Ultra-miniaturized and high-Q embedded loaded coaxial cavity resonators and filters

Abstract

The rapid development in modern wireless communication systems and remote sensing and their applications towards portable and compact systems are inevitable. Microwave filters are critical components in the RF front-ends for band selection and unwanted signal rejection. These filters generally occupy a large area in the RF front-ends. In addition, RF filters’ in-band losses significantly impact the overall system performance, such as noise figure and dynamic range. Thus, compact and high-quality factor RF filters are greatly desirable. Embedded coaxial substrate integrated waveguide filters have shown great potential compared to competing technologies in terms of high-Q, compact size, planar form, and wide spurious free region. This dissertation develops the theory and design of ultra-miniaturized high-Q embedded loaded coaxial substrate integrated waveguide filters with potential application in modern communication systems. In this research, high-Q and compact embedded loaded coaxial substrate integrated waveguide (ELCSIW) resonators are designed and investigated. Integrating air-filled parallel plate capacitive loading into an embedded coaxial cavity enables the achievement of a highly miniaturized resonator with a 99 % miniaturization factor compared to the conventional substrate integrated waveguide resonator. In-depth analysis of the resonant frequency, miniaturization factor, and unloaded quality factor is presented. A second-order embedded loaded coaxial substrate integrated waveguide filter with a resonant frequency of 1.1 GHz is developed and implemented. The filter obtains a simulated insertion loss of 0.26 dB and a wide spurious free stopband up to seven times its resonant frequency. The overall ELCSIW filter size is 0.07λg × 0.26λg. The proposed ELCSIW filter demonstrates superiority in compactness, quality factor, and stopband rejection compared to other miniaturized counterparts. Moreover, the proposed ELCSIW filter showed a high power handling capability of 113 Watts through theoretical investigation. A mixed (electric and magnetic) inter-resonator coupling mechanism is developed and integrated into the ELCSIW filter to generate a transmission zero to improve the stopband rejection. The arbitrary location of the transmission zero is determined by the intensity of each coupling mechanism. Moreover, a novel post-fabrication frequency tuning mechanism for the embedded loaded coaxial substrate integrated waveguide filter is developed. The tuning mechanism is constructed of additional embedded capacitive loading strip lines that can be altered to compensate for fabrication tolerances. The circuit model and the working mechanism of the tuning circuit are explained and illustrated. The novel tuning method shows a capability of a post-fabrication tuning range of 10 % for the proposed ELCSIW filter. This research illustrates the feasibility of designing and integrating low-loss, compact, low-cost, and lightweight filters for future communications and radar systems.