624 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 22, NO. 12, DECEMBER 2012

Compact Wide Bandwidth Balun Based on ModifiedAsymmetric Broadside Coupled Lines

Hai Hoang Ta, Binh Le Pham, Student Member, IEEE, and Anh-Vu Pham, Senior Member, IEEE

Abstract—We present the design and fabrication of a compact,wide bandwidth balun on a multilayer organic substrate. Thebalun is designed using asymmetrical broadside coupled lineson a patterned ground structure. The experimental results showthat the balun has a measured insertion loss less than 1 dB from0.5–9 GHz and better than 2.7 dB up to above 20 GHz. Themeasured phase and amplitude imbalances are less than 8 degreeand 1 dB respectively from 1 GHz to above 20 GHz. The balun hasa compact size of 5.5 mm 3.5 mm 0.3 mm.

Index Terms—Asymmetric broadside coupled lines, multilayer,patterned ground structure, wide bandwidth balun.

I. INTRODUCTION

B ALUN is an important component for converting bal-anced signals into unbalanced signals in advanced

circuits. They are widely used in RF/microwave circuits suchas balanced mixers, push-pull amplifiers, and high speed digitalsystems to convert single-ended signals into differential ones.To operate at low frequencies, either coaxial transmission linesloaded with ferrite cores or large spiral inductors are used toconstruct the balun [1]. These types of baluns are usually bulky,become lossy at high frequencies, and have limited bandwidth.For high frequency applications, planar Marchand baluns canbe used to achieve wide bandwidths [2], [3]. These types ofbaluns are based on quarter-wavelength transmission lines andbecome large at low frequencies. Several techniques have beenreported on the miniaturization of Marchand baluns [4]–[6].Other topologies include using a transition between distincttransmission line media such as coplanar-waveguide to slotlines[7], coplanar-waveguide to coplanar-stripline (CPW-to-CPS)[8] or coplanar-waveguide to air gap overlay parallel plates [9].In this letter, we present a wide bandwidth balun design using

modified asymmetric broadside coupled lines on a patternedground structure. The patterned ground structure is loaded withtwo symmetrical rows of thin conductors to improve the bal-anced performance of the balun over a wide bandwidth. Theexperimental results show that the balun achieves a measuredinsertion loss better than 1 dB from 0.5 GHz to 9 GHz. Themeasured phase and amplitude imbalances are better than 8 de-grees and 1 dB respectively from 1 GHz to above 20 GHz.

Manuscript received October 29, 2012; accepted November 07, 2012. Dateof publication November 30, 2012; date of current version December 13, 2012.This work is supported in part by the Vietnam Education Foundation, and Ag-ilent Technologies.The authors are with the School of Electrical and Computer Engineering,

University of California at Davis, Davis, CA 95616 USA (e-mail: hhta@uc-davis.edu).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.2012.2228180

Fig. 1. 3-D view of the balun.

II. DESIGN OF THE BALUN

Fig. 1 shows a 3-D view and dimensional parameters of thebalun. The balun is built on a 3-metal layer substrate that hasa dielectric constant of 3.56 and a loss tangent of 0.01. Thedielectric thicknesses are mm and mm.The broadside coupled lines are realized on the top- andsecond-metal layers of the dielectric substrate with the topconductor meandered. The bottom-metal layer has a patternedground structure with two symmetrical rows of periodicallyloaded thin conductors.Similar to the analysis of a symmetric coupled-line balun

[10], the port impedances and amplitude balance ratio are re-lated to the average pi- and c-impedance of an asymmetric cou-pled-line as

(1)

(2)

where, and are the average pi- and c-mode characteristicimpedance of the asymmetric coupled-line. As shown in [11],asymmetric coupled lines have two pi-mode characteristic im-pedances and two c-mode characteristic impedances

. The average c-mode and pi-mode characteristic im-pedances are and .For our balun, the unbalanced and balanced port terminations

are 50 and 100 , respectively, and from (1) is calculated tobe 35 . The center frequency of our design is at 5 GHz, wherethe asymmetric coupled-line is at a quarter-wavelength in thepi-mode. From (2), to have an amplitude imbalance less than1 dB at the center frequency, the c-mode to pi-mode impedance

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TA et al.: COMPACT WIDE BANDWIDTH BALUN BASED ON MODIFIED ASYMMETRIC BROADSIDE COUPLED LINES 625

Fig. 2. Average p-mode characteristic impedance as and change.

Fig. 3. Average p-mode characteristic impedance as changes.

Fig. 4. pi-mode impedance ratio versus while fixing other parameters.

Fig. 5. Prototype of the balun (a) Top view; (b) Bottom view.

