Ferran Martín

Microwave filters with improved stopband based on sub-wavelength resonators

The main aim of this paper is to demonstrate the potentiality of sub-wavelength resonators, namely, split-ring resonators, complementary split-ring resonators, and related structures to the suppression of undesired spurious bands in microwave filters, a key aspect to improve their rejection bandwidths. The main relevant characteristics of the cited resonators are their dimensions (which can be much smaller than signal wavelength at resonance) and their high-Q factor. This allows us to design stopband structures with significant rejection levels, few stages, and small dimensions, which can be integrated within the filter active region. By this means, no extra area is added to the device, while the passband of interest is virtually unaltered. A wide variety of bandpass filters, implemented in both coplanar-waveguide and microstrip technologies, have been designed and fabricated by the authors. The characterization of these devices points out the efficiency of the proposed approach to improve filter responses with harmonic rejection levels near 40 dB in some cases. It is also important to highlight that the conventional design methodology for the filters holds. For certain configurations, the presence of the resonators slightly lowers the phase velocity at the frequencies of interest with the added advantage of some level of reduction in device dimensions.

Tunable metamaterial transmission lines based on varactor-loaded split-ring resonators

In this paper, it is demonstrated that varactor-loaded split-ring resonators (VLSRRs) coupled to microstrip lines can lead to metamaterial transmission lines with tuning capability. Both negative permeability (mu<0) and double (or left-handed) negative media have been designed and fabricated with tuning ranges as wide as 30%. The negative effective permeability is provided by the VLSRRs in a narrow band above their resonant frequency, which can be bias controlled by virtue of the presence of diode varactors. To achieve a negative effective permittivity in the left-handed structure, metallic vias emulating shunt inductances are periodically placed between the conductor strip and the ground plane. The lumped-element equivalent-circuit models of the designed structures have been derived. It has been found that these models provide a good qualitative description of device performance. Since the VLSRR microstrip line and the line loaded with both VLSRRs and vias exhibit stopband and bandpass behavior, respectively, the ideas presented in this study can be applied to the design of narrowband tunable frequency-selective structures with compact dimensions. This is the first time that a tunable left-handed transmission line, based on SRRs, is proposed

The heat shock stress response after brain lesions: Induction of 72 kDa heat shock protein (cell types involved, axonal transport, transcriptional regulation) and protein synthesis inhibition

The cerebral stress response is examined following a variety of pathological conditions such as focal and global ischemia, administration of excitotoxins, and hyperthermia. Expression of 72 kDa heat shock protein (Hsp70) and hsp70 mRNA, the mechanism underlying induction of hsp70 mRNA involving activation of heat shock factor 1, and inhibition of cerebral protein synthesis are different aspects of the stress response considered here. The results are compared with those in the literature on induction, transcriptional regulation, expression, and cellular location of Hsp70, with a view to getting more insight into the function of the stress response in the injured brain. The present results illustrate that Hsp70 can be expressed in cells affected at various degrees following an insult that will either survive or dic as the brain lesion develops, depending on the severity of cell injury. This indicates that, under certain circumstances, synthesized Hsp70 might be necessary but not sufficient to ensure cell survival. Other situations involve uncoupling between synthesis of hsp70 mRNA and protein, probably due to very strict protein synthesis blockade, and often result in cell loss. Cells eventually will die if protein synthesis rates do not go back to normal after a period of protein synthesis inhibition. The stress response is a dynamic event that is switched on in neural cells sensitive to a brain insult. The stress response is, however, tricky, as affected cells seem to need it, have to deal transiently with it, but eventually be able to get rid of it, in order to survive. Putative therapeutic treatments can act either selectively, potentiating the synthesis of Hsp70 protein and recovery of protein synthesis, or preventing the stress response by deadening the insult severity.

