelectronics. The feasibility of realizing highly miniaturized microwave niobium devices allows for the integration of superconducting digital microelectronics circuits and analog microwave devices on a single chip. Microwave devices such as bandpass filters, lowpass filters, bandstop filters, quadrature hybrids, and resistive loads are all demonstrated experimentally. New tunable filter designs are presented that can make use of MEMS switches. A manifold-coupled switched multiplexer that allows for 2^N possible states is presented. The tunable multiplexer has N filters connected to two manifolds and has embedded switches, which detune certain resonators within the filters to switch between ON and OFF states for each channel. The new concept is demonstrated with a diplexer design and two 3-pole coplanar filters. The concept is further developed through test results of a fabricated HTS triplexer and electromagnetic simulations to demonstrate a superconducting manifold-coupled switched triplexer. Another filter design is presented that makes use of switches placed only on the resonators of the filters. This filter design has N possible states and the absolute bandwidth can be kept constant for all N states. Finally, the integration of HTS circuits and MEMS devices is investigated to realize low loss tunable microwave filters. The hybrid integration is first performed through the integration of an HTS microstrip filter and commercially available RF MEMS switches. A fabrication process to monolithically integrate MEMS devices and high temperature superconducting circuits is then investigated. The fabrication process includes a titanium tungsten layer, which acts as both a resistive layer and an adhesion for the dielectric layer, an amorphous silicon dielectric layer, a photoresist sacrificial layer, and the top gold layer. The fabrication process is built up on a wafer with a thin film of a high temperature superconducting material covered with a thin film of gold. Several processes are tested to ensure that the superconducting properties of the thin film are not affected during the MEMS fabrication process

RF MEMS-Based Frequency Dependent Power Limiters

Microwave power limiters are passive devices that are widely used to protect receivers from large interfering signals. Conventional radio frequency (RF) power limiters attenuate both desired and undesired signals in the entire frequency band. The need for frequency-dependent power limiters (FDPLs) arises in cases where the system dynamic range is frequency-dependent and the threshold level is dependent on the frequency band. RF micro-electro-mechanical systems (RF MEMS) switches self-actuate under high RF power and are well-known for their linearity and very low insertion loss. A FDPL can benefit from all the advantages of RF MEMS switches, particularly the linearity. In this study, a novel approach to FDPLs is proposed. RF MEMS switches are integrated with bandpass filters to form a power limiter where the output RF power is limited to specific levels that can vary with the frequency band. Both non-planar and planar versions of FDPLs are presented using circulators and hybrids, and RF MEMS-based power limiters are analyzed theoretically and experimentally for one frequency band. The limiter attenuates the high power signal only within the bandwidth of the integrated filter. The design of the proposed power limiter is expanded to achieve power limiting for various frequency bands. The flatness of the threshold level can be set to the desired value by controlling the return loss of the filters used in the FDPL circuit. Measured results for an FDPL circuit are presented, demonstrating that the limiting power level can be controlled by adjusting the dc bias of the MEMS switches. The commercially available electrostatically actuated switches OMRON and Radant are employed for the realization of the FDPL. Additionally, tunable FDPL circuits are fabricated and measured, demonstrating the feasibility of realizing adaptive frequency-dependent power limiters. In order to improve the controllability of the proposed MEMS-based FDPL circuits, both electrostatically-actuated and thermally-actuated switches are investigated theoretically and experimentally for use in power limiter applications. It is concluded that MEMS thermally- actuated switches provide better controllability of the self-actuation RF power level through adjustment of the bias voltage to the thermal actuator. Additionally, a novel concept for realizing an RF power limiter for protecting superconductor digital receivers is proposed. A lumped-element niobium-based filter is used as a protection circuit. It consists of lumped-element resonators formed using spiral inductors and metal- insulator-metal capacitors integrated on a multilayer niobium process. The circuit operates as a bandpass filter at low power levels, allowing RF signals to pass, and as a reflector at high power levels. The lumped-element filter circuit is studied in detail to explain the performance of the filter at high power levels. It is concluded that some of the lumped-element inductors switch from being inductors when operating at low power levels to being capacitors when operating at high power levels. When the lumped-element inductors switch to capacitors, the filter circuit that consists of L-C resonators switches to a circuit that consists of capacitors, causing the input power to be reflected back. Both theoretical and experimental results are presented to verify this phenomenon. In addition to applications in power limiters, the concept can be employed to realize transmit/receive (T/R) switches in order to isolate the transmit circuit from the receive circuit

