Ultra-wideband microstrip to CB-CPW transition applied to broadband filter M. Nedil, T.A. Denidni and A. Djaiz A new ultra-wideband microstrip to conductor backed coplanar waveguide (CB-CPW) transition structure is presented and implemen- ted. Results show good performance in terms of bandwidth, which is about 12 GHz. Using this transition, a new ultra-wideband filter was designed and fabricated. Simulated and measured results show that the filter can cover the band from 3.1 to 10.6 GHz (10 dB bandwidth). Introduction: Planar transmission lines such as microstrip and coplanar waveguide (CPW) have been applied in various microwave and millimetre-wave circuits. In some multilayered structures, these transmission lines coexist and are even combined to develop new circuit components [1]. For instance, multilayer microwave integrated circuits require more flexibility to use both microstrip and CPW circuit technologies. To ensure compatibility between these techno- logies, low-loss, wideband, and compact transitions between micro- strip and CPW lines are necessary. In practice, additional conducting planes in the CPW line are often present below the substrate in order to electromagnetically separate the circuit from its environment [2]. In the same perspective, vialess CB-CPW to microstrip line transi- tions using electromagnetic (EM) coupling of three-line couplers have also been proposed [3–5]. However, to our knowledge a broadside CB-CPW to microstrip transition has not been proposed. In this Letter, a novel ultra-wideband microstrip to CB-CPW transi- tion using slot coupling is proposed. The transition uses a conductor- backed coplanar waveguide, which offers superior characteristics over the CPW such as reduced size [6]. As an application, this transition was applied to design a new multilayer ultra-wideband filter. This filter offers a wider out-of-band rejection bandwidth and has good perfor- mance in terms of group delay. Fig. 1 Layout of proposed microstrip to CB-CPW transition and cross- section a Layout of proposed microstrip to CB-CPW transition b Cross-section Microstrip to CB-CPW transition: Fig. 1 shows the geometrical layout of the proposed two-port microstrip to CB-CPW transition. This transition is characterised by an aperture etched on the common ground plane of the two-layered structures to provide a fed-through coupling between the upper microstrip line and the lower CB-CPW line. In this structure, the upper microstrip conductor is vertically coupled with the lower CB-CPW line via a coupling slot located in the common ground plane as shown in Fig. 1. The transition was designed using the EM simulator IE3D, which is part of Zeland software package [7]. The top and bottom 50O transmission lines were computed using HP’s LineCalc. The coupling line widths and line lengths were varied in order to optimise the transition match. It is noted that the design offers good transition and tight coupling via the geometry of the slot coupled in the ground plane between the microstrip feed line to CB-CPW line section. The transition was designed using a Duroid substrate (RT=Duroid 5880) with e r ¼ 2.2 and thickness of 0.254 mm. The dimensions of the transition are L S ¼ 5.15 mm, L ¼ 4.75 mm, W ¼ 2.7 mm, W 1 ¼ 4.5 mm, W 2 ¼ 4.3 mm, S ¼ 0.2 mm, G ¼ 0.69 mm, e ¼ 0.2 mm. The main drawback of the CB-CPW technology is the parallel-plate modes, which are considered as unwanted bulk modes [8]. This indicates that the minimum parasitic resonant frequency from the parasitic parallel-plate modes of the CB-CPW can be predicted based on a simple rectangular patch model as reported in [8]. The calculated lowest order mode resonance frequency is f 11 (  20.8 GHz) which is obtained while the dimension of the width and the length of the ground are W g ¼ 5 mm and L g ¼ 20 mm, respectively. In this case, it is noted that the leaky wave phenomenon does not affect the performance of the proposed transition, which allows avoiding the use of via. Based on this design, an experimental prototype was fabricated and tested. Fig. 2 shows the simulated and measured results. From these, it can be seen that simulated and experimental data show good agreement and the microstrip to CB-CPW transition offers a very wide bandwidth of  12 GHz, which is more superior than the transition reported in [9]. As an application of this new transition, an ultra-wideband bandpass filter was introduced, where a very wide bandwidth was achieved, which is enough to cover ultra-wideband application (more than fixed by FCC [10]). Fig. 2 Simulation and experimental results of proposed transition —— simulated - - - -measured UWB filter design: The most challenging problem in the design of an UWB filter is the 110% fractional bandwidth requirement of the Federal Communications Commission (FCC)’s decision to permit the unlicensed operation band from 3.1 to 10.6 GHz in 2002 [10]. Few works on UWB passband filters have been proposed [11–13]. In this contribution, a new bi-layer UWB filter based on the proposed transition is presented in this Section. Fig. 3 shows the layout of the proposed UWB filter. This filter is composed of two microstrip– CB-CPW transitions and a section of CB-CPW transmission line as a multiple-mode resonator (MMR) between the two transitions [9]. At the centre frequency of the concerned UWB passband, both side sections of this MMR (microstrip to CB-CPW transition) are identical and they are chosen as one quarter-wavelength (l g =4) while the middle section is set as one half-wavelength (l g =2). Fig. 3 Layout of proposed UWB filter ELECTRONICS LETTERS 12th April 2007 Vol. 43 No. 8