Browsing by Subject "Resonators."

Now showing 1 - 4 of 4

Open Access
Design of novel printed microwave band-reject filters using split-ring resonator and complementary split-ring resonator
(2008) Öznazlı, Volkan
Filters are one of the fundamental microwave components used to prevent the transmission or emission of signals with unwanted frequency components. In general, they can be considered as an interconnection of resonator structures brought together to accomplish a desired frequency response. Up to GHz frequencies, these resonator structures are usually constructed using lumped elements such as discrete capacitors and inductors. At microwave frequencies, discrete components lose their normal charcteristics and resonators can be realized using distributed structures like quarter- or half-wavelength transmission line stubs. However, filters built using this approach are generally big, especially when high frequency selectivity is desired. Recently, sub-wavelength structures, namely split-ring resonator (SRR) and complementary split-ring resonator (CSRR), have attracted the attention of many researchers. Interesting properties of the periodic arrangements of these structures have led to the realization of left-handed materials. Furthermore, high-Q characteristics of these structures enabled the design of highly frequencyselective devices in compact dimensions. In this thesis, these two resonator structures are investigated in detail. A deep exploration of their resonance mechanisms and transmission properties is provided along with a brief survey of related literature. However, the main focus of the thesis is the design of band-reject filters based on these resonator structures. Experimental results based on measuring the scattering paramaters of fabricated prototypes are supported with computer simulations. Band-reject filters based on SRR and CSRR are compared and discussed. It is observed that both filter types have some advantages and disadvantages which make them suitable for different applications. Finally, an electronically switchable split-ring resonator structure based on PIN diodes is presented. It is demonstrated that by employing microwave PIN diodes across the slits of an SRR, the magnetic response of a SRR particle can be eliminated. This leads to the design of filters whose rejection bands can be removed electronically
Open Access
Experimental demonstration of transmission enhancement through subwavelength apertures at microwave frequencies
(2012) Ateş, Damla
Metamaterials are artificial materials with novel electromagnetic characteristics. They are used in many applications including imaging, super lenses, cloaking, transmission enhancement, beaming and recently in nano applications. One of the major building blocks is the split ring resonators (SRR). We can construct metamaterials by using a single or an array of the SRRs. In this thesis, enhanced transmission through subwavelength apertures, which is one of the applications of metamaterials, is obtained by using various split ring resonators configurations. We demonstrated transmission enhancement with Connected Split Ring Resonators (CSRRs), Omega-like Split Ring Resonators and Stack-like Split Ring Resonators through circular and rectangular subwavelength apertures experimentally and numerically at the microwave frequencies. We report the highest experimental transmission enhancement results in the literature so far. Besides high factors, we also obtained multi-peak resonant characteristics with Stack-like SRR designs. Furthermore, we analyzed these various SRR samples numerically in order to understand the resonance behavior. We also discuss the effects of shorting the loops, omitting the components of the SRRs and aperture geometry to the resonance frequency. Finally, we applied Tight Binding methods to analyze the multi-peak characteristics of the Stack-like SRR design.
