Substrate Integrated Waveguide (SIW) and Superconducting Filters

Abstract

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.