A New Uwb Directional Coupler Based on a Combination Between Cb-cpw and Microstrip Technologies

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    A New Ultra wideband Directional coupler Based on a Combination between CB-CPW and Microstrip Technologies Mourad Nedil and   Tayeb A. Denidni Université de Québec, INRS-EMT, Place Bonaventure, 800, de La Gauchetière Ouest, bureau 6900, Montréal (Québec), H5A 1K6.  Abstract — In this paper, a novel ultra wideband directional coupler employing conductor backed coplanar waveguide CB-CPW and microstrip multilayer slot coupling technique is presented and implemented. The coupler uses two different transmission lines CB-CPW and microstrip, printed on two stacked substrate layers and coupled through a rectangular slot etched on a common ground plane located between these lines. Firstly, an analysis technique was used to obtain the coupler even and odd mode characteristic impedances. Secondly, using this approach, a new design of the directional coupler was performed. Simulation and experimental results show a good performance in terms of bandwidth, which covers the entire ultra-wideband operation (3.1-10.6 GHz). Index Terms— directional coupler, coupler analysis, CB-CPW/microstrip lines. I. I NTRODUCTION  In recent years, there has been growing interest in Ultra-wideband (UWB) systems, since the Federal Communications Commission (FCC)’s decision to permit the unlicensed operation band from 3.1 to 10.6 GHz in 2002 [1]. UWB has emerged as a very promising technology for future short-range data communication and localization systems. The wide bandwidth available in UWB systems can lead to many potential benefits such as a high data rate and greater immunity to multipath fading. In this area, various studies are under progress, especially in UWB couplers, which is one of the key passive components in the design of microwave circuits and systems. Directional couplers are fundamental and indispensable components used in microwave integrated circuits (MIC’s) applications. Indeed, these components are often used in microwave systems to combine or divide RF signals, and they are commonly used in many applications, such as antenna feeds, balanced mixers, modulators and so on. Tight-coupling directional couplers are often required in the design of various multiport circuits or beam-forming printed antenna array. In practice, the most convenient form of these couplers should be one which can easily be integrated in the same circuit. However, in many multilayered structures, microstrip and coplanar waveguide CPW transmission lines coexist and are even combined to form circuit components like filters [2]. Some applications, such as multilayer microwave integrated circuits, require the flexibility to use both integrated microstrip and CPW circuits. Few research has been reported on designing directional couplers with combining CPW and microstrip technologies [3-6]. A 3-dB coupler using microstrip-to-CPW via-hole transitions has been reported in [3]. Another double-sided branch-line coupler using CPW-to-microstrip via-hole transitions has been proposed in [4]. Miniaturization of microstrip rat race coupler by incorporating a broadside-coupled structure and stepped-impedance line sections is implemented in [5]. The most interesting configuration using broadside-coupled CPW/microstrip lines has been proposed in [6]. In this paper, a new structure of wideband multilayer directional coupler using a broadside CB-CPW/microstrip slot-coupled is proposed and analyzed. The use of multilayer technology in this design is considered as an alternative method to conventional single layer to develop design couplers with tight coupling and