Electrically Small Resonators for Metamaterial and Microwave Circuit Design

A new branch in microwave engineering arose just few years ago with the emergence of metamaterials in 2000 (Smith et al., 2000). The implementation of the first artificial medium with negative effective dielectric permittivity and magnetic permeability opened the door to the experimental study of a new kind of media: left-handed media. The possibility of the artificial implementation of such media allowed the corroboration of many of their electromagnetic properties, predicted years before by Viktor Veselago (Veselago, 1968). Since the year 2000, the interest stirred up by these new materials has given rise to numerous works in a wide range of scientific branches. The possibilities that metamaterials offer to create artificial media with controllable characteristics has permitted the creation of a growing number of completely new applications. Undoubtedly, the most innovative and spectacular application of such artificial media is their use in the implementation of cloaking structures to achieve invisibility, which can be accomplished thanks to the engineering of the refraction index of the different layers of the cloaking shield (Schurig et al., 2006). Within the vast number of new applications of metamaterials, one of the most productive ones is the implementation of microwave devices by means of artificial transmission lines. The following sections will deal with one of the approaches devoted to this purpose: the resonant-type approach. Different subwavelength resonators employed in the design of metamaterial transmission lines based on the resonant-type approach will be studied. The equivalent circuit models of different kinds of metamaterial transmission lines, as well as the parameter extraction methods employed as design and corroboration tools will be also presented. In closing, a selection of application examples of resonant-type metamaterial transmission lines in the design of microwave devices will be presented. 19

Miniaturization and Dual-Band Operation in Planar Microwave Components by Using Resonant-Type Metamaterial Transmission Lines

In this work, it is shown that dual-band microwave components based on plusmn90deg metamaterial transmission lines can be miniaturized by implementing the artificial lines by means of complementary spiral resonators (CSRs). Such lines exhibit a composite right/left handed (CRLH) behavior. Dual-band operation is achieved by designing the lines to provide a phase of -90o at the lower operating frequency (within the left handed band) and +90o phase shift at the upper frequency (within the right handed band). Size reduction, as compared to conventional implementations, is achieved by implementing the lines with a single unit cell, and thanks to the small electrical size of CSRs. To illustrate the possibilities of the approach, a dual-band branch-line coupler based on metamaterial transmission lines implemented by means of CSRs is presented. The device has been designed to be functional at the mobile GSM bands (f1= 0.9 GHz, f2= 1.8 GHz). As compared to the conventional branch line, a 40% size reduction is obtained. The approach is fully compatible with planar technology.

Implementation of negative μ medium in coplanar waveguide technology

In this paper, a novel structure in coplanar waveguide (CPW) technology which exhibits an equivalent negative magnetic permeability is described. Such a structure consists in a conventional coplanar waveguide that is loaded with split ring resonator (SRR) cells. Due to the configuration of the magnetic field components in the coplanar waveguide, by adequately placing the SRR cells, quasi-static resonance occurs. In the vicinity of such resonance frequency, the magnetic permeability exhibits a negative value in a certain frequency range. Full wave simulation results as well as measurement from fabricated prototypes validate initial assumptions, providing a new method to implement band rejection filters with very small size.

Characterization of Metamaterial Transmission Lines with Coupled Resonators Through Parameter Extraction

Since resonant-type metamaterial transmission lines were proposed (Martin et al., 2003), this kind of transmission lines have been of significant importance in the development of new and innovative microwave devices. The small size and novel characteristics of these transmission lines based on sub-wavelength resonators allows the miniaturization and improvement of existing devices (Bonache et al., 2006a, 2006b; Gil et al., 2007a, 2007b), as well as the design of components with new functionalities (Siso et al., 2008, 2009). Due to the complicated layouts that these designs usually involve, having an accurate equivalent circuit model is an important assist during the design process. Besides, the application of parameter extraction methods (Bonache et al., 2006c) to obtain the values of the electrical parameters of the circuit model makes possible the characterization of both the transmission line and, therefore, the microwave device. The circuit models and parameter extraction methods presented in this chapter have been widely verified and their accuracy permits even their application for automatic layout generation based on space mapping techniques (Selga et al., 2010), which is a large and useful advance in the design of such structures.

Simulation of EBG structures in coplanar waveguides with the aid of FDTD

In the paper, electromagnetic bandgap (EBG) structures in coplanar waveguide (CPW) technology are presented. In order to design this type of circuits, a custom Finite Difference Time Domain (FDTD) code is employed. Due to the fact that simulation is performed in the time domain, a wide frequency response can be obtained in a reasonable amount of time, being a numerically efficient technique. Simulation of several proposed designs are presented and these results are validated with measurements from fabricated prototypes. The results show that the use of FDTD techniques is adequate for the design of EBG devices in planar circuit technology in general.