IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 56, NO. 8, AUGUST 2008 1995

A Dual-Band Balun Using Broadside-CoupledCoplanar Striplines

Lap K. Yeung, Member, IEEE, Wang-Chi Cheng, and Yuanxun E. Wang, Member, IEEE

Abstract—In this paper, a dual-band balun is presented. Basedon the traditional Marchand balun configuration, this deviceutilizes a pair of 4 coupled-line sections and a single 4

open-circuit stub to achieve a dual-band balun-type function.An experimental prototype, operating at both 2.4- and 5.8-GHzbands, has been realized on a traditional printed circuit boardusing the broadside-coupled asymmetric coplanar stripline tech-nique. Experimental measurements show good agreement withfull-wave computer simulations, showing good potentials fordual-band/multiband RF front-end modules.

Index Terms—Balun, coplanar stripline (CPS), coupled trans-mission line, front-end module.

I. INTRODUCTION

T HE ever-increasing demand for advanced wireless com-munication applications necessitates RF transceivers

operating at multiple separated frequency bands. For example,high-speed wireless local are networks (LANs), offering upto 54-Mbit/s wireless access service, operate at both 2.4- and5-GHz bands. To accommodate such dual-band RF signal re-ception and transmission, devices such as dual-band antennas,dual-band baluns, and dual-band filters are currently gainingwide attention. Among various passive devices, the filter isconsidered as one of the most important component types,and therefore, most of the research efforts [1]–[6] have beenfocusing on this particular area, especially on the filter typesthat utilize stepped-impedance resonators to achieve dual-bandbehavior. A recent research publication [7] has demonstratedthe use of a transmission line stub to achieve dual-band filteringby introducing a narrow rejection band within a passband.

While a lot of attention has been paid to the filter, not muchwork has been done on the balun even though it is also a keyRF front-end functional passive device. The balun is a devicefor converting a balanced signal to an unbalanced one, or vice

Manuscript received February 4, 2008; revised April 25, 2008. First publishedJuly 25, 2008; last published August 8, 2008 (projected). This work was sup-ported by the Hong Kong Applied Science and Technology Research InstituteCompany Ltd.

L. K. Yeung was with the Hong Kong Applied Science and Technology Re-search Institute Company Ltd., Shatin, NT, Hong Kong. He is now with theElectrical Engineering Department, University of California at Los Angeles,Los Angeles, CA 90095 USA (e-mail: lkyeung@msn.com).

W.-C. Cheng is with the Advanced Wireless Technology Group, Hong KongApplied Science and Technology Research Institute Company Ltd., Shatin, NT,Hong Kong.

Y. E. Wang is with the Electrical Engineering Department, University of Cal-ifornia at Los Angeles, Los Angeles, CA 90095 USA.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMTT.2008.927390

Fig. 1. Proposed dual-band balun.

versa. A balanced signal consists of two signal components withthe same magnitude, but 180 out-of-phase. Many analog cir-cuits, such as mixers, amplifiers, and multipliers, require a bal-anced input or output to achieve noise or high-order harmonicsreduction. A dual-band balun [8] using a tapered-line struc-ture has been recently proposed. It is essentially a conventionalMarchand balun with its uniform coupled-line sections replacedby their tapered counterparts.

In this paper, a novel dual-band balun is presented. It simplyconsists of two coupled-line sections and a single open-circuit stub. An experimental prototype has been realized usinga broadside-coupled asymmetric coplanar stripline (CPS) con-figuration [9] and its measured results serve to validate our pro-posed concept.

II. THEORY

A. Dual-Band Marchand Balun

Fig. 1 shows the schematic of our proposed dual-band balun.It consists of a traditional Marchand balun and an open-circuitstub. In essence, the Marchand balun is overcoupled so that ithas a wide operating bandwidth, but not a very good return loss( 12 dB). At the same time, a narrow rejection at the center ofthis wide operating band is introduced by a stub to achieve adual-band behavior. Notice that the return loss at two particularfrequency bands is improved by the introduction of this stub.Based on this principle, the design procedure for this device canbe summarized in the following steps.Step 1) Choose two desired operating frequencies, namely,

and .Step 2) Design a conventional Marchand balun operating at

the frequency .Step 3) Overcouple this balun such that its bandwidth ex-

tends slightly over the frequencies and .Step 4) Add a open-circuit stub to introduce a strong

rejection at frequency .

0018-9480/$25.00 © 2008 IEEE

1996 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 56, NO. 8, AUGUST 2008

Fig. 2. Circuit simulations of an example dual-band balun and its associatedovercoupled Marchand balun.

Step 5) Adjust and of the coupled-line sections, aswell as of the stub, until a desired performanceis obtained.

