Jordi Selga

Broadband Resonant-Type Metamaterial Transmission Lines

In this letter, it is shown that, contrary to previous assumptions, the broadband characteristics of metamaterial transmission lines are not exclusive of the so called CL loaded composite right/left handed (CRLH) structures. In the balance condition, the typical frequency gap between the left handed (LH) and right handed (RH) transmission bands of these CRLH lines collapses, the characteristic impedance varies smoothly in the vicinity of the transition frequency, and broadband response results. However, through an appropriate design, similar behavior can be obtained in resonant type metamaterial transmission lines, namely transmission lines loaded with complementary split rings resonators. A detailed analysis of the structures, based on the equivalent circuit model is provided, and implications of balancing are pointed out. In this letter, it is clearly demonstrated that broadband balanced CRLH lines can also be implemented by means of the resonant type approach

Recent Advances in Metamaterial Transmission Lines Based on Split Rings

This paper is focused on metamaterial transmission lines based on split rings. Specifically, the considered lines are those based on the hybrid approach, where complementary split ring resonators (CSRRs) are combined with series gaps and shunt inductive stubs, and those implemented by loading a host line with open split ring resonators (OSRRs) and open complementary split ring resonators (OCSRRs). The dispersion characteristics and the characteristic impedance of such lines, essential for design purposes, are analyzed to the light of the lumped element equivalent circuit models of the lines. Finally, it is shown that hybrid lines are useful for the design of power splitters with filtering capability, and OSRR/OCSRR-loaded lines are of interest for the design of wideband bandpass filters. The achieved performances are satisfactory and device dimensions small.

Design of Planar Wideband Bandpass Filters From Specifications Using a Two-Step Aggressive Space Mapping (ASM) Optimization Algorithm

This paper is focused on the automated and unattended optimization of a type of planar wideband bandpass filters by means of aggressive space mapping (ASM). The considered filters are microstrip filters implemented through a combination of shunt connected stepped impedance resonators (SIRs) and grounded stubs coupled through admittance inverters. The most relevant and novel aspect of this paper is the fact that the filter layout is automatically generated from filter specifications, i.e., central frequency, fractional bandwidth, in-band ripple and order, without the need of any external aid to the design process. To achieve this layout generation, filter optimization has been divided into two independent ASM processes. The first one generates the filter schematic (circuit element values) providing the required specifications. This first iterative process is necessary since, due to the narrow band operation of the admittance inverters, the target specifications are achieved by compensating the effects of such narrow band operation. The purpose of the second ASM algorithm is to automatically generate the layout from the filter schematic resulting from the first ASM process. To validate the new two-step ASM optimization tool, two sets of filter specifications (inputs of the developed tool), are considered. The generated filter layouts exhibit frequency responses that satisfy the specifications, and are in excellent agreement with the responses of the schematics.

High-pass Filters Implemented by Composite Right/Left Handed (CRLH) Transmission Lines Based on Complementary Split Rings Resonators (CSRRs)

It was recently shown (1) that it is possible to achieve broadband composite right/left handed (CRLH) transmission lines by using complementary split rings resonators (CSRRs). Making use of balanced CRLH/CSRR-based metamaterial transmission lines, high pass fllters are designed. Good performance and small dimensions are achieved. The results open the door to new design possibilities in microwave circuitry by using this type of CRLH metamaterial transmission lines. DOI: 10.2529/PIERS060802072849 In microwave engineering, two main approaches exist to obtain left-handed transmission lines. One of them is the dual transmission line approach (2{4), which is based on loading transmission lines with series capacitances and shunt inductances. The other one is the resonant-type approach (5,6), which combines resonant sub-wavelength particles, such as split rings resonators (SRRs) or their dual counterparts (CSRRs), with shunt inductances or series capacitances etched on a host transmission line. Traditionally, it was supposed that the use of structures based on the resonant- type model was limited to narrow band applications. However, as occurs in the dual transmission line model, resonant-based left handed structures exhibit also a right-handed transmission band at higher frequencies due line parasitics (7). This allows us to obtain a composite right/left handed (CRLH) behaviour. The line can be tailored in order to collapse the typical frequency gap present between the left handed and right handed transmission bands. Namely, it is possible to make coin- cident the upper and the lower limits of the left and the right-handed bands, respectively (balance case), with the result of a continuous transition between both transmission bands. Although up to now it was not exploited, the balance case is also achievable by means of the resonant approach, what makes possible to obtain broad-band responses by using CRLH resonant-type transmission lines. The basic cell of the structure is represented in Fig. 1(a). It consists on a CSRR etched on the ground plane of a microstrip line, and located underneath a capacitive gap etched on the top layer (conductor strip). The CSRR provides the negative value of the dielectric permittivity in the vicinity of its resonance frequency, whereas the negative value of the magnetic permeability needed to obtain the left-handed behaviour is related to the efiect of the series capacitance due to the gap. This structure is modelled by the equivalent T-circuit model shown in Fig. 1(b) (8). The CSRR is modelled by the resonant tank LC-CC, which is electrically coupled to the line through the capacitance C. The capacitance Cg models the series gap, and L accounts for the line inductance. The right handed transmission band is due to line parasitics (L and C), but also to the CSRR (which behaves capacitively at high frequencies). The analysis of the equivalent circuit model reveals the behaviour of the structure. The phase shift per cell, `, (dispersion relation) and Bloch impedance, ZB, are given by:

Split rings-based differential transmission lines with common-mode suppression

A novel microstrip differential transmission line with common-mode noise suppression is proposed and experimentally validated. It is implemented by periodically etching complementary split ring resonators (CSRRs) in the ground plane. For the differential signals, the symmetry of the structure efficiently cancels the electric field components axial to the CSRRs, and these particles have no effect on signal transmission. However, the CSRRs are activated under common mode excitation, with the result of a stop-band behavior. For the designed and fabricated prototype device, over 20 dB suppression of common-mode noise is achieved over a frequency range from 1.18 GHz to 1.74 GHz.

