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Design of a Wideband Balun Using Parallel Strips Jin Shao, Student Member, IEEE, Rongguo Zhou, Member, IEEE, Chang Chen, Xiao-Hua Wang, Hyoungsoo Kim, Member, IEEE, and Hualiang Zhang, Member, IEEE
Abstract—A wideband balun designed by exact synthesis is presented in this letter. The designed balun has a symmetrical four-port structure with one port open-ended. It consists of a wideband impedance transformer and a broadband phase inverter, which is realized by connecting bottom and top layer parallel strip through two via holes. To design the wideband impedance transformer, an -plane highpass prototype has been exactly synthesized with third-order Chebyshev response. The proposed design procedure is very flexible, and it can be easily scaled for other frequency bands. A wideband parallel strip balun operating from 0.72 to 2.05 GHz is fabricated and tested to verify the design concepts. The measured results match well with the design theory. Index Terms—Balun, even-odd mode, exact impedance transformer, phase inverter, wideband.
synthesis,
I. INTRODUCTION
B
ALUNS are used to convert unbalanced signals to balanced one, and vice versa. They have been widely used in differential-mode RF/Microwave circuits and antenna feeding networks. Baluns can be divided into two categories, namely, active and passive baluns. For active baluns, it will consume more energy. As for passive baluns, they can be further classified as lumped-type and distributed-type [1]–[7]. The disadvantage of lumped-type balun is that it often exhibits poor phase differences and magnitude response between the two signals. The distributed-type balun features low loss and low cost. Therefore, this type of balun is attractive for practical applications. To meet the requirements of modern wireless communication, the designs of passive components are facing new challenges, including size reduction [1], [2], multi-band operation [3], [4], and bandwidth enhancement [5]–[10]. Size miniaturization can be achieved by adding lumped components in distributed-type balun [1], or by using multi-layer structure [2]. By adding two additional stubs and providing two coupling path, designs of dual-band baluns have been introduced in [3], [4]. For bandwidth enhancement, a broadband balun was designed based on composite right/left-handed transmission line [5]. In [6], [7],
Manuscript received September 22, 2012; accepted January 29, 2013. Date of publication February 13, 2013; date of current version March 07, 2013. J. Shao is with the Electrical Engineering Department and Computer Science and Engineering Department, University of North Texas, Denton, TX 76203 USA. R. Zhou, H. Kim, and H. Zhang are with the Electrical Engineering Department, University of North Texas, Denton, TX 762013 USA (e-mail: hualiang.
[email protected]). C. Chen is with the Department of EEIS, University of Science and Technology of China, Hefei, Anhui, China. X.-H. Wang is with the School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu, China. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LMWC.2013.2246150
Fig. 1. General schematic of the proposed wideband parallel-strip balun (size: 71 mm by 62 mm).
a three-line balun and a balun filter with wide bandwidths were introduced by using exact synthesis. In [8]–[10], by applying 180 CPW phase inverter [8] and parallel-strip phase inverter [9], [10], the wideband operations have also been reached. In this letter, a new wideband parallel-strip balun designed by exact synthesis is presented. The proposed balun consists of a broadband phase inverter and a wideband impedance matching network. It is found that it has many advantages such as: the designed procedure is very flexible and easy to be applied for other frequency bands; the proposed structure can be also applied to design other microwave components with wide bandwidth [12]. To verify the design concept, a parallel strip balun working from 0.72 to 2.05 GHz is designed, fabricated, and tested. The measurement results agree well with the simulation results. II. GENERAL STRUCTURE AND DESIGN Fig. 1 shows the schematic diagram of the proposed parallel strip balun. It has a symmetrical four port structure with one port open-ended. The broadband phase inverter is designed by parallel strips [11]. To realize the phase inverter function, the upper and lower layer of the parallel strip is linked through two via holes, as shown in the inset of Fig. 1. In this way, very wide[9], [10], [12]) phase band (e.g., fraction inverter can be achieved. To ensure the proper balun operation, it is found that the proposed balun needs to meet a transmission stop condition under the even-mode excitation, and an impedance matching condition under the odd-mode excitation [1]. Our designed structure is aimed to match these two requirements. Since the designed balun has a symmetrical structure, it can be analyzed by even-odd mode method. Fig. 2 illustrates the even- and odd-mode equivalent circuits of the proposed balun. At even-mode [Fig. 2(a)], due to the effect of the wideband phase inverter, the proposed structure is shorted at the port 1, providing the desired transmission stop. At
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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 23, NO. 3, MARCH 2013
Fig. 2. (a) Equivalent circuit under even-mode excitation, (b) equivalent circuit under odd-mode excitation.