Following the above procedure, a schematic example ofand has been designed.

This balun operates at 2.4 and 5.8 GHz with its return loss betterthan 20 dB at these frequencies. Fig. 2 shows its responses,as well as the responses of its corresponding overcoupledMarchand balun.

As suggested from the simulation results, the Marchand balunwithout the open-circuit stub is designed at frequency of4.1 GHz. It is overcoupled so as its bandwidth extends slightlyover the range of 2.4–5.8 GHz. Notice that the return loss isonly around 12 dB over this range because of the overcouplingeffect. By adding a stub, which introduces a strong rejection at4.1 GHz, the operating band is thus divided into two with morethan 20-dB return loss at both 2.4- and 5.8-GHz bands. To fur-ther understand the open-circuit stub effects, we can treat theMarchand balun as a four-port network [10] and carry out theeven- and odd-mode analysis to obtain its return-loss expressionat the unbalanced port. In addition, by enforcing the boundarycondition , we have

(1)

where and have the same meanings as those de-fined in [10], and is the reflection coefficient introduced bythe open-circuit stub. It is this reflection condition that allowsa simultaneous matching at two separate frequencies. Ideally,we would like to have at and so that a perfectmatch is achieved at the unbalanced port. However, in practice,a sufficiently small value of is acceptable by minimizingthe numerator term

(2)

or, equivalently,

with (3)

Fig. 3. jS j minimization by varying Z . (a) Dual-frequency matching.(b) Overall return loss.

over the two desired frequencies by choosing suitable values ofand . This chosen set of parameters can then be

used as the starting point for Step 5). Fig. 3 depicts the varia-tions of (with and ) to min-imize for the example mentioned above. It can be seen that

is almost zero at 2.4 and 5.8 GHz when is set to 100 .In general, good matching at dual frequencies [under the condi-tion of Step 2)] can be achieved by correctly adjusting the threeimpedance parameters.

B. Broadside-Coupled Asymmetric CPS

According to the circuit level design, the even- and odd-modeimpedances for each coupled-line section are 100 and 22 ,respectively. In fact, these values represent a pair of stronglycoupled lines that cannot be easily realized by a conventionaledge-coupled microstrip pair. To overcome such a problem, abroadside-coupled asymmetric CPS section is used instead. Thereason for using asymmetric CPSs is that half of the line couldbe part of a large ground plane. Fig. 4 shows the configurationof a pair of broadside-coupled asymmetric CPS.

Due to symmetry, fundamental properties of a broadside-cou-pled asymmetric CPS are characterized by its even- and odd-mode equivalent structures. Analytical expressions for the cor-responding even- and odd-mode impedances can be obtainedby the conformal mapping technique similar to the one used foranalyzing broadside-coupled coplanar waveguide (CPW) [11],[12]. Mathematical details have been given in [9], and the re-sults are repeated here with

(4a)

YEUNG et al.: DUAL-BAND BALUN USING BROADSIDE-COUPLED CPSs 1997

Fig. 4. Broadside-coupled asymmetric CPS with its even- and odd-modeequivalent structures.

(4b)

and

(5a)

(5b)

These equations provide an analytical means to obtain variouscharacteristics of a pair of broadsided-coupled asymmetric CPS.

C. Dielectric Covering and Nonuniform Separation Gap

In inhomogeneous media, a broadside-coupled asymmetricCPS section has a large difference between its even- and odd-mode phase velocities, which is undesirable for some applica-tions. In order to reduce such a difference, dielectric covering ornonuniform gap separation techniques [13] can be employed. Inthe dielectric covering method, both the top and bottom of thecoupling structure are separately covered with a layer of dielec-tric. Fig. 5 shows an example configuration of a broadside-cou-pled dielectric-covered asymmetric CPS. For this configuration,both the even- and odd-mode effective permittivities should bemodified to

(6)

(7)

Fig. 5. Broadside-coupled asymmetric CPS covering with dielectric.

Fig. 6. Even- and odd-mode characteristic impedances of a broadside-coupled:(a) asymmetric CPS. (b) Dielectric-covered asymmetric CPS versus s=h.

and the two characteristic impedances should also be modi-fied accordingly. Analytical results of the even- and odd-modecharacteristic impedances for both the broadside-coupled asym-metric CPS and dielectric-covered asymmetric CPS are plottedin Fig. 5(a) and (b), respectively, versus the normalized gapwidth, , for and and .

According to Fig. 6, it is seen that the required parameterset and can only be achievedby the dielectric-covered configuration with and

when the typical FR4 dielectric material is used. A

1998 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 56, NO. 8, AUGUST 2008

Fig. 7. (a) Physical layout of an example dual-band balun using broadside-coupled dielectric-covered asymmetric CPSs. (b) Its simulation results usingmultilayer transmission-line and full-wave models.