to reduce in dimensions. To validate the proposed approach, a prototype circuits were analyzed, designed and fabricated. Simulations and measurements were performed, and the obtained results show a good agreement. II. C OUPLER A NALYSIS  Fig. 1 shows the layout of the proposed slot-coupled directional coupler. It allows coupling two different lines CB-CPW/microstrip placed in different layers through a slot etched on the common ground plane between these layers. This component has the following property: if Port 1 is fed, then the signal travels to Port 2 (direct), and consequently, Port 3 is coupled while Port 4 is isolated. The input power is split equally (3 dB off) between the two output ports, and the two signals presents 90 °  out of phase. For this type of couplers, a numerical analysis takes into account the odd and even modes of propagation. These modes are illustrated in Fig. 2. They can be isolated by assuming an electrical wall for the odd mode and a magnetic wall for the even mode. The even modes propagate when equal currents, in amplitude and phase, flow on the two coupled lines, while the odd mode is obtained when the currents have equal amplitudes but opposite phases [7]. The coupled region is characterized by two characteristic impedances,  Z  0,o  and  Z  0,e . The characteristic impedance in the coupling section is given by the combination of the impedances of both modes. The analysis of this coupler was done using ANSOFT HFSS [8], a commercial computer software package by considering a rectangular shaped slot coupling region. The characteristic impedances are computed by HFSS based on the obtained electric and magnetic field quantities in the model. It provides 978-1-4244-1780-3/08/$25.00 © 2008 IEEE1219    0,51,01,52,0010203040506070    O   d   d  -  m  o   d  e  c   h  a  r  a  c   t  e  r   i  s   t   i  c   i  m  p  e   d  a  n  c  e ,   Z    0 ,  e  ,   (      Ω    ) S (mm) G=1 mm G=2 mm G=3 mm G=4 mm 0,51,01,52,00102030405060708090100    E  v  e  n  -  m  o   d  e  c   h  a  r  a  c   t  e  r   i  s   t   i  c   i  m  p  e   d  a  n  c  e ,   Z    0 ,  e  ,   (      Ω    ) S (mm) G=1 mm G=2 mm G=3 mm G=4 mm 0,51,01,52,020304050607080    O   d   d  -  m  o   d  e  c   h  a  r  a  c   t  e  r   i  s   t   i  c   i  m  p  e   d  a  n  c  e ,   Z    0 ,  e  ,   (      Ω    ) S (mm) W m = 1 mm W m = 2 mm W m = 3 mm W m = 4 mm the calculation of the characteristic impedance of the mentioned transmission lines that are considered as  Z  0,o  and  Z  0,e , respectively. Fig. 1: Layout of directional slot-coupled coupler. Fig. 2: Odd and even-mode electric field distribution These characteristic impedances are generally function of the following variables: center conductor strip G , slot width S  , microstrip line width W  m , substrate thickness h,  and slot coupled width W  S  . Based on the work reported in [9], the input characteristic impedance  Z  0  and the coupling coefficient K   are defined as: eo  Z  Z  Z  ,0,00  =  (1) oeoe  Z  Z  Z  Z K  ,0,0 ,0,0 +−=   (2) As a function of the slot width S,  the computed even- and odd-mode characteristic impedances for the proposed coupler are plotted in Fig. 3 and Fig. 4, respectively. Fig. 3 demonstrates the curves for the odd-mode and even-mode characteristic impedances  Z  0,o  and  Z  0,e   versus slot width of CB-CPW line S  , varied from 0.1 to 2 mm. The curves were computed for the strip width (CB-CPW) G  varied from 1 to 4 mm, and fixed values of substrate thickness h = 0.254 mm and slot coupled width W  S  = 5mm. As a result, the characteristic impedance  Z  0,o   decreases when S   increases as observed in Fig. 3a, and  Z  0,e  increases when S   increases as shown in Fig. 3b. Fig. 