Available online at www.sciencedirect.com Procedia Technology 4 (2012 ) 466 471 C3IT-2012 Wide-Band Bandpass Filter Using CRLH Transmission Line and Floating Slot Approach Pratik Mondal a, A.Roy a, S.K.Parui a a Department of Electronics and Telecommunication Engineering Bengal Engineering and Science University, Shibpur, Howrah-711103, India Abstract This paper presents construction of a composite right/left-handed (CRLH) transmission line (TL) using microstrip coupled lines. For getting high coupling in comparison to conventional edge coupled microstrip line, the floating slot in the ground plane is used. Two configuration named as SOS(shortopen-short) the OSO(open-short-open) model are studied for realization of band pass filter. The floating ground equalizes the even and odd mode phase velocities and solves the difficulties arised in the fabrication process as it relaxes the required physical dimensions like coupled gaps between lines. Finally a bandpass filter having bandwidth of 56.7% and selectivity of 40 db/ghz has been designed using OSO model and the experimental results are verified with EM simulated results. 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of C3IT Open access under CC BY-NC-ND license. Keyword: Composite right/left handed transmission line(crlh); Open-short-open; Short-open-short; Floating slot; band pass filter 1. Introduction Composite right/left-handed (CRLH) transmission lines (TLs) are TL based on metamaterials, which have recently led to numerous novel concepts and applications [1]. The CRLH transmission line developed a new class of highly functional microwave devices such as power dividers, couplers and filters [2]. For conventional transmission line i.e.; for right handed transmission line, a series inductor and a shunt capacitor generally is used. If the position of the inductor and the capacitor interchanges, the obtained structure is referred as left-handed transmission line. The models of a purely right-handed and lefthanded transmission line shown in Fig.1. The left-handed transmission line is commonly known as metamaterials. Left handed transmission lines do not exist naturally, only they can be designed artificially using lumped elements and periodically loading a normal transmission line. The CRLH TL is generally implemented by loading a host TL periodically with series capacitances and shunt inductances, and exhibits both the left-handed (LH) and right-handed (RH) propagation bands. As the right-handed, parasitic effects cannot be avoided, when the realization of the left-handed TL is done, Composite 2212-0173 2012 Published by Elsevier Ltd. doi:10.1016/j.protcy.2012.05.074 Open access under CC BY-NC-ND license.
Pratik Mondal et al. / Procedia Technology 4 ( 2012 ) 466 471 467 right/left-handed (CRLH) transmission lines represent a practical left-handed TL [4]. The CRLH TL concept was introduced because of purely LH TL cannot be able to exist due to parasitic. (b) (c) Fig1. Schematic of the Right handed TL (b) Left handed TL (c) CRLH TL The broadside coupled coplanar waveguides was chosen to achieve tight coupling with practical dimensions, which reflects an enhancement in the bandwidth [3-4]. In this paper, a new type of bandpass filter has been realized using CRLH structure and floating slot on ground plane. It is shown that by proper design of the slotted ground, the coupling between the two microstrip lines can be increased dramatically keeping practical dimensions for the coupled line width and spacing. Nomenclature CRLH-TL: Composite right/left handed transmission line OSO: open-short-open SOS: short open-short 2. Composite Right/Left handed Transmission line The normal-modes of symmetrical coupled lines are normal & coupled mode. Normal modes are then classified as even and odd mode. The coupling between symmetrical lines can be determined in terms of characteristic impedance and phase velocities and for asymmetrical lines it can be designed as C and parameters. The coupled microstrip lines dielectric medium is not homogeneous, because of that propagation velocities for the even and odd modes are also different. So, the coupling between two microstrip lines will be described using homogeneous dielectric medium equations, where the electrical lengths are the same for both the modes.the current and voltage of the four port network as shown in Fig. 2 can be related as,v=[z][i]. Fig.2: Schematic diagram of pair of coupled lines The port 3 and port 4 terminated with short (V 3 =0) and open (I 4 =0) circuits; respectively to realize the unit cell approach and the network becomes a two-port network. By eliminating I 3 from the equations we can get the ABCD transmission matrix
468 Pratik Mondal et al. / Procedia Technology 4 ( 2012 ) 466 471 It has been observed from the above equation that, CRLH cell can be represented as series impedance and parallel admittance and can be represented by either two configurations depending on the transformer position as shown in Fig. 3. The equivalent circuit parameters in Fig. 3 are given by, Fig3. Realization1 (b) Realization2 (b) The parallel admittances Y p and Y p acts as inductive susceptance for the coupled line electrical length < /2 and capacitive for > /2.Similarly for Z s and Z s. To cancel the presence of the ideal transformer,the CRLH unit cell may be constructed by cascading the above said circuit with its mirrored image. Here we found two configurations, either the unit cell shown in Fig. 4 and its equivalent circuit is shown in Fig. 4(b) as T-network, or the circuit of Fig. 5 and its equivalent circuit is shown in Fig. 5(b) as II network. The configuration in Fig. 4is known as OSO (open-short- open ports) and the configuration of Fig. 5 is SOS(short-open-short port).the design equations for the OSO and the SOS terminating impedance (i.e. Zo = 50 ohm) are (b)(c) Figure 4: OSO Coupled line configuration. (b) cascaded OSO configuration (c) equivalent T -Network