TABLE IDESIGN PARAMETERS FOR THE DUAL-BAND BALUN OF FIG. 7(a)

100- even-mode impedance is simply too low for the noncov-ered counterpart. Based on these results, a dual-band balun [seeFig. 7(a)] has been designed with parameters shown in Table I.

Circuit-level and full-wave electromagnetic simulations havebeen carried out and the results are shown in Fig. 7(b). Fromthis figure, it is seen that this balun operates at around 2.4 and5.8 GHz with approximately 20-dB return loss in each band.Notice also that a signal rejection appears at around 4.1 GHz,as expected. The dielectric covered asymmetric CPS balun re-quires three layers of dielectric. If a single-layer substrate is de-sired, the nonuniform asymmetric CPS technique can be usedinstead. In this case, the separation gap varies along an asym-metric CPS section (see Fig. 8). Since the odd-mode impedanceis practically constant over a wide range of ratio, varying thegap separation only affects the even-mode behavior, and there-fore, achieves reduction of the difference between even- andodd-mode velocities.

Fig. 8. Dual-band balun model using broadsided-coupled nonuniform asym-metric CPSs.

TABLE IIDESIGN PARAMETERS FOR THE DUAL-BAND BALUN OF FIG. 8

Fig. 9. Prototype dual-band balun using broadsided-coupled nonuniformasymmetric CPS. (a) Top view. (b) Bottom view.

III. PHYSICAL IMPLEMENTATION

For our experimental prototype using the nonuniform asym-metric CPS technique, a dual-band balun with parameters

and is implemented. Ac-cording to the Fig. 6, the initial parameter values should be

YEUNG et al.: DUAL-BAND BALUN USING BROADSIDE-COUPLED CPSs 1999

Fig. 10. Measured and simulated return losses of the experimental prototypedual-band balun using nonuniform asymmetric CPS technique.

Fig. 11. Prototype dual-band balun with a different port arrangement. (a) Topview. (b) Bottom view.

those shown in Table II. The layout dimensions of the fabri-cated prototype, including the nonuniform gap separation, areoptimized through a full-wave solver to obtain a desired overallperformance. Fig. 9 shows the prototype dual-band balun imple-menting on a FR4 printed circuit board (PCB). Notice that thebalanced 100- slotline of port 2 is terminated into a resistor of100 .

IV. EXPERIMENTAL RESULTS

Measured and simulated return losses are shown in Fig. 10.These results suggest that the prototype performs, in general, aspredicted, except with a slightly shift of frequency. This dis-crepancy is most likely due to the inaccuracy in the fabrica-tion process. Nevertheless, the two operating frequencies can beshifted up by simply adjusting the lengths of those asymmetricCPS sections. It is seen from this figure that a return loss of atleast 16-dB return loss is achieved in each operating frequencyband. In order to check the amplitude and phase balance of theproposed dual-band balun, another prototype with a differentport configuration has been fabricated. Instead of using a 100-slotline, two 50- microstrips have been used in this prototype,which is shown in Fig. 11. Measured insertion loss, amplitudebalance, and phase balance are shown in Fig. 12. In the 2.4-GHzband, both amplitude and phase balances are good. However,the amplitude balance deteriorates significantly in the 5.8-GHzband because the effect of unequal even- and odd-mode phasevelocities has not been fully compensated.

Fig. 12. Measured performances of the dual-band balun shown in Fig. 10.(a) Insertion loss. (b) Amplitude and phase balance in 2.4-GHz band. (c) Am-plitude and phase balance in 5.8-GHz band.

V. CONCLUSION

The concept of a simple dual-band balun has been presented.The fundamental concept behind this proposed dual-band balunis that a strong rejection is deliberately introduced within theoperating frequency band of an overcoupled Marchand balun soas to divide the operating band into two, as well as improvingits return loss at two separate frequencies simultaneously. Asthe overcoupled Marchand balun cannot be easily realized by

2000 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 56, NO. 8, AUGUST 2008

edge-coupled microstrip pairs, broadside-coupled asymmetricCPS sections are used instead. Besides presenting its funda-mental theory and working mechanism, an experimental pro-totype has also been realized to validate the proposed concept.Notice that, in inhomogeneous media, the difference betweeneven- and odd-mode velocities for broadsided-coupled asym-metric CPS sections is simply too large to construct a gooddual-band balun. To overcome this problem, two methods havebeen suggested. The one using the nonuniform asymmetric CPStechnique has been employed in our prototype, which has a rea-sonable performance, showing good potential for different ap-plications.

ACKNOWLEDGMENT

The authors are very grateful to L. Fung, for providing sup-port on preparing the experimental prototype.

