IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 12, DECEMBER 2007 843 A Broadband Substrate Integrated Waveguide (SIW) Planar Balun Zhen-Yu Zhang and Ke Wu, Fellow, IEEE Abstract A broadband planar balun is presented in this work that makes use of a substrate integrated waveguide (SIW) technique using a printed circuit board process. The proposed balun structure is able to operate at millimeter wave frequencies and it does not require any tight line coupling sections as frequently used in monolithic microwave integrated circuit balun design. In addition, this balun can be integrated with other planar topologies including nonplanar circuits made of the same substrate for achieving high efficiency. This balun structure consists of a 3 db SIW power divider and microstrip lines that are placed on both sides of the substrate at balanced ports to obtain an 180 phase shift. The concept is validated by simulations and measurements. Our measured results suggest that a 10 db return loss at unbalanced port can easily be achieved across a 42% bandwidth from 19 to 29 GHz. Measured amplitude and phase imbalance between two balanced ports are within 1 db and 5, respectively. Index Terms Broadband structure, millimeter wave techniques, planar balun, substrate integrated waveguide (SIW). I. INTRODUCTION ABALUN is a device or circuit that converts signals between an unbalanced circuit structure and a balanced counterpart. A large number of analog radio frequency (RF) and microwave circuits require balanced inputs and outputs in order to reduce noise and high order harmonics as well as improve dynamic range of circuits. With the inherent features of generating balanced signals, baluns are widely used in many wireless communication systems for realizing critical building blocks such as balanced mixer, push pull amplifier and antenna feed networks. Although various types of baluns have been reported for applications in connection with microstrip circuits in the form of microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs), a planar balun structure with favourable performance has been a challenging research topic over the past several decades [1] [4]. Among them, the 180 hybrids and multisection half-wave baluns are frequently used in microwave circuits as they can easily be realized in many design procedures and fabrication processes, Manuscript received April 15, 2007; revised August 20, 2007. This work was supported in part by the National Sciences and Engineering Research Council of Canada. The authors are with the Poly-Grames Research Center, Département de Génie Électrique, Ecole Polytechnique de Montreal, Montreal, QC H3T 1J4 Canada (e-mail: zhang.zhenyu@polymtl.ca; ke.wu@polymtl.ca). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LMWC.2007.910479 and also they provide satisfactory performances, namely, low voltage standing wave ratio (VSWR), small insertion loss and good amplitude/phase imbalance between balanced ports. However, they fail to maintain those advantageous features over a wide frequency range. Furthermore, they are hardly employable when the operating frequency goes into millimeter-wave frequency band. The Marchand balun is a known broadband solution but its performances largely depend on a tight coupling of the coupled line sections. In addition, it is difficult to design with the conventional printed circuit board (PCB) process at frequencies beyond 20 GHz. Due to those challenging issues in the design of planar baluns, non-planar baluns are usually used instead [5], [6]. However, the use of a non-planar topology may prevent itself from its seamless integration with planar circuits, especially with respect to surface mounted chip structures. In this letter, a broadband substrate integrated waveguide (SIW) balun is proposed and presented, which is made in planar form. The SIW concept has been proposed and demonstrated in many previous publications [7], [8]. The fundamental point is that the usually non-planar hollow rectangular waveguide can effectively be synthesized in planar substrate with metallized slots or trenches or even arrays of metallic via posts. The proposed broadband SIW-based planar balun consists of a 3 db SIW power divider [7] for equal power splitting and microstrip lines which are placed on different sides of a dielectric substrate at balanced ports to obtain a 180 phase shift. Since the SIW power divider presents broadband characteristics, the newly proposed balun structure can provide a broadband frequency response, which is a critical feature for many wireless communication systems. On the other hand, the planarity of balanced ports can further be achieved by using a metallic via hole to conduct one of the differential signals from one side to the other. As such, the new balun can easily be designed and fabricated on a double-sided PCB substrate. Compared to previously reported structures, the proposed balun can easily be used at millimeter wave frequencies and also the structure does not require tight coupling sections as usually made in many MMIC balun structures. Of course, the proposed topology can be integrated with other planar structures including nonplanar circuits within the same substrate for achieving high efficiency at low cost. To demonstrate the design methodology and structural features, a prototype of the balun is presented in this work. The design was made through the use of a commercial software package (Ansoft HFSS v 10.1 [9]) and validated by experiments. 1531-1309/$25.00 2007 IEEE
