IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 27, NO. 1, JANUARY 2017 31 Compact Balanced Dual-Band Bandpass Filter Based on Modified Coupled-Embedded Resonators Fulya Bagci, Member, IEEE, Armando Fernández-Prieto, Member, IEEE, Aintzane Lujambio, Jesús Martel, Senior Member, IEEE , Joaquín Bernal, Member, IEEE, and Francisco Medina, Fellow, IEEE Abstract—A new compact balanced dual-band bandpass filter based on coupled-embedded resonators with modified ground plane is presented in this work. Common-mode is rejected within the two differential passbands by symmetrically introducing four coupled U-shaped defected ground structures below the resonators. Common-mode rejection is significantly improved when compared with the standard (solid ground plane) filter with similar geometry thanks to the introduction of four extra trans- mission zeros. Due to the symmetry, the differential mode is not significantly affected by the presence of the U-shaped resonators. Circuit-model data, full-wave simulations and measurements are provided to verify the benefits of the proposed dual-band filter. Index Terms— Balanced filter, common-mode suppression, dual-band filter. I. I NTRODUCTION R F/MICROWAVE balanced bandpass filters (BPFs) have attracted the interest of the microwave community in recent years due to their enhanced signal to noise ratio, noise immunity, low crosstalk and low electromagnetic interfer- ence (EMI) when compared with their classical single-ended counterparts. Nowadays, with the rapid growth of multi- band wireless communication systems, multiband BPFs with very demanding specifications concerning both differential mode (DM) transmission and common-mode mode (CM) sup- pression are required. Several proposals regarding dual-band balanced BPFs can be found in the recent literature [1]–[9]. In [1]–[4], [7] balanced dual-band operation is obtained by means of electrically-coupled resonators. In such designs, Manuscript received May 22, 2016; revised July 21, 2016; accepted Octo- ber 10, 2016. Date of publication December 14, 2016; date of current version January 6, 2017. This work has been supported in part by the Spanish Ministerio de Economía y Competitividad with European Union FEDER Funds (contract TEC2013-41913-P), by the Spanish Junta de Andalucía (project P12-TIC-1435), and by TUBITAK, The Scientific and Technological Research Council of Turkey, under grant 22B2219 F. Bagci is with the Department of Electronics and Electromagnetism, Physics Faculty, University of Sevilla, Av. Reina Mercedes s/n, Seville 41012, Spain and also with the Department of Engineering Physics, Faculty of Engineering, Ankara University, Besevler, Ankara 06100, Turkey. A. Fernández-Prieto and F. Medina are with the Department of Electronics and Electromagnetism, Physics Faculty, University of Sevilla, Av. Reina Mercedes s/n, Seville 41012, Spain (e-mail: armandof@us.es). A. Lujambio is with the Skylife Engineering, Parque Tecnológico Aeroes- pacial de Andalucía, Early Ovington Street, 24, Nave 15-16, Seville 41309, Spain. J. Martel is with the Department of Applied Physics II, ETS Architecture School, Av.Reina Mercedes s/n, Seville 41012, Spain. J. Bernal is with the Dept. of Applied Physics III, ETS Ingenieros, University of Sevilla, Av. de los Descubrimientos, Seville 41092, Spain. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LMWC.2016.2629962 CM suppression is improved by adding extra lumped capac- itors, inductors, resistors and/or open-circuited stubs. Other recently proposed approaches to DM filter design with CM suppression make use of asymmetrical coupled lines [5] or substrate integrated waveguide (SIW) technology [8]. These designs provide good DM performance and CM rejection, but they suffer from the problem of having large electrical size and, moreover, the use of a large number of via-holes. In [6] balanced dual-band filtering is achieved by using coupled-embedded resonators. In this implementation the CM is rejected by cascading with the filter a pair of CM rejection differential-line stages based on the use of a low-pass DGS structure. Unfortunately, this solution increases the design complexity and the overall electrical size. Coupled comple- mentary split-ring resonators (C-CSRR) have very recently been proposed [9] for the design of compact balanced dual- band filters with good CM rejection. In this letter, a new compact balanced dual-band BPF based on coupled-embedded resonators with a modified ground plane is proposed. Design methodology and experimental validation of the structure are illustrated with a specific filter example operating at two wireless local area network (WLAN) bands. Measured and simulated results show the benefit of the proposed configuration. II. PROPOSED DGS STRUCTURE The layout of the proposed balanced dual-band BPF is shown in Fig.1. The top layer geometry is based on the single- ended and differential dual-band filters reported in [10] and [6] respectively. It consists of the combination of two different sub-filters that enable the generation of two different DM passbands. These bands can be independently tuned. The bottom layer is formed by four U-shaped slot resonators. These resonators have been used in [11], [12] to improve the CM rejection of differential lines. In this letter the DGS resonators are embedded within the printed filter area, thus yielding a more compact design. When operating in DM, the AA ′ symmetry plane (see Fig.1) is a virtual short-circuit. The DGS is then grounded at both ends and hence it hardly affects the response of this mode. On the contrary, for CM operation the AA ′ plane behaves as a virtual open-circuit. It is then expected a strong disturbance of the CM response. Let us first focus our attention on the role of the U-shaped DGSs when used below a differential line pair [see Fig.2(a)]. Each resonator can be modeled (CM excitation) as a parallel LC -tank circuit with a couple of electrically short lines connected at both sides [11] (see Fig.2). 1531-1309 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. Authorized licensed use limited to: Universidad de Sevilla. Downloaded on May 25,2020 at 15:11:49 UTC from IEEE Xplore. Restrictions apply.