REFERENCES

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[2] C. Quendo, E. Rius, and C. Person, “An original topology of dual-bandfilter with transmission zeros,” in IEEE MTT-S Int. Microw. Symp. Dig.,2003, vol. 2, pp. 1093–1096.

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[4] S. F. Chang, Y. H. Jeng, and J. L. Chen, “Dual-band step-impedancebandpass filter for multimode wireless LANs,” Electron. Lett., vol. 40,no. 1, pp. 38–39, Jan. 2004.

[5] H. M. Lee, C. R. Chen, C. C. Tsai, and C. M. Tsai, “Dual-band cou-pling and feed structure for microstrip filter design,” in IEEE MTT-SInt. Microw. Symp. Dig., 2004, vol. 3, pp. 1971–1974.

[6] J. T. Kuo and H. S. Cheng, “Design of quasi-elliptic function filterswith a dual-passband response,” IEEE Microw. Wireless Compon. Lett.,vol. 14, no. 10, pp. 472–474, Oct. 2004.

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[8] Y. Guo, Z. Y. Zhang, L. C. Ong, and M. Y. W. Chia, “A novel LTCCminiaturized dualband balun,” IEEE Microw. Wireless Compon. Lett.,vol. 16, no. 3, pp. 143–145, Mar. 2006.

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[10] K. S. Ang, Y. C. Leong, and C. H. Lee, “Analysis and design ofminiaturized lumped-distributed impedance-transforming baluns,”IEEE Trans. Microw. Theory Tech., vol. 51, no. 3, pp. 1009–1017,Mar. 2003.

[11] R. N. Simons, Coplanar Waveguide Circuits, Components, and Sys-tems. New York: Wiley, 2001.

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[13] F. Tefiku and E. Yamashita, “Novel directional couplers using broad-side-coupled coplanar waveguides for double-sided printed antennas,”IEEE Trans. Microw. Theory Tech., vol. 44, no. 2, pp. 275–282, Feb.1996.

Lap K. Yeung (S’00–M’02) received the B.Eng. de-gree in electrical and information engineering fromthe University of Sydney, Sydney, Australia, in 1998,the M.Eng. degree in electronic engineering fromthe Chinese University of Hong Kong, Shatin, HongKong, in 2002, and is currently working toward thePh.D. degree at the University of California at LosAngeles (UCLA).

During 1999, he was with the CommonwealthScientific and Industrial Research Organization(CSIRO), Sydney, Australia, where he was a Re-

search Engineer involved in the numerical modeling of different antennastructures. From 2003 to 2006, he was with the Chinese University of HongKong, where he is involved in various LTCC multichip-module (MCM)designs and the development of numerical algorithms for analyzing multilayerembedded RF modules.

Wang-Chi Cheng received the B.Eng., M.Phil., andPh.D. degrees in electronic engineering from theChinese University of Hong Kong (CUHK), Shatin,Hong Kong, in 1999, 2001, and 2004, respectively.

In May 2005, he joined the Electrical andElectronic Engineering Department, Nanyang Tech-nological University (NTU), Singapore, where hewas a Research Fellow. He is currently a SeniorEngineer in charge of ultra-wideband (UWB) trans-ceiver integrated circuit (IC) design with the HongKong Applied Science and Technology Research

Institute (ASTRI) Ltd., Shatin, Hong Kong. His major research interestsduring his M.Phil. and Ph.D. studies were in the field of low-voltage receiverfront-end circuits design with CMOS technology. His current research interestsinclude 802.11 A/B wireless local area network (WLAN) receivers and UWBtransceiver design.

Dr. Cheng was a reviewer for the IEEE MICROWAVE AND WIRELESS

COMPONENTS LETTERS in 2004. He is a referee of the IEEE InternationalSolid-State Circuits Conference (ISSCC) in 2008.

Yuanxun E. Wang (S’96–M’99) received the B.S.degree in electrical engineering from the Universityof Science and Technology of China (USTC), Hefei,China, in 1993, and the M.S. and Ph.D. degrees inelectrical engineering from the University of Texasat Austin, in 1996 and 1999, respectively.

From 1999 to 2002, he was a Research Engineerand Lecturer with the Department of ElectricalEngineering, University of California at Los Angeles(UCLA), prior to joining the faculty. In November2002, he became an Assistant Professor with the

Electrical Engineering Department, UCLA. His research is in the general areaof microwave systems with emphasis on antennas and power amplifiers. He is atechnical consultant in radar and microwave systems for several companies. Hehas authored or coauthored over 100 refereed journal and conference papers.His research interests concern the fusion of signal-processing and circuittechniques into microwave system design for novel architectures and circuitconfigurations.