Design and optimization of the cascaded band-stop filters using vertically coupled open-loop ring resonators

In this paper, we proposed several compact bandstop filters using C-open-loop ring topology. In the first step a minimize bandstop using single C-ring resonator is simulated optimized and realized. The triple ring BPF consists of two cascaded C-ring resonators, which are placed on the RO4003 substrate, while the third ring is etched as DGS resonator on the ground plane. The both top resonators are connected with input and output through microstrip feed lines. The both filters are simulated, optimized and partially realized using MWR simulator and Anritsu E5072A vector network analyzer VNA. In order to reduce the size and to improve the filter characteristics, novel compact filter topologies are designed basing on the previous structures. The proposed multi-band band stop filters consist of several open-loop ring resonators placed vertically behind each other. Using this idea, the filter topologies with design flexibility, close size and excellent results are reached. The novel compact multi-band band stop filters produce several stopband along a frequency range from DC to 9 GHz, in which each separate band exhibits an acceptable and useful bandwidth. Each stopband has regenerated two reflexion zeros, what leads to a good sharpness factors in the transition domains. Good agreement between the experimental results, full-wave simulation has been achieved. This new filter idea can be very attractive for the nowadays multilayer and compact radio frequency integrated circuit design.

I. Introduction

In recent years, the demand for compact microstrip filters is increasing due to the recently expanding microwave and mobile communication systems. So far, a few effective techniques have been successfully developed to miniaturize the filter size and to transform a simple filter structure to multi-band filter without additional components. Nowadays multi-standard wireless communication systems require multiband operating transceivers. Dual-band and multi-band filters are in demand for these wireless systems. Beside the multiband operation, low cost, simplicity, small occupied area and performance are desired features for these filters. Over these years, microstrip planar multi-band filters have been widely proposed as an approach solution to solving the most difficult technical challenges in microwave and wireless communication areas [1]–[3]. In order to increase the number of bands of a structure, they were firstly implemented by the cascaded two filters at different frequencies, with the inherent disadvantage of increased total size [3]. Also they were realized by using a combination of a wide-band bandpass filter and a band stop filter [4]. However, the size of topology remained a big challenger. Alternatively, other techniques as filter with open stubs are used to regenerate transmission zeros in order to separate the passbands and lead to create several separate bands [3], [5]. Stepped-impedance resonators (SIR) are utilized to realize the dual-stopband characteristics, and reduce size of filters. Stepped impedance resonator (SIR) can adjust the second passband by regulation the impedance ratio and electric lengths of SIRs. By correctly choosing the relevant impedance or strip width ratio, the dual-band topologies employing SIRs can be produced [6]. On the other hand, few researchers have investigated the design of triple-band filters. Boutejdar, et. aI., Ren, et. al. and Hejazi, et. al. proposed filter using vertical and horizontal cascaded DGS-Resonators [7]–[11] and hairpin slots [12], [13]. The disadvantages in this works are the complicated used DGS shapes, the undesired coupling between the DGS resonators and the large size of the used filter structures. Quendo, et. al. proposed the dual- behavior resonators as good candidate for the design of tri-band filters [14]. Mokhtari, et. al. and Boutejdar et. al. proposed a design methodology to these kind of filters using cross-coupled networks represented by coupling matrix [15]–[17]. However, in both proposed topologies, the disadvantage of large circuit size remains a real challenge. In [15], the coupling of two tri-band dual- behavior resonators DBRs was needed to achieve a second order triple-band filter response with six poles (two poles in each band) and five transmission zeros (single zero between each transmission band and two transmission zeros to the left and to the right of all passbands). This cascaded topology shows an undesired increase in the size of the proposed filter. In [15], six hairpin resonators were needed to achieve a filter with triple-band features with six poles (two in each band) and four transmission zeros (two transmission zeros between each two bands).