odd-mode [Fig. 2(b)], the wideband phase inverter is equivalent to an open-circuit. Additional structure is needed to perform as a wideband impedance matching network to match the to . In this letter, the impedance matching network is designed by exact synthesis with third order Chebyshev response. This method has advantages of wide bandwidth response, and it can be easily applied to other frequency bands. In Section III, more details about the design method of exact synthesis will be introduced. III. EXACT SYNTHESIS To design a wideband impedance matching network, a high-pass -plane prototype with third-order Chebyshev response has been proposed. It has advantages such as low element number requirement. Fig. 3(a) shows the non-redundant prototype of the -plane high-pass network with four elements (including the transformer), which is corresponding to a wide band-pass characteristic in the -plane. The design target of this -plane high-pass prototype is to satisfy the following specifications: commensurate frequency at , 0.045 dB pass-band ripple, , and a low cutoff frequency at . After carrying out the synthesis procedure [13], the driving point impedance of the synthesized prototype can be derived as
(1) Based on the function given in (1), the normalized design parameters of the prototype can be extracted as: , , and . For the purpose of practical realization, the transformer in the prototype needs to be eliminated. The specific implementation procedure can be summarized as follows. 1) First, we divided into two parallel inductors and [ didn’t shown in Fig. 3(b)]. 2) Second step is to create a new transformer (0.707:1) by applying Kuroda Identities. , , will be transferred
Fig. 3. (a) Non-redundant prototype of the high-pass S-plane network, (b) prototype after applying Kuroda Identities, (c) prototype with only transmission lines.
to , , and [as shown in Fig. 3(b)]. In this way, two transformer (0.707:1) and (1:0.707) can be cancelled out with each other. 3) Third step is to implement the prototype using quarterwave length transmission lines (at ). and can be replaced by transmission line and . By applying Richards Transformation, and can be realized by shunt short-ended stub and [as shown in Fig. 3(c)]. The parameters in Fig. 3(b) can be calculated as: , , and . The final prototype is designed with four transmission lines as shown in Fig. 3(c). These four transmission lines are all quarterwave length at commensurate frequency. The impedances of , , , and are 62.03 , 51.00 , 43.86 , and 105.82 , respectively. This wideband impedance matching network with third-order Chebyshev response can be easily scaled for other frequency bands by simply changing the commensurate frequency of these four transmission lines. IV. SIMULATION AND EXPERIMENTAL RESULTS To verify the design concept, a wideband parallel strip balun working from 0.72 to 2.05 GHz with center frequency at 1.39 GHz was fabricated on a Duroid 5880 substrate with a dielectric constant of 2.2 and substrate thickness of 0.787 mm. The size of the proposed balun is 71 mm by 62 mm. The top view of the fabricated balun is illustrated in Fig. 4. Fig. 5 shows the calculated, simulated (using Mentor Graphics’ IE3D) and measured results of , , and of the proposed balun. The calculated and are identical, and they are larger than from 0.67 to 2.11 GHz. Both the calculated and the measured are below from 0.72 to 2.05 GHz, which indicates a 96% fractional bandwidth. The maximum measured return loss is 28.1 dB, and the minimum measured insertion loss is 3.1 dB. The measured amplitude differences between and
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manually drilled four via holes to connect the upper and lower layer of parallel strips. Overall, the measurement results have clearly shown the wideband operation of the proposed balun, validating our design concept. V. CONCLUSION
Fig. 4. Photo of the fabricated balun.
A new broadband parallel strip balun has been presented in this letter. The proposed balun has a symmetrical structure, and it consists of a broadband phase inverter and a wideband impedance matching network. By adopting parallel strip transmission lines, the broadband phase inverter has been obtained. The wideband impedance matching network is designed by exact synthesis. This wideband matching network can be easily applied to other frequency bands. It can also be applied to design other wideband microwave components such as a wideband Wilkinson power divider. To verify the design concept, a wideband parallel strip balun working from 0.72 to 2.05 GHz is fabricated and measured. Good agreement has been achieved between the simulation and measurement results. REFERENCES
Fig. 5. Calculated, simulated and measured S-parameters of the proposed parallel strip balun.
Fig. 6. Simulated and measured phase difference between the two output ports and ). (
are within 1 dB from 0.74 to 2.13 GHz (shown in Fig. 5). Fig. 6 shows the simulated and measured phase differences between and . Within the 10 dB return loss bandwidth, the measured phase difference between the two output ports is smaller than 4.8 . The fabrication tolerance contributes to the discrepancy between the theoretical and measured results, as we
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