Pratik Mondal et al. / Procedia Technology 4 ( 2012 ) 466 471 469 (b) (c) Figure 5: SOS Coupled line configuration. (b) cascaded SOS configuration (c) equivalent -Network The floating slot at the ground plane solves the difficulties arising in the design procedure and the fabrication process as it relaxes the requirements on physical dimensions of the lines. In this design, adding the floating slot in the ground conductor as tends to equalize the even and the odd phase velocities and also increase the coupling dramatically. 2. Implementation of CRLH structure The CRLH structure is implemented by using edge coupled microstrip lines with a ground plane aperture. For getting high coupling instead of using conventional edge coupled microstrip lines the floating is used which makes it possible to design unit CRLH bandpass filter and maintained all connections in the same plane.the floating ground solves the difficulties arised in the design and the fabrication process as it relaxes the required physical dimensions of the lines and also equalizes the even and odd phase velocities. As the CRLH transmission line network is a combination of the right and left-handed network, it is a bandpass filter. In this design as illustrated in Fig.6, a substrate of dielectric constant, r = 4.4 and thickness, h = 1.59 mm and loss tangent = 0.02 is used. Here, the gap between two coupled lines S g =0.5mm is considered. To realize Zoe = 184.46 and Zoo = 73.78, the values of W = 0.68mm, S g =0.5mm and the gap between two slot Ss=5 mm are obtained. The length of the coupled line of the unit cell is taken as 26 mm and length of the floating slot as 25.5 mm for both the SOS and OSO model. (b) Figure 6: Schematic diagram of theproposed OSO Coupled CRLH unit cell (b) SOS coupled CRLH unit cell 3. Results and Discussions The unit CRLH cell of the both OSO and SOS model has been simulated using MoM based IE3D software and the simulated S-parameters are shown in Fig 7. The center frequency at 1.76 GHz and 3dB bandwidth of 0.99 GHz with insertion loss of -0.783 is observed in OSO model. Also the Selectivity 47.36 db/ghz for rising edge and 38.63 db/ghz for falling edge is observed. The center frequency at 1.74 GHz and 3dB bandwidth of 0.18GHz is observed in SOS model. The selectivity for SOS model is
470 Pratik Mondal et al. / Procedia Technology 4 ( 2012 ) 466 471 18.88 db/ghz for rising edge and 35.411 db/ghz for falling edge. The simulated results for the SOS unit cell is 1.09 GHz to 2.18 GHz at 20dB with insertion loss of -1.99dB. (b) Fig.7: simulated s-parameters OSO modelled CRLH unit cell (b) SOS modelled CRLH unit cell A prototype filter has been fabricated for OSO model on FR-4 substrate which has dielectric constant of 4.4 and thickness of 1.59mm. The fabricated unit has been measured using Agilent make vector network analyzer (model N5230A). The measured results in Fig.-8 shows center frequency at 1.762 GHz and 3dB bandwidth of 1GHz in comparison to the simulated results obtained as center frequency at 1.76 GHz and 3dB bandwidth of 0.99 GHz. As the CRLH transmission line network is a combination of the right and left-handed network, it is a bandpass filter ranges from 0.62 GHz to about 3.68GHz at 30dB with an insertion loss of -0.823dB. In experimental results we obtained the fractional bandwidth of 56.7% for the 3dB bandwidth of 1GHz and center frequency of 1.76GHz in OSO model. The EM-simulated, experimentally measured and circuit model results are compared as shown in Fig. 9. (b) Fig. 8: Photographh of the proposed unit CRLH cell (OSO model) Bottom Layer (b) Top Layer Fig. 9: Comparative study between measured & simulated s-parameters
Pratik Mondal et al. / Procedia Technology 4 ( 2012 ) 466 471 471 4. Conclusion We propose to design a bandpass filter using composite right/left-handed (CRLH) transmission lines (TL) and floating slot in ground plane. We verified this idea through implementation on coupled microstrip lines with slotted ground. It is found that OSO model is for wide band application whereas the SOS model may be used for the narrow band application. The vias for both the modelling are almost identical except the positions are different. It offers low pass band insertion loss and wide bandwidth. Such CRLH filters may be found suitable in various applications. Reference 1. F. Bongard, J. Perruisseau-Carrier, J.R. Mosig. Enhanced CRLH Transmission Line Performances Using a Lattice Network Unit Cell. vol. 19. IEEE Microwave and Wireless Components Letters; 2009, p. 431 433 2. Sun Mingming, Qin Weiping, Wang Haimeng, Chen Dong. Resonator and Bandpass Filter Using CRLH Transmission Line Based on Microstrip-Coplanar-Waveguide Structure. vol. 3.Int.Microw and Millimeter Wave Technology; 2008,p. 1530-1592 3. Aya F. Abdelaziz, Tamer M. Abuelfadl, and Osman L. Elsayed. Realization of Composite Right/Left-Handed Transmission Line Using Broadside Coupled Coplanar Waveguides. IEEE Antennas and Propagation Symposium; 2009, p. 1-4 4. A. F. Abdelaziz, T. M. Abuelfadl, and O. L. Elsayed. Realization of composite right/left-handed transmission line using coupled lines. PIER 92.Progress In Electromagnetics Research; 2009, p. 299 315