TABLE IDIMENSIONAL PARAMETERS SUMMARY

ratio should be greater than 17.39 or . Further-more, as the ratio rises, the frequency bandwidth of thebalun increases. We aim to design our asymmetric coupled-line

to achieve i) , ii) a quarter-wavelength at 5 GHzin pi-mode and iii) as large as possible to achieve awide-bandwidth balun.In order to derive the average c- and pi-mode characteristic

impedances, we have simulated the coupled-line as a 4-port de-vice using theHigh Frequency Structure Simulator (HFSS) [12].Each port is terminated with 50 . We then convert the 4-portS-parameters to the differential-mode and common-mode S-pa-rameters [13]. The input impedance of the coupled-line can bedetermined as [14]

(3)

where is the simulated differential-mode or thecommon-mode of the coupled-line and is 100for the differential-mode and 25 for the common-mode.Knowing the common- and differential-mode input imped-ances, we can calculate the average pi-mode and c-modecharacteristic impedances of the coupled line [14]. Thevariations of the and versus dimensions of the balun areshown in Figs. 2 and 3.As can be seen in Fig. 2, the average pi-mode character-

istic impedance is inversely proportional to both andwhen other dimensions are fixed. Fig. 3 shows that the magni-tude of the average c-mode characteristic impedance and thec-mode-to-pi-mode impedance ratio is almost con-stant when the ground is configured as two symmetrical rowsof periodically loaded thin conductors and is signifi-cantly larger compared to the continuous ground case .The and can be increased by enlarging . How-ever, the trade-off is that the size of the balun is also increased.Since the asymmetric broadside-coupled line has two dif-

ferent pi-mode characteristic impedances for the top and lowertraces, the phase and amplitude imbalances of the balun degradeas the difference increases. To solve this issue, we propose to useameandered line for the top conductor. This meandered line cre-ates offset sections that help to modify the pi-mode character-istic impedances of both top and lower conductors. As explainedin [15], by choosing proper values for the trace widths of the topand lower conductors as well as the offset between them, onecan produce equal pi-mode characteristic impedances. The vari-ation of the pi-mode characteristic impedance ratio aschanges is shown in Fig. 4. is extracted from simulated Z-pa-rameters based on the equations shown in [11]. The simulationdata in Fig. 4 demonstrates that the offset can be chosen toachieve equal pi-mode characteristic impedances. Table I showsthe summary of the dimensions of the balun to satisfy our designgoals. The quarter-wavelength at 5 GHz is verified by observingthe 90 degree phase of the simulated . The entire balun wassimulated using HFSS. The prototype and the dimensions of thebalun are shown in Fig. 5.

III. EXPERIMENTAL RESULTS

The electrical performance of the balun was measured on aCascade Microtech RF probe station with an Agilent E83642-port network analyzer. The probes were calibrated usingthe standard Thru-Reflect-Line calibration on Picoprobe CS-9

626 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 22, NO. 12, DECEMBER 2012

Fig. 6. Measurement results of the balun (a) S11 and insertion loss; (b) Ampli-tude and phase imbalances.

TABLE IICOMPARISON OF BALUN DESIGNS

substrate [16]. Fig. 6 shows the measured and simulated re-sults of the , insertion loss, and phase and amplitudeimbalances of the balun. As shown in Fig. 6(a), the balun hasa measured input return loss better than 10 dB from 0.5 to11 GHz. The measured insertion loss (IL) of the balun canbe calculated from the measured and by the equation

(dB). Fig. 6(a) shows that themeasured insertion loss of the balun is better than 1 dB in theband from 0.5 to 9 GHz. The insertion loss rises in the bandof 9–20 GHz due to the worsening mismatch of the transitionbetween the coplanar waveguide pads and the transmission lineformed by the top-meandered and lower-straight conductors.The increase in the radiation loss due to the defected groundstructure also becomes significant in the high frequency range.Fig. 6(b) shows that the balun achieves a measured phaseimbalance better than 8 degrees and amplitude imbalance betterthan 1 dB from 1 GHz to above 20 GHz. The misalignment be-

tween the top and lower conductors in the fabricated prototypecauses some deviations in the measured results as comparisonwith the simulated ones. Table II shows the comparisons of thisdesign with published planar PCB broadband compact balunsin terms of bandwidth, phase imbalance, amplitude imbalanceand size.

IV. CONCLUSION

We present the design and development of a wide bandwidthbalun. The balun is designed based on modified asymmetricbroadside coupled lines on a patterned ground structure. Themeasurement results show that the balun achieves a bandwidthratio of 9:1 (from 1 to 9 GHz). Within this operating band, thebalun has phase imbalance better than 8 degrees and ampli-tude imbalance better than 1 dB. The balun has a total size of5.5 mm 3.5 mm 0.3 mm.

ACKNOWLEDGMENT

This work is supported in part by Vietnam Education Foun-dation and Agilent Technologies.

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