Tunable microwave filters using ferroelectric thin films

Frequency agile microwave devices based on Barium Strontium Titanate (BST) thin films have gained a lot of interest in recent years. The frequency agility of the ferroelectric devices is based on the external DC electric field controlled permittivity of BST thin film. In this research work, several tunable microwave filters incorporating BST thin film varactors operating in a frequency range between 1 GHz and 25 GHz are designed, tested and analysed. A lumped element lowpass filter incorporating integrated meander line inductors and BST parallel plate capacitors is implemented on a high resistivity silicon substrate and demonstrates 32.1 % tuning of the cut-off frequency at 15 V. A combline bandpass filter employing integrated BST parallel plate varactors as tuning elements is implemented on a MgO substrate and shows a reasonable tuning from about 8 GHz to 12 GHz with 10 V bias of only one resonator. Two pole and four pole coupled resonator bandpass filters with discrete BST or GaAs varactors as tuning elements are implemented in a frequency range of 1 - 3 GHz. The filters based on BST parallel plate capacitors show an insertion loss in line with the GaAs filters, which is also the lowest insertion loss of BST filters ever reported. Future work on improving the BST film and metal film loss at tens of gigahertz range is also discussed

Nonlinear mechanisms in passive microwave devices

Premi extraordinari doctorat curs 2010-2011, àmbit d’Enginyeria de les TICThe telecommunications industry follows a tendency towards smaller devices, higher power and higher frequency, which imply an increase on the complexity of the electronics involved. Moreover, there is a need for extended capabilities like frequency tunable devices, ultra-low losses or high power handling, which make use of advanced materials for these purposes. In addition, increasingly demanding communication standards and regulations push the limits of the acceptable performance degrading indicators. This is the case of nonlinearities, whose effects, like increased Adjacent Channel Power Ratio (ACPR), harmonics, or intermodulation distortion among others, are being included in the performance requirements, as maximum tolerable levels. In this context, proper modeling of the devices at the design stage is of crucial importance in predicting not only the device performance but also the global system indicators and to make sure that the requirements are fulfilled. In accordance with that, this work proposes the necessary steps for circuit models implementation of different passive microwave devices, from the linear and nonlinear measurements to the simulations to validate them. Bulk acoustic wave resonators and transmission lines made of high temperature superconductors, ferroelectrics or regular metals and dielectrics are the subject of this work. Both phenomenological and physical approaches are considered and circuit models are proposed and compared with measurements. The nonlinear observables, being harmonics, intermodulation distortion, and saturation or detuning, are properly related to the material properties that originate them. The obtained models can be used in circuit simulators to predict the performance of these microwave devices under complex modulated signals, or even be used to predict their performance when integrated into more complex systems. A key step to achieve this goal is an accurate characterization of materials and devices, which is faced by making use of advanced measurement techniques. Therefore, considerations on special measurement setups are being made along this thesis.Award-winningPostprint (published version