Open Access
Novel implantable distributively loaded flexible resonators for MRI
(2011) Gökyar, Sayım
Magnetic resonance imaging (MRI) is an enabling technology platform for imaging applications. In MRI, the imaging frequency falls within the radio frequency (RF) range where the tissue absorption of electromagnetic power is conveniently very low (e.g., compared to X-ray imaging), making MRI medically safe. As a result, MRI has evolved into a major imaging tool in medicine. However, in MRI, it is typically difficult to receive a magnetic resonance signal from tissue near a metallic implant, which hinders imaging of the implant device neighborhood to observe, monitor, and make assessment of the recovery and tissue compatibility. This can be accomplished by using locally resonating implants, but such implantable local resonators compatible with MRI that simultaneously feature reasonable chip size are currently not available (although there are some MRI-guided catheter applications). In this thesis, we proposed and developed a new class of implantable chip-scale local resonators that operate at radio frequencies of MRI, despite their small size, for the purposes of enhancing the signal-to-noise ratio (SNR) and thus the resolution in their vicinity. Here we addressed the scientific challenge of achieving low resonance frequency while maintaining chip-scale size suitable for potential MR-compatible implants. Using only biocompatible materials (gold, nitrides, and silicon or polyimide) within a substantially reduced footprint (miniaturized by 2 orders of magnitude), we demonstrated novel chip-scale designs based on the basic concept of split ring resonators (SRRs). Different than classical SRRs or those loaded with lumped elements (e.g., thin-film lumped loading), however, in our designs we loaded the SRR geometry in a distributive manner with a micro-fabricated dielectric thin-film layer to increase effective capacitance. For a proof-of-concept demonstration, we fabricated 20 mm ´ 20 mm resonators that operate at the resonance frequency of 130 MHz (compatible with 3 T MRI system) when distributively loaded with the capacitive film, which would otherwise operate around 1.2 GHz as a classical SRR of the same size if not loaded. It is worth noting that this resonance frequency of 130 MHz would normally require a classical SRR of 20 cm ´ 20 cm, a chip size 100-fold larger than ours. Designing and fabricating flexible thin-film resonators, we also showed that this architecture can be tuned by bending and is appropriate for non-planar surfaces, which is often the case for in vivo imaging. The phantom images indicated that, depending on the resonator configuration, these novel self-resonating structures increase SNR of the received signal by a maximum factor of 4 to 150 and over an enhancement penetration up to 10 mm into the phantom. This corresponds to a resolution enhancement in the 2D image by a factor of 2 to 12, respectively, under the same RF power. These in vitro experiments prove that it is possible to operate our local resonators at reduced frequencies via the help of distributive loading on the same chip. These findings suggest that proposed implantable resonator chips make promising candidates for self-resonating MR-compatible implants.
Open Access
Novel wireless RF-bioMEMS implant sensors of metamaterials
(2010) Melik, Rohat
Today approximately one out of ten patients with a major bone fracture does not heal properly because of the inability to monitor fracture healing. Standard radiography is not capable of discriminating whether bone healing is occurring normally or aberrantly. To solve this problem, we proposed and developed a new enabling technology of implantable wireless sensors that monitor mechanical strain on implanted hardware telemetrically in real time outside the body. This is intended to provide clinicians with a powerful capability to asses fracture healing following the surgical treatment. Here we present the proof-of-concept in vitro and ex vivo demonstrations of bio-compatible radio-frequency (RF) micro-electro-mechanical system (MEMS) strain sensors for wireless strain sensing to monitor healing process. The operating frequency of these sensors shifts under mechanical loading; this shift is related to the surface strain of the implantable test material. In this thesis, for the first time, we developed and demonstrated a new class of bio-implant metamaterial-based wireless strain sensors that make use of their unique structural advantages in sensing, opening up important directions for the applications of metamaterials. These custom-design metamaterials exhibit better performance in remote sensing than traditional RF structures (e.g., spiral coils). Despite their small size, these meta-sensors feature a low enough operating frequency to avoid otherwise strong background absorption of soft tissue and yet yield higher Q-factors (because of their splits with high electric field density) compared to the spiral structures. We also designed and fabricated flexible metamaterial sensors to exhibit a high level of linearity, which can also conveniently be used on non-flat surfaces. Innovating on the idea of integrating metamaterials, we proposed and implemented a novel architecture of ‘nested metamaterials’ that incorporate multiple split ring resonators integrated into a compact nested structure to measure strain telemetrically over a thick body of soft tissue. We experimentally verified that this nested metamaterial architecture outperforms classical metamaterial structures in telemetric strain sensing. As a scientific breakthrough, by employing our nested metamaterial design, we succeeded in reducing the electrical length of the sensor chip down to λo/400 and achieved telemetric operation across thick soft tissue with a tissue thickness up to 20 cm, while using only sub-cm implantable chip size (compatible with typical orthopaedic trauma implants and instruments). As a result, with nested metamaterials, we successfully demonstrated wireless strain sensing on sheep’s fractured metatarsal and femur using our sensors integrated on stainless steel fixation plates and on sheep’s spine using directly attached sensors in animal models. This depth of wireless sensing has proved to suffice for a vast portfolio of bone fracture (including spine) and trauma care applications in body, as also supported by ongoing in vivo experiments in live animal models in collaboration with biomechanical and medical doctors. Herein, for all generations of our RF-bioMEMS implant sensors, this dissertation presents a thorough documentation of the device conception, design, modeling, fabrication, device characterization, and system testing and analyses. This thesis work paves the way for “smart” orthopaedic trauma implants, and enables further possible innovations for future healthcare.