Differential Bandpass Filters with Common-Mode Suppression based on Stepped Impedance Resonators (SIRs) Paris Vélez 1 , Jordi Naqui 1 , Armando Fernández-Prieto 2 , Miguel Durán-Sindreu 1 , Jordi Bonache 1 , Jesús Martel 2 , Francisco Medina 2 and Ferran Martín 1 1 GEMMA/CIMITEC, Departament d’Enginyeria Electrónica, Universitat Autónoma de Barcelona, 08193 Bellaterra, Spain. (email: paris.velez@uab.cat ) 2 Grupo de Microondas, Universidad de Sevilla, 41012 Sevilla, Spain. (email: medina@us.es ) Abstract — A novel strategy for the design of common-mode suppressed differential (or balanced) filters, based on stepped impedance resonators (SIRs), is presented. The differential mode band pass response is achieved by coupling parallel LC resonators, implemented by a patch capacitance and a grounded inductance, through admittance inverters. Such inverters are implemented by means of 90 o transmission lines, whereas the grounded inductances are implemented by means of mirrored stepped impedance resonators (SIR). For the differential mode, the symmetry plane is a virtual ground, the wide strip section of the SIR is effectively grounded, and the SIR behaves as a shunt inductance. However, for the common mode, where the symmetry plane is an open (magnetic wall), the SIR is a shunt connected series resonator, providing a transmission zero, which can be used for the rejection of the common mode in the differential filter pass band. The equivalent circuit model of the proposed structure is validated through electromagnetic simulation and experimental data of order-3 and -5 Chebyshev differential bandpass filters. Moreover, guidelines for the design of balanced filters with wide bandwidths, including ultra- wideband (UWB) bandpass filters, are provided. Index Terms —Differential filters, stepped impedance resonator (SIR), microstrip, common-mode suppression. I. INTRODUCTION The design of differential filters with common-mode noise suppression is of high interest in balanced circuits, where high immunity to environmental noise, interference and crosstalk between different elements are key advantages over their single-ended counterparts. Several strategies for the implementation of balanced bandpass filters with common- mode rejection have been proposed [1]-[10]. Essentially, such filters are designed by using symmetry properties. Namely, they are symmetric structures, where the electric wall of the symmetry plane for the differential mode makes the structure to exhibit band pass functionality. However, through a proper design, it is possible to achieve common-mode rejection in the differential filter pass band thanks to the effects of the magnetic wall for the common mode. Based on these ideas, moderate or narrow band [6],[7], dual-band [2]-[5], and ultra-wideband (UWB) [9],[10] balanced filters have been recently reported. Most of these common-mode suppressed balanced filters are based on distributed elements, and filter optimization requires parametric analysis. This is the case, for instance, of the UWB balanced filters implemented by means of branch line sections with open-circuited stubs attached along the symmetry plane [9], or by means of open-ended parallel coupled lines [10]. In this paper, a very simple circuit model (that combines transmission line sections and lumped elements) for the implementation of narrow and moderate bandwidth balanced bandpass filters with common mode suppression is proposed. Then, it is demonstrated that such circuit model can be synthesized to a very good approximation in microstrip technology by implementing the lumped elements through patch capacitances and stepped impedance resonators (SIRs). The advantages of this approach are: (i) easy design, and the possibility to implement standard response filters, such as balanced Chebyshev band pass filters, (ii) compact size, (iii) high and wideband common-mode rejection, and (iv) simple fabrication and good isolation (since vias are not required and the ground plane is not etched). It will also be pointed out that by alleviating the requirement of keeping the ground plane unaltered, it is potentially possible to achieve very wide differential filter bandwidths and small size, yet preserving the design methodology, based on the equivalent circuit model. II. PROPOSED DIFFERENTIAL FILTER WITH COMMON MODE SUPPRESSION AND CIRCUIT MODEL The proposed circuit for the implementation of balanced bandpass filters with common mode suppression is depicted in Fig. 1(a), where the transmission line sections present between the lumped elements are 90 o lines at the central filter frequency, f o . For the differential mode, the symmetry plane is an electric wall, and the capacitances C z are grounded. The resulting structure is thus the canonical circuit of a bandpass filter, consisting on a cascade of parallel LC resonators coupled through admittance inverters (Fig. 1b) [11]. For the common mode, the symmetry plane is a magnetic wall, and the equivalent circuit (Fig. 1c) exhibits a stop band behavior. Indeed, except for the presence of the capacitances C p , such circuit is the canonical circuit of a stopband filter. As long as the admittance inverters exhibit their functionality in a narrow (or moderate) band, the synthesis of balanced filters with 978-1-4673-2141-9/13/$31.00 ©2013 IEEE Authorized licensed use limited to: Universidad de Sevilla. Downloaded on June 01,2020 at 14:22:42 UTC from IEEE Xplore. Restrictions apply.