Substrate Integrated Waveguide (SIW) and Superconducting Filters

Substrate integrated waveguides (SIW) provide an excellent compromise between size and loss reduction for applications in planar circuits. SIW filters provide a better Q-factor than microstrip filters and a significant reduction in size compared to waveguide filters. The use of multi-band filters has become increasingly more common because they provide the opportunity to reduce the total footprint in both RF transmitters and receivers. This thesis investigates the design process of a single-band quasi-elliptic and dual-band SIW filter. We use several methods to design the single-band SIW, and compare the simulated results of each. These filters are designed on 0.508mm thick Rogers4003C substrate, fabricated, and measured. The introduction of negative cross-coupling in SIW structures is achieved by using etched coplanar waveguide (CPW) lines. This negative cross-coupling allows for the introduction of transmission zeros in both designed filters. We carefully investigate the transition technology to ensure that we achieve a wideband match between microstrip and SIW. The thickness of the substrate provides some challenges in the matching, so we take extra consideration to overcome this. The second part of this thesis explores the design of lumped element superconducting bandpass filters. When designing filters in the kHz and MHz range, several challenges arise. The first is the ability to use certain software: Sonnet and HFSS both have a limited ability to simulate low-frequency components. More specifically, Sonnet demonstrates an inability to accurately simulate inductors, while simulation times in HFSS are prohibitively long. Momentum thus proves to be the best EM simulator for this task. The second challenge is the need to miniaturize these filters. At such low frequencies, the filter’s footprint is quite large, therefore the reduction in size is extremely important. We implement traditional methods, such as stacked spiral inductors and vertically integrated capacitors, and achieve further size reduction by modifying the circuit topology to reduce the components with the largest footprints. We also introduce transmission zeros to improve the upper and lower band rejection. We then design a three-pole classical Chebyshev filter and a three-pole quasi-elliptic filter that uses a miniaturized circuit topology. Finally, we design a 10% six-pole superconducting slotline resonator filter. Slotline resonators provide an excellent quality factor, even at higher frequencies. A CPW-to-slotline transition is implemented so that the device can be measured using a ground-signal-ground probe. The resonators implemented use dual-spiral inductors and interdigital capacitors. This allows for flexibility when choosing the resonant frequency. All superconducting filters are fabricated using the MIT-Lincoln Lab (MIT-LL) multilayer niobium fabrication process

Modeling and design of superconducting microwave passive devices and interconnects

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1994.Includes bibliographical references (p. 157-163).by Laurence H. Lee.Ph.D

Reconfigurable and multi-functional antennas

This thesis describes a research into multi-frequency and filtering antennas. Several novel antennas are presented, each of which addresses a specific issue for future communication systems, in terms of multi-frequency operation, and filtering capability. These antennas seem to be good candidates for implementation in future multiband radios, cognitive radio (CR), and software defined radio (SDR). The filtering antenna provides an additional filtering action which greatly improves the noise performance and reduces the need for filtering circuitry in the RF front end. Two types of frequency reconfigurable antennas are presented. One is tunable left-handed loop over ground plane and the second is slot-fed reconfigurable patch. The operating frequency of the left handed loop is reconfigured by loading varactor diodes whilst the frequency agility in the patch is achieved by inserting switches in the coupling slot. The length of the slot is altered by activating the switches. Compact microstrip antennas with filtering capabilities are presented in this thesis. Two filtering antennas are presented. Whilst the first one consists of three edge-coupled patches, the second filtering antenna consists of rectangular patch coupled to two hairpin resonators. The proposed antennas combine radiating and filtering functions by providing good out of band gain suppression

Design, Modelling and Implementation of Several Multi-Standard High Performance Single-Wideband and Multi-Wideband Microwave Planar Filters

The objectives of this work are to review, investigate and model the microwave planar filters of the modern wireless communication system. The recent main stream of microwave filters are classified and discussed separately. Various microwave filters with detailed applications are investigated in terms of their geometrical structures and operational performances. A comprehensive theoretical study of microwave filters is presented. The main types of microwave filters including the basic low-pass filters such as Butterworth and Chebyshev filters are fully analysed and described in detail. The transformation from low-pass prototype filters to high-pass filters, band-pass filters and band-stop filters are illustrated and introduced. Research work on stepped impedance resonator (SIR) and asymmetric stepped impedance resonator (ASIR) structure is presented. The characteristics of λg/4, λg/2 and λg (λg is the guided wavelength of the fundamental frequency in the free space) type SIR resonators, and the characteristic of asymmetric SIR resonator are categorized and investigated. Based on the content mentioned above, novel multi-standard high performance asymmetric stepped impedance resonator single-wideband and dual-wideband filters with wide stopbands are proposed. The methodologies to realize wide passband and wide stop-band filters are detailed. In addition, multi-standard high performance triplewideband, quadruple-wideband and quint-wideband filters are suggested and studied. The measurement results for all prototype filters agree well with the theoretical predictions and simulated results from Ansoft HFSS software. The featured broad bandwidths over single/multiple applicable frequency bands and the high performances of the proposed filters make them very promising for applications in future multistandard wireless communication