Multisection Vialess Baluns with Coupled-Line Impedance Transformers Sheng Sun a), Lei Zhu a)* and Kian Sen Ang b) a)

School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore b) DSO National Laboratories, Singapore * Corresponding e-mail: [email protected]

Abstract — Multisection vialess baluns are proposed by cascading several λ/4 edge-coupled lines. Without needing any via-hole or ground connection, these vialess baluns are simply implemented on microstrip structures. With the backside aperture, the even-mode impedance of coupled microstrip lines is raised beyond 280 Ω. Taking advantage of multisection coupled line approach, the eventual even-mode impedance can achieve as small as 2 Ω, thus satisfactorily meeting the requirements of λ/4 coupled line baluns with open-circuited terminals. Three- and five-section vialess baluns are designed and the predicted performances are validated by experiment. In a wide frequency range (1.8–3.6 GHz), the measured amplitude imbalance is less than 0.4 dB and phase imbalance is within ±5°. Index Terms — Vialess baluns, coupled lines, multisections, even-/odd-mode impedance and backside-aperture.

(a)

I. INTRODUCTION

(b)

Today, much interest has been focused on the design of various planar and compact baluns for applications in microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs) [1]. They function as a three-port transformer which transforms a balanced signal to two unbalanced signals with same amplitude and out-of-phase. Compared to active baluns with consequential noise and nonreciprocal characteristics [2], passive baluns have become more popular in numerous applications such as balanced mixer, amplifier, phase shifters, antenna feed networks and 180° hybrids [1]–[5]. The design of baluns generally covers the consideration of amplitude and phase balance performance, all-ports matching, and isolation between the two balanced outputs. Based on these requirements, a variety of planar balun structures have been presented so far, for instances, planar Marchand baluns [6]–[8], quarter-wave coupled line baluns [1], and planar transmission line baluns composed of a Wilkinson divider and a 180° phase shifter [9]. However, most of these baluns inquire the additional installment of via-holes at the isolation port. It results in some problematic issues, such as bad repeatability, complexity, increased cost in fabrication as well as parasitic effects of via-holes to electrical performance. In [10], a class of multisection λ/4 coupled-line baluns with a short-circuited terminal is proposed by cascading several coupled and uncoupled line sections. This multisection

Fig. 1. Three-section vialess balun. (a) 3D physical layout with λ/4 edge-coupled microstrip lines. (b) cascaded coupled-line model..

approach can be effectively applied to reject even-mode excitation. In this context, the edge-coupled microstrip lines with backside aperture are employed to increase the coupling factor and also even-mode impedance. As commented in [10], these baluns may be constructed by their dual circuits with an open-circuited terminal if the coupled slot-lines are utilized to make even-mode impedance lower than odd-mode impedance. Of course, the short-circuited terminal can be replaced by a λ/4 open-circuited line [11]. However, no reported work to date has built up such a kind of baluns without needing any short-circuited via-holes. In this work, multisection baluns with λ/4 edge-coupled microstrip lines are developed without including any via-holes and its three-section geometry is shown in Fig. 1(a). The aperture is formed underneath the middle coupled-line to tremendously enhance its even-mode impedance as requested. After the proposed multisection vialess baluns are analyzed and designed, the three- and five-section balun circuits are fabricated for experimental demonstration of their attractive features in terms of amplitude balance, phase balance and input return loss.

II. MULTISECTION VIALESS BALUNS Fig. 1 depicts the three-dimensional (3-D) geometry and equivalent cascaded coupled-line model of the proposed threesection vialess balun. It consists of three cascaded λ/4 uncoupled/coupled line sections with an unbalanced input (P1) and two balanced outputs (P2 & P3). The targeted Sparameters of this three-port balun are regulated as S11 = 0

(1a)

S21 = − S31

(1b)

Following the derivation in [10, 12], the required coupled-line even- and odd-mode impedances ( Z 0e & Z 0o ) for the opencircuited case should be satisfied with, Z 0e = 0 Z 0o = 2 Z in Z out = 70.7 Ω

(2a)

Fig. 2. Even- and odd-mode impedances versus aperture widths of a edge-coupled microstrip lines at the center frequency.

