## 1 Introduction  
In recent years, the use of differential signals has gained increasing attention for both digital high-speed and analog microwave circuit applications [1; 2]. This interest in differential devices is mainly due to their higher immunity to environmental noise, better electromagnetic compatibility, lower level of electromagnetic interference (EMI) and better signal to noise ratio performance, when compared to their single-ended counterparts. Despite all these advantages, differential devices can suffer from the presence of common-mode (CM) noise, mainly caused by amplitude unbalance and time skew of the differential signals. Therefore, to ensure the integrity of the differential signal over the frequency range of interest, strong CM rejection is highly desired. Different types of microwave devices in their differential version can be found nowadays in the literature: power dividers and combiners [3; 4; 5; 6], diplexers [7] or passive equalizers [8]. However, differential-mode balanced bandpass filters (DM-BPFs) are, undoubtedly, the devices that have attracted the most attention in the literature. DM-BPFs with single/multiple differential passbands are required to have good differential-mode (DM) transmission within the passbands (low insertion loss, IL), good out-of-band rejection (high selectivity) and high CM rejection level (at least within the differential passbands). During the last decade, much effort has been focused on the design of balanced single-band BPFs, in such a away that a huge number of works can be found in the specialized literature. To mention just a few, see for example, [2; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20] and the references therein. Nevertheless, much less research has been done on balanced dual-band BPFs [27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42] when compared with single-ended versions. It is well known that balanced dual-band BPFs are key components for multi-band systems operating under several wireless standards from Institute of Electrical and Electronics Engineers (IEEE), such as IEEE 802.11 (Wi-Fi 2.45/5.3 GHz) and IEEE 802.16 (WiMAX - 2.45/3.5 GHz), as well as for Global Navigation Satellite System (GNSS) systems such as GPS (L1/L2 - 1.575/1.227 GHz) [43]. Regarding the methods developed to design balanced dual-band BPFs, the use of electrically coupled resonators is by far the most commonly used strategy, usually leading to good common-mode rejection. For example, in [27] a fourth-order dual-band balanced BPF based on half-wavelength resonators is presented. Although the filter offers good DM performance and good CM noise suppression, it is difficult to control the bandwidths of the two differential-passbands independently. In addition, it requires the introduction of lumped-elements to properly reject the common-mode signal since for the unloaded resonators CM rejection is poor. In [28; 31; 32; 33; 34], stepped impedance resonators (SIRs) are used to perform balanced dual-band operation. The impedance and length ratios of the SIRs were adjusted to realize the desired dual-band response. However, since the level of CM noise rejection for electrically coupled SIRs is relatively poor, additional elements such as lumped inductors/capacitors/resistors [28], common-mode rejection stages based on differential lines with defected ground structures (DGS) [31; 32], or open-circuit stubs [33; 34] must be added to the resonators in order to improve their CM performance. In all the aforementioned cases, both DM and CM exhibit good performance, but at the expense of increasing the filter size and the complexity of the design process, since two different devices must be designed: the filter itself and the additional elements to suppress common-mode transmission. As in the cases exposed in [31; 32], DGSs have also been used in [29; 35] to improve CM rejection. In those papers, a multilayer structure was proposed with the ground plane located in the middle.