4 shows the effect of characteristic impedance  Z  0,o  and  Z  0,e   on slot width of CB-CPW line S  , established for various values of microstrip width W  m , and fixed strip width G  = 4mm, substrate thickness h = 0.254 mm and slot coupled width W  S  = 5mm. In this case,  Z  0,o  and  Z  0,e  decrease while the microstrip width W  m  increases. (a) (b) Fig. 3: Odd- and Even-mode characteristic impedance as a function of S   and G  (a) 1 2 3 4 S S G W S   h h W m  Bottom Layer Bottom Layer Slot-coupled region W  g   L g   (b) Electrical Wall C’ C Magnetic Wall C’ C (a) 978-1-4244-1780-3/08/$25.00 © 2008 IEEE1220    0,51,01,52,00,00,10,20,30,40,50,60,70,80,91,0    C  o  u  p   l   i  n  g  c  o  e   f   f   i  c   i  e  n   t S (mm) G=1 mm G=2 mm G=3 mm G=4 mm 0,51,01,52,00,00,10,20,30,40,50,6    C  o  u  p   l   i  n  g  c  o  e   f   f   i  c   i  e  n   t S (mm) Wm= 1 mm Wm= 2 mm Wm= 3 mm Wm= 4 mm 34567891011-60-50-40-30-20-100   m  a  g  n   i   t  u   d  e   (   d   B   ) Frequency (GHz) S(1,1) S(1,2) S(1,3) S(1,4) 34567891011-60-50-40-30-20-100    M  a  g  n   i   t  u   d  e   (   d   B   ) Frequency (GHz) S(1,1) S(1,2) S(1,3) S(1,4) 0,51,01,52,00102030405060708090100110    E  v  e  n  -  m  o   d  e  c   h  a  r  a  c   t  e  r   i  s   t   i  c   i  m  p  e   d  a  n  c  e ,   Z    0 ,  e  ,   (      Ω    ) S (mm)  W m =1 W m =2 W m =3 W m =4  (b) Fig. 4: Odd- and Even-mode characteristic impedance as a function of S   and W  m  (a) (b) Fig. 5: Coupling coefficient as a function of S, G and W  m  The computed coupling coefficient K   is plotted in Fig. 5. Fig. 5a and Fig. 5b present the influence of the coupling coefficient on the slot width S   for various strip conductor G  and W  m  for a fixed value of the   substrate thickness h = 0.254 mm, microstrip width W  m = 4mm and slot coupled width W  S  = 5mm. From both figures, it can be seen that, the coupling coefficient increases when S, W  m   and G  increase. All of these results were obtained at the center frequency of 6 GHz. III. C OUPLER  D ESIGN   AND  P ERFORMANCES  The obtained results from the coupler analysis are used to design a new 3 –dB CPW/microstrip multilayer rectangular slot-coupled directional coupler as a practical example. The top and bottom 50 Ω  transmission lines were designed using a Duroid substrate (RT/ Duroid 5880) having a dielectric constant of ε   r  =2.2 and a thickness of h = 0.254 mm . (a) (b) Fig. 6: Scattering parameters of the proposed coupler (a) simulated, (b) measured The initial dimensions of the rectangular shaped slot coupling are obtained for  Z  0,o = 30 Ω  and  Z  0,e = 85 Ω  at 6 GHz when the coupling coefficient equal 0.5. These initial 978-1-4244-1780-3/08/$25.00 © 2008 IEEE1221    345678910110306090120150180    P   h  a  s  e   (   d  e  g   ) Frequency (GHz) Simulated Measured parameters were simulated with  IE3D  [10] an electromagnetic simulator and the optimized values are estimated and implemented. The optimum rectangular-shaped slot has G = 3.5  mm, W  m  =3.6, S = 1.5 mm, W = 5 mm and  L = 9.1 mm. The length  L  of the coupler was designed to be a quarter wavelength at 6 GHz.   However, in the CB-CPW technology, the main drawback is the parallel-plate modes, which are considered as unwanted bulk modes [11]. The minimum parasitic resonance frequency from the parasitic parallel-plate modes of the CB-CPW, which can be predicted, based on a simple rectangular patch theorem [11] as illustrated in the following equation: 22 2       +      = ggr mn  LnW mc f  ε    (3) where c  is the velocity of light, ε   r    is the relative permittivity, and where Wg  (= 9 mm) and  Lg  (= 15 mm) are the width and length of the ground of the coupler