(2b)

where the Z in & Z out are the impedances at the balanced and unbalanced ports, respectively, that are usually set to 50 Ω. Eq. (2a) indicates that the even-mode impedance must be zero or extremely small [10]. This is not valid for any coupled line since Z 0e is always greater than Z 0o of non-zero quantity. In order to reasonably approach the two conditions in Eq. (2), multisection coupled-line structure is implemented to take advantage of its effective or cumulative impedances. For the three-section coupled-line balun in Fig. 1(b), the two effective impedances can be expressed in terms of those of each individual section, i.e., Z 0ei & Z 0oi , where i=1, 2, 3.

Z 0e =

Z 0e1Z 0e3 Z 0e 2

(3a)

Z 0o =

Z 0o1Z 0o3 Z 0o 2

(3b) Fig. 3. Effective even- and odd-mode impedances versus aperture widths of a three section balun as shown in Fig. 1.

In these equations, Z 0e1 and Z 0e3 are set relatively low, whereas Z 0e 2 is set as high as possible to lower the effective impedance of Z 0e . Eq. (4) gives the relevance between evenand odd-mode impedances of a single coupled-line section.

Z 0ei > Z 0oi (Coupled cases)

(4a)

Z 0ei = Z 0oi (Uncoupled cases)

(4b)

of such a three-section case can achieve less than 10 Ω if the Z 0e 2 in the second section can be raised beyond 250 Ω. In this case, the effective Z 0o approximately equals to 85 Ω as can be found in Fig. 3. Of course, as the coupled-line sections are increased, the effective Z 0e can further rise up. For a fivesection case, the effective Z 0e can be reduced below 2 Ω. For a three-section case, Z 0o 2 of the second section is enforced by Eq. (5) that is derived from (2b), (3b) and (4).

Considering these constraints, the first and third sections are formed as uncoupled parallel lines with the impedances of Z 0e1 = Z 0o1 = Z 0e3 = Z 0o3 =50 Ω. A backside aperture is etched out underneath the second coupled-line, aiming to raise the value of Z 0e 2 to a great extent. Fig. 2 plots the extracted evenand odd-mode impedances of this aperture-backed coupledline as a function of aperture width using the Sonnet EM Suite. As the aperture is widened from 1.9 to 25.9 mm, Z 0e gains a quick increment from 87.7 to 280.7 Ω. At the same time, Z 0o seems to be almost unaffected. Based on our calculation using Eq. (3a), we can estimate that effective Z 0e

Z 0o 2 =

Z 0o1Z 0o3 = 35.4 Ω Z 0o

(5)

Similarly, for a five-section case, the odd-mode impedances of second and fourth sections should be selected based on,

Z 0o 2 = Z 0o 4 =

2

Z 0o1Z 0o3 Z 0o5 = 42.0 Ω Z 0o

(6)

Fig. 4. Top-view photograph of the fabricated three-section vialess microstrip-line balun.

Fig. 6. Phase balance of the three-section vialess balun.

Fig. 7. Top-view photograph of the fabricated five-section vialess microstrip-line balun.

Fig. 5. Simulated and measured S-magnitude response of the threesection vialess balun.

To further improve the amplitude and phase imbalance, a five-section vialess balun is designed and fabricated with the strip width of 0.60 mm, slot width of 0.20 mm and the aperture width of 24.57 mm. In this case, the effective Z 0e & Z 0o become 1.5 Ω and 90.3 Ω, respectively. Fig. 7 shows the top-view photograph of this fabricated balun circuit. As can be observed from the measured results in Fig. 8 and 9, the amplitude imbalance is approximately close to 0.4 dB in the frequency range of 1.8–3.6 GHz. The return loss is below 22 dB at the center frequency of 2.5 GHz. The measured phase imbalance varies within ±5° in the band of 1.6–3.6 GHz.

III. EXPERIMENTAL RESULTS Based on the above technique, two multisection vialess baluns are optimally designed using the Agilent Momentum simulator and they are fabricated on the RT/Duriod 6010 with a thickness of 1.27 mm and a dielectric constant of 10.8. Fig. 4 shows the top-view photograph of the fabricated three-section vialess balun. The strip and slot widths of the central coupled-line are 0.90 mm and 0.11 mm, respectively, and the aperture width equals to 24.57 mm. Under these dimensions of choice, the effective Z 0e and Z 0o achieve 9.2 Ω and 85.0 Ω , respectively, so as to make up a three-section vialess balun with reasonably good performances. Fig. 5 plots the predicted and measured S-magnitude response, exhibiting a good agreement between them. At the center frequency of 2.5 GHz, good input matching is achieved with the return loss higher than 17 dB. Meanwhile, the fractional bandwidth is found about 46% at the 10-dB return loss. As the two most concerned parameters, the measured amplitude imbalance is approximate to 1.8 dB in the frequency range of 2–3 GHz, while the measured phase imbalance achieves 183° at 2.5 GHz and varies within the degree of ±13° in the operating bandwidth as shown in Fig.6.