844 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 12, DECEMBER 2007 Fig. 1. Proposed broadband SIW planar balun. Fig. 2. Electrical field plots on the A plane. II. GEOMETRY AND DESIGN OF BROADBAND SIW PLANAR BALUN STRUCTURE The SIW structure is known to have broadband monomode characteristics compared to its conventional waveguide counterpart as the SIW usually supports only modes. This is because the synthesis of bilateral metallic walls is made with discontinued metallized via or slot arrays in the thin dielectric substrate, which does not allow the guidance of the TM modes. Therefore, the monomode bandwidth is guaranteed with mode related to mode. In addition, the SIW yields better loss characteristics compared with other schemes like microstrip and coplanar waveguide (CPW) lines. It can be used at millimeter wave frequencies where loss becomes critical. Fig. 1 shows the structure of the proposed broadband SIW planar balun, in which the SIW section is realized by using metallic slot arrays. The dark area stands for the metallization parts on the bottom plane of the double-sided PCB substrate and the gray area stands for the metallization on the top plane. The proposed balun consists of a 3 db SIW power divider for equal power splitting and microstrip line sections which are placed on different sides of the PCB substrate at balanced ports so as to obtain an 180 phase shift. To further demonstrate this idea with a better explanation, an intersection plane A is chosen, which is marked in Fig. 1. Fig. 2 illustrates the electrical fields on plane A, from which it can be found that the E-field orientations of balanced ports are different from each other, which means an 180 phase difference. The transition between the SIW and microstrip line is referred to a well-described tapered line structure given in [10]. The only difference is that the ground plane of the transitions is also modified in the form of tapered section so that the transitions can provide a better performance. The planarity of balanced ports is further achieved using a metallic via hole transition to conduct one of the differential signals from one side to the other. As shown in Fig. 3, the via-hole transition and the tapered ground plane were carefully designed to ensure that the RF signal can Fig. 3. Physical dimensions of the experimental prototype of the proposed SIW planar balun (w = 1.74 mm, w = 0.6 mm, w = 2.14 mm, w = 5.2 mm, w = 5.2 mm, l = 12 mm, l = 3 mm, l = 7 mm, l = 5.6 mm, l = 4.25 mm, s = 0.3 mm, s = 0.6 mm, s = 0.15 mm, x = 1.8 mm, x = 1.6 mm, y = 0.4 mm, r = 1.6 mm, r = 0.75 mm, = 0.6 mm and = 0.25 mm). be smoothly transferred to the output port with very low insertion loss and little discontinuity. All the interconnections that generate the discontinuities were made into tapered shape so that the discontinuity and inductance effects were minimized. The optimization was done with the aid of Ansoft HFSS v 10.1. The general design considerations of the tapered lines can be referred to the previous publication [6]. To demonstrate the design methodology and electrical performances, an experimental prototype of the proposed balun is designed and fabricated at 24 GHz. The substrate used in our work is RO6002 with a thickness of 10 mil and a dielectric constant 2.94. Fig. 3 shows the physical dimensions of the designed broadband SIW planar balun. The via hole transition part is enlarged to provide a clear illustration. The gray area in this case stands for the metallization on the top plane of the substrate while the dashed-line area stands for the metallization on the bottom plane of the substrate. The SIW is realized by using metallic slots. The balanced ports are extended to different directions just for the measurement consideration. III. SIMULATION AND MEASUREMENT RESULTS Fig. 4(a) (b) display the simulated and measured results for the amplitude response and phase balance of the proposed planar balun, in which the solid line stands for the simulated results and the dashed line stands for the measured results. This balun is simulated by using Ansoft HFSS v 10.1. The measurements are carried out by using an Aristu test fixture model 3680 K with the maximum frequency up to 40 GHz and an HP 8510C vector network analyzer. The calibration work is
ZHANG AND WU: BROADBAND SIW PLANAR BALUN 845 IV. CONCLUSION A broadband SIW planar balun implemented on a PCB is proposed and presented in this letter. The balun structure consists of a 3 db SIW power divider for equal power splitting and microstrip lines which are placed on different sides of the PCB substrate at balanced ports to obtain an 180 phase shift. To demonstrate the design concept, an experimental prototype is fabricated. The balun structure is simulated and validated by the measurement. The merits of this balun are concluded by the fact that it can easily operate at millimeter wave frequencies and does not require any tight coupling sections as usually used in many MMIC balun structures. In addition, the balun can be integrated with other planar circuits including nonplanar and multilayered structures for achieving high circuit efficiency. It can be anticipated that the proposed balun will have a broad range of RF and millimeter-wave applications. REFERENCES Fig. 4. S-parameters of the proposed balun: (a) amplitude responses and (b) phase responses. done by using a set of TRL calibration standards. It is observed that a good agreement is achieved between the measurement and the simulation. Measured results show that better than 10 db return loss of the unbalanced port is well achieved across the whole bandwidth of interest from around 19 to 29 GHz, or 42% of bandwidth. Within the bandwidth of operation, the measured amplitude and phase imbalance between the two balanced ports are respectively within 1 db and 5, except that the phase imbalance reaches 8 at around 28 GHz. [1] N. Marchand, Transmission-line conversion transformers, Electronics, vol. 17, no. 12, pp. 142 145, Dec. 1944. [2] R. Sturdivant, Balun designs for wireless mixers, amplifiers and antennas, Appl. Microw., pp. 34 44, 1993. [3] M. Basraoui and S. N. Prasad, Wideband, planar, log-periodic balun, in IEEE MTT-S Int. Dig., 1998, pp. 785 788. [4] Z. Y. Zhang, Y. X. Guo, L. C. Ong, and M. Y. W. Chia, Improved planar marchand balun with a patterned ground plane, Int. J. RF Microw. CAE, vol. 15, no. 3, pp. 307 316, May 2005. [5] S. A. Maas, Microwave Mixer, 2nd ed. Norwood, MA: Artech House,, 1993, pp. 219 312. [6] J. W. Duncan and V. P. Minerva, 100:1 Bandwidth balun transformer, IRE Proc., vol. 48, pp. 156 164, 1960. [7] S. Germain, D. Deslandes, and K. Wu, Development of substrate integrated waveguide power dividers, in Proc. IEEE Canadian Conf. Elect. Comput. Eng., 2003, pp. 1921 1924. [8] D. Deslandes and K. Wu, Single-substrate integration technique of planar circuits and waveguide components, IEEE Trans. Microw. Theory Tech., vol. 51, no. 2, pp. 593 596, Feb. 2003. [9] Ansoft Corporation: Ansoft HFSS, Version 10.1 2006. [10] D. Deslandes and K. Wu, Integrated microstrip and rectangular waveguide in planar form, IEEE Microw. Wireless Compon. Lett., vol. 11, no. 2, pp. 68 70, Feb. 2001.
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