Broadside Coupled-Line Balun

Broadside Coupled-Line Balun

Guan-Yu Chen, Jwo-Shiun Sun and Sen-Yi Huang

Department of Electronic Engineering, National Taipei University of Technology, Taiwan E-mail: s1669012@https://www.360docs.net/doc/5315186849.html,.tw and jss@https://www.360docs.net/doc/5315186849.html,.tw

YD Chen, Director, HTC

Antenna and Wireless System Integration Dept., High Tech Computer, Corp (HTC), Taiwan

Abstract-This paper presents a practical and simple broadside coupled-line balun with multi-layers profile microstrip line and broadside coupling structure to enhance the coupled coefficient and balanced output bandwidth. Designed broadside coupled-line balun design with a symmetric quarter wavelength coupled-line is developed to increase wider bandwidth efficiency and the return loss is better than the traditional planar marchancd coupled-lines balun. The structure of the designed balun is fairly simple, practical and easily fabricated. The measured results show good agreements with those analyses data.

I. INTRODUCTION Balun is a word derived from terms of balanced to unbalanced transmission lines that join a balanced structures and unbalanced structures transition [1]. Balanced line has two conductors with equal potential and 180 degrees phase difference, such as a twisted pair cable, coplanar strip line and slot line. Unbalanced line has just one conductor and a ground which unbalanced current flows through a ground plane, such as a coaxial cable and microstrip line. Baluns are used to connect balanced antennas like dipole antenna to unbalanced transmission lines like microstrip line or coaxial cable. And the balun will be the converter that allows the network analyzer to interface with the dipole antenna. The phase difference between the signals is 180 degrees. The balun must shift the phase of one of the signals so that the two signals are in phase and then combine the two signals to produce an unbalanced signal that the network analyzer can receive and interpret.

Balun can also be used in power or signal isolators along the transmission line to reject the flow of induced RF current. A Marchand balun having a pair of coupled lines of quarter wavelength for dividing and combining signals with the same amplitude and opposite phase. Form a mixer, and a par of mixers are combined with 90 degree and 180 degree splitters to provide circuits with superior rejection of specific spurious signals as well as RF and LO leakage over. Balun provides impedance transformation and matching network for structures transition [2]. For converting balanced input to unbalanced output or vice versa that is to enable transmission lines and equipment of different impedances to be matched and connected for maximum power transfer with minimum effects to the transmitting signal such as insertion loss, waveform distortion and minimum reflection of the signal applied in microwave applications as balanced mixers, push-pull amplifiers, antennas fed networks or doublers [3], etc.

In this paper, the design is a balun transformer that converts a single-ended (or unbalanced) signal to a differential (or balanced) signal. The balun is a printed metal pattern on a multi-layer circuit board in conjunction with several low cost chip capacitors and a low cost chip inductor for circuits integration and co-design. To make the balun compact, it is electrically lengthened through the use of LC loading, which reduces the required physical size. The balun transformer is a Marchand balun modeling that is implemented using printed broadside coupling transmission lines. The balun has a plurality of coupled transmission lines to improve tolerances to variations in PCB fabrication. The purpose of this thesis is to design and implement the match circuit of broadside coupled-line balun, which is widely applied on MIC (Microwave Integrated Circuit) of wireless communication systems. Balun are also a kind of coupler with power divider and matching impedance function circuit, the circuit is designed in this paper for ISM band working frequencies at 2.45GHz center frequency. AWR high frequency circuit simulate

program is applied to analysis its characteristics of 3dB coupler at 2.45GHz center frequency bands. Implementation of the practical passive circuits and the measured data on FR4 boars are shown in this paper. The simulated data and the measured results show good agreements.

II. DESIGN

Operation principle Based on Marchand model, two coaxial lines [4] consist of the characteristic impedances a and b . The outer conductors of these two lines are coupled to each other from the balanced lines of characteristic impedance ab and shielding connect to short with coaxial lines of a and b , respectively. The characteristic impedance of outer conductors a and b , open coaxial line of b with electrical length Z Z Z Z Z Z Z Z l β of a θ and b θ degree are designed at center frequency. The coupled coaxial transmission lines impedance consist of an open circuit stub and a short circuit stub in parallel with balanced load impedance .

Z L Z ()a L L Z Z j Z Z ?+=θθθsin cot sin 22 1

while and .

a b Z Z =L ab Z Z =

Enhanced technology The conventional microstrip coupled-lines balun [5] is ordinary in strong coupling coefficient by high even mode impedance, as shown in Fig. 1. Therefore, the gap in between the coupled lines of the balun by narrow space for coupling is not easy to be realized in microstrip configuration. The improved design [6] of the symmetric parallel connection has been proposed. In this paper, the coupled coefficient of the design is higher than the traditional ones and easy to implement for system integration. The symmetric multi-layers coupled-line balun is designed as shown in Fig. 2. The broadside coupling method to connect a symmetric broadside coupled-line with quarter wavelength to split the strong coupling of balanced output is applied. The bandwidth can be wider than that conventional method. The bandwidth and high even mode impedance is strictly solved in broadside coupling structure with multi-layers method in this design.

Figure 1. The planar Marchand microstrip balun with edge

coupling structure

Figure 2. The broadside Marchand coupled-line balun with

multi-layer