ULTRA WIDE BAND PASS FILTERS (UWBPF) BASED ON COMPLEMENTARY SPLIT RINGS RESONATORS J. Bonache, 1 F. Martı ´n, 1 J. Garcı ´a-Garcı ´a, 1 I. Gil, 2 R. Marque ´ s, 2 and M. Sorolla 3 1 Departament d’Enginyeria Electro ` nica Universitat Auto ` noma de Barcelona 08193 Bellaterra (Barcelona), Spain 2 Departamento de Electro ´ nica y Eletromagnetismo Facultad de Fı´sica Universidad de Sevilla Av. Reina Mercedes s/n 41012 Sevilla, Spain 3 Departamento de Ingenierı ´a Ele ´ ctrica y Electro ´ nica Universidad Pu ´ blica de Navarra 31006 Pamplona (Navarra), Spain Received 24 January 2005 ABSTRACT: In this work, a new strategy for the design of microstrip filters with ultra-wide bandwidths is proposed. This is based on the combination of square-shaped complementary split-ring resonators (CSRRs) etched in the ground plane and grounded stubs. Coupling be- tween adjacent resonators is achieved by means of /4 lines, acting as impedance inverters. To illustrate the potentiality of the approach, a prototype device with 90% fractional bandwidth has been designed for operation at C-band. The measured fractional bandwidth (87%), in-band losses (1.3 dB), and out-of-band rejection (30 dB), obtained on a device scaled down in frequency, point out the high performance achiev- able with this design methodology. Moreover, the first frequency para- sitic (spurious) does not appear up to three times the central filter fre- quency. Due to the small dimensions, wide bandwidths, and compatibility with planar circuit technology, it is believed that the ap- proach presented in this work can be of actual interest to the design of filters for ultra-wideband applications. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 283–286, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop. 20966 Key words: split-ring resonators (SRRs); complementarity; microstrip filters; ultra-wideband systems 1. INTRODUCTION Recently, split-ring resonators (SRRs), originally proposed by Pendry [1] (see Fig. 1), have attracted a great deal of interest for the design of negative permeability and left-handed (LH) effective media [2]. The main relevant characteristic of these resonators is their electrical size, which can be made very small, due to the distributed capacitance between concentric rings. The small elec- trical size is a necessary condition for the synthesis of effective (continuous) media based on periodic arrangements of these res- onators. In planar technology, the authors have designed and fabricated SRR-based LH transmission lines [3], and have applied this concept to the design of narrow band-pass filters in coplanar- waveguide technology [4, 5]. The improvement of filter perfor- mance and dimensions by combining SRRs with shunt connected metal wires and series gaps has been reported recently [6]. Highly selective symmetric responses and small dimensions (that is, length l = /3, with  the signal wavelength at the central filter frequency) have been demonstrated in a fabricated prototype de- vice with 5% fractional bandwidth. Figure 1 also depicts the complementary SRR (CSRR). This particle, recently proposed by some of the authors [7], is the negative image of the SRR and is the constitutive element for the synthesis of negative-permittivity media. Since this particle must be driven by an axial time-varying electric field, rather than by an axial magnetic field (as is the case for SRRs), negative permittiv- ity, as well as LH transmission lines, are preferably (although not exclusively) implemented in microstrip technology. To this end, CSRRs should be etched in the ground plane, underneath the conductor strip, where the electric field intensity is orthogonal to the plane of the particle. Following this idea, narrow band-pass structures with backward-wave propagation have been recently fabricated, with comparable performance to those periodic band- pass filters implemented in CPW by using SRRs [8]. In the cited SRR and CSRR band-pass filter prototypes, the filter bandwidth has not been adjusted to any targeted specification. In other words, rather than following a well-established method- ology aimed at the design of band-pass structures with controllable bandwidth, we have simply pursued the synthesis of narrowband SRR (or CSRR) loaded transmission lines, with the optimum result of a 5% fractional bandwidth, measured on a device designed for operation at the C-band [6]. In this work, band-pass filters based on CSRR-loaded microstrip lines with fractional bandwidths exceed- ing 50% are presented for the firs time. The filter stages consist of square-shaped CSRRs, etched on the ground plane, combined with shunt stubs, which are grounded by means of metallic vias. In order to enhance and control the bandwidth, these stages are coupled by means of J = 1 admittance inverters, which are implemented by means of /4 lines. Due to the CSRR/stub com- bination, the 3-dB bandwidth of the shunt resonators can be made very wide, and filter bandwidths as high as 90% are potentially achievable. Specifically, the fabricated prototype exhibits a frac- tional bandwidth FBW = 87%. It is worth mentioning that the lower edge of the first spurious band is allocated at approximately three times the central frequency. These ultra-wide bandwidths and stop-band rejection are not simultaneously obtainable with con- ventional distributed approaches [9], unless combined low-pass/ high-pass filter stages are cascaded [10]. It is believed that the proposed structures can be of practical interest for ultra-wide band-pass filter (UWBPF) applications, where high-data-rate transmission is required. 2. SYNTHESIS OF UWBPF BY MEANS OF CSRRs A typical layout for the proposed filters is depicted in Figure 2 (corresponding to a 3 rd -order implementation). The grounded stubs are modeled by shunt-connected inductors, L p , while CSRRs are described by parallel resonant tanks (with inductance L r and capacitance C r ) electrically coupled to the line through the capac- itance of the substrate, C c . The inductor/CSRR combination forms the shunt resonator that should be tailored in order to achieve the required filter specifications. These resonators are coupled by admittance inverters (with normalized admittance J = 1), that are implemented by means of /4 lines. Thus, the equivalent circuit model for the basic cell of the filter is that depicted in Figure 2(b). This circuit model fits to the generalized band-pass filter network [see Fig. 2(c)] that results from frequency and element transfor- mation of the low-pass filter prototype [11]. In order to determine Figure 1 Topology of (a) the SRR proposed by Pendry and (b) CSRR (metal regions are depicted in grey) MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 46, No. 3, August 5 2005 283