respectively (see Fig. 1). Using the above equation, the lowest order mode resonance frequency  f  11  of 13.6 GHz is calculated. As a result, this parasitic resonance frequency is shifted to a higher frequency regime. In this case, it can be noted that the leaky wave phenomenon does not affect the performance of the proposed directional coupler, which allows avoiding the use of via in the circuit. Fig. 7: Simulated and measured of phase difference To validate the proposed design, an experimental coupler prototype was fabricated and measured using an HP8772 network analyzer. The simulated and measured of the return loss and the insertion loss are shown in Fig. 6. From these results, it can be concluded that a bandwidth of ∼  7 GHz is achieved from 3.1 to 10.6 GHz. The average value of the coupling for the direct port and the coupled port is 3.5 dB. The return loss and isolation are better than 20 dB within the operating band. The simulated and measured phase shifts between the two ports are plotted in Fig. 7. The phase difference between the direct and coupled ports is approximatively 90 °  across the operating band, which supports the proposed approach. A data comparison of the simulated and experimental results shows a good agreement. IV.   C ONCLUSION  In this paper, multilayer directional couplers using broadside CB-CPW/microstrip slot-coupled has been analyzed and designed. The coupling is controlled by microstrip and CB-CPW lines and a coupling slot. The coupler has been fabricated and measured. It has shown UWB behavior across the band from 3.1 to 10.6 GHz. An excellent agreement between simulations and measurements was obtained over the entire operating frequency band (3.1–10.6 GHz). R EFERENCES   [1]   “Revision of part 15 of the Commission’s Rules Regarding Ultra-Wideband Transmission Systems,” ET-Docket 98-153, First note and order Federal Communications Commission, Feb. 14, 2002. [2]   C. Warns, W. Menzel and H. Schumacher, “Transmission lines and passive elements for multilayer coplanar circuits on silicon,” IEEE Trans. Microw. Theory Tech., vol. 46, no. 5, pp. 616–622. May 1998. [3]   J. C. Chiu, J. M. Lin, M. P. Houng and Y. H. Wang, “A PCB-compatible 3-dB coupler using microstrip-to-CPW via-hole transitions,” IEEE Microw. Wirel. Compon. Lett., vol. 16, no. 6, pp. 369 - 371, June 2006.   [4]   J. H. Lee and H. Y. Lee, “Novel quadrature branch-line coupler using CPW-to-microstrip transitions,”  IEEE MTT-S Int. Microw. Symp. Dig. , June 2000. vo1. 2, pp. 621 - 624. [5]   J. T. Kuo, Y. C. Chiou and J. S. Wu, “Miniaturized Rat Race Coupler with Microstrip-to-CPW Broadside-Coupled Structure and Stepped-Impedance Sections,”  IEEE MTT-S Int. Microw. Symp. Dig. , June 2007. pp. 169 – 172. [6]   F. Mernyei and H. Matsuura, “A new broadside-offset coupler using CPW and microstrip lines,” European Microwave Conference, Oct. 1993. pp. 617 – 620. [7]   M. Bona, L. Manholm, J.P. Satarski, and B. Svensson, “Low-Loss Compact Butler Matrix for a Microstrip Antenna,”  IEEE Trans. on Microwave Theory and Tech , vol. 50, pp. 2069-2075, Sept. 2002. [8]   ANSOFT HFSS, version 9. [9]   Rainee N. Simons, Coplanar Waveguide Circuits, Components, and Systems, Wiley 2001. [10]   IE3D 8.2, Zeland Software, Inc. Fremont, CA. [11]   W. H. Haydl, “On the Use of Vias in Conductor-Backed Coplanar Circuits,”  IEEE Trans, on Microwave Theory and Tech. , vol. 50, no. 6, pp. 1571-1577, June 2002. [12]   M. Nedil, T.A. Denidni and A. Djaiz, “Ultra-wideband microstrip to CB-CPW transition applied to broadband filter,”  IET, Electronics Letters, Vol. 43, no. 8, pp. 464 – 466, April 2007. 978-1-4244-1780-3/08/$25.00 © 2008 IEEE1222
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