IV. CONCLUSION In this work, a multisection vialess balun with λ/4 edgecoupled microstrip lines has been presented without needing any short-circuited via-holes, in comparison with its dual baluns in [10]. The backside aperture is formed underneath the coupled strip conductors, yielding the high even-mode impedance as inquired. By cascading several λ/4 uncoupledand coupled-line sections, the low effective even-mode impedance is achieved, allowing us to make up an opencircuited vialess baluns composed of multisection λ/4 coupled-line sections. The three- and five-section balun

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REFERENCES [1] R. Mongia, I. Bahl, and P. Bhartia, RF and Microwave CoupledLine Circuits, Norwood, MA: Artech House, ch. 11, 1999. [2] A. M. Pavio, R. H. Halladay, S. D. Bingham, and C. A. Sapashe, “Double balanced mixers using active and passive techniques,” IEEE Trans. Microwave Theory & Tech., vol. 36, no. 12, pp. 1948-1957, December 1988. [3] K. W. Hamed, A. P. Freundorfer, and Y. M. M. Antar, “A monolithic double-balanced direct conversion mixer with an integrated wideband passive balun,” IEEE J. Solid-State Circuits, vol. 40. no. 3, pp. 622-629, March 2005. [4] J. –W. Lee and K. J. Webb, “A low-loss planar microwave balun with an integrated bias scheme for push-pull amplifiers,” 2001 IEEE MTT-S Int. Microwave Symp. Dig., vol. 1, pp. 197200, June 2001. [5] K. S. Ang and Y. C. Leong, “Converting baluns into broad-band impedance-transforming 180 hybrids,” IEEE Trans. Microwave Theory & Tech., vol. 50, no. 8, pp. 1990-1995, August 2002. [6] C. Cho and K. C. Gupta, “A new design procedure for singlelayer and two-layer three-line baluns,” IEEE Trans. Microwave Theory & Tech., vol. 46, no. 12, pp. 2514-2519, December 1998. [7] M. Chongcheawchamnan, C. Y. Ng, K. Bandudej, A. Worapishet, and I. D. Robertson, “On miniaturization isolation network of an all-ports matched impedance-transforming marchand balun,” IEEE Microwave Wireless Components Lett., vol. 13, no. 7, pp. 281-283, July 2003. [8] W. M. Fathelbab and M. B. Steer, “New classes of miniaturized planar marchand baluns,” IEEE Trans. Microwave Theory & Tech., vol. 53, no. 4, pp. 1211-1220, April 2005. [9] J. Rogers and R. Bhartia, “A 6 to 20 GHz planar balun using a Wilkinson divider and lange couplers,” 1991 IEEE MTT-S Int. Microwave Symp. Dig., vol. 2, pp. 865-868, June 1991. [10] K. S. Ang, Y. C. Leong, and C. H. Lee, “Multisection impedance-transforming coupled-line baluns,” IEEE Trans. Microwave Theory & Tech., vol. 51, no. 2, pp. 536-541, February 2003. [11] R. K. Settaluri and A. Weisshaar, “A broadside-edge-coupled vialess balun,” 2003 IEEE MTT-S Int. Microwave Symp. Dig., vol. 2, pp. 1251-11254, June 2003. [12] Y. C. Leong, K. S. Ang, and C. H. Lee, “A derivation of a class of 3-port baluns from symmetrical 4-port networks,” 2002 IEEE MTT-S Int. Microwave Symp. Dig., vol. 2, pp. 1165-1168, June 2002.

Fig. 8. Simulated and measured S-magnitude responses of the fivesection vialess balun.

Fig. 9. Phase balance of the five-section vialess balun.

circuits are designed and fabricated. Compared to the threesection vialess balun, the five-section one has much better balance performance with the amplitude imbalance less than 0.4 dB and phase imbalance in between ±5° in the frequency band of 1.8–3.6 GHz. ACKNOWLEDGEMENT The authors would like to thank Ms. Huamin Shi with the Motorola Electronics Pte Ltd, Singapore, and Mr. Saiwai Wong with the Nanyang Technological University, Singapore, for their assistance in microwave measurement.

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