Synthesis of Split-Rings-Based Artificial Transmission Lines Through a New Two-Step, Fast Converging, and Robust Aggressive Space Mapping (ASM) Algorithm

This paper is focused on the synthesis of artificial transmission lines based on complementary split ring resonators (CSRRs). The considered structures are microstrip lines with CSRRs etched in the ground plane and microstrip lines loaded with both CSRRs and series capacitive gaps. An aggressive space mapping (ASM) optimization algorithm, able to automatically generate the layout of these artificial lines, has been developed. The tool has been optimized in order to achieve fast convergence and to provide accurate results. The main relevant aspects of the proposed algorithm (based on a novel two-step ASM optimization approach) are: 1) the capability to provide the implementable circuit elements of the equivalent circuit model of the considered artificial lines and 2) the ability to converge in a few (unprecedented) iteration steps, due to a new procedure to generate the initial layouts (which are very close to the final ones). First, the software is tested through the synthesis of several CSRR-based microstrip lines, and then some practical application examples of such artificial lines are reported to illustrate the potential of the proposed synthesis tool.

Low-pass and high-pass microwave filters with transmission zero based on metamaterial concepts

In this work, highly selective filters based on periodic arrays of electrically small resonators are pointed out. The high-pass filters are implemented in microstrip technology by etching complementary split ring resonators (CSRRs), or complementary spiral resonators (CSRs), in the ground plane, and series capacitive gaps, or interdigital capacitors, in the signal strip. The structure exhibits a composite right/left handed (CRLH) behavior and, by properly tuning the geometry of the elements, a high pass response with a sharp transition band is obtained. The low-pass filters, also implemented in microstrip technology, are designed by cascading open complementary split ring resonators (OCSRRs) in the signal strip. These low pass filters do also exhibit a narrow transition band. The high selectivity of these microwave filters is due to the presence of a transmission zero. Since the resonant elements are small, filter dimensions are compact. Several prototype device examples are reported in this paper.

Automated synthesis of resonant-type metamaterial transmission lines using aggressive space mapping

A technique for the automated generation of the layout of microstrip lines loaded with complementary split ring resonators (CSRRs) is proposed. This synthesis strategy presents a great practical interest, due to the reduction in terms of computational efforts, that is achieved without any significant lack of accuracy in the final solution. The application makes use of an aggressive space mapping (ASM) algorithm based on a constrained Broyden-based input approach. In this case, the parameter extraction (PE) does not require of any optimization, since the circuital parameters of the metamaterial transmission line can be determined following a straightforward method.

Automated Design of Common-Mode Suppressed Balanced Wideband Bandpass Filters by Means of Aggressive Space Mapping

The automated and unattended design of balanced microstrip wideband bandpass filters by means of aggressive space mapping (ASM) optimization is reported in this paper. The proposed filters are based on multisection mirrored stepped impedance resonators (SIRs) coupled through quarter-wavelength transmission lines, acting as admittance inverters. Such resonant elements provide transmission zeros useful for the suppression of the common mode in the region of interest (differential filter pass band) and for the improvement of the differential-mode stopband (rejection level and selectivity). Due to the limited functionality of the inverters, related to the wide fractional bandwidths, the automated filter design requires a two-step process. With the first ASM, the filter schematic satisfying the required specifications (optimum filter schematic) is determined. Then, the layout is synthesized by means of a second ASM algorithm. Both algorithms are explained in detail and are applied to the synthesis of two filters, as illustrative (and representative) examples. With this paper, it is demonstrated that the two-step ASM optimization scheme (first providing the optimum schematic and then the layout), previously applied by the authors to wideband single-ended filters, can be extended (conveniently modified) to common-mode suppressed differential-mode bandpass filters. Thus, the value of this two-step ASM approach is enhanced by demonstrating its potential for the unattended design of complex filters, as those considered in this paper.

Size reduction and spurious suppression in microstrip coupled line bandpass filters by means of capacitive electromagnetic bandgaps

This paper is focused on the application of electromagnetic bandgaps based on capacitive loading to the implementation of microstrip coupled line bandpass filters with reduced size and spurious suppression. Size reduction is due to the slow-wave effect caused by the loading capacitances of the different coupled lines, whereas spurious suppression is related to periodicity. By properly designing the capacitively-loaded coupled line sections of the filter, implemented by means of square patches in practice, it is possible to significantly reduce filter size and simultaneously achieve spurious cancellation. As an example, an order-3 Chebyshev bandpass filter with 30% length reduction (as compared to the conventional counterpart) and spurious rejection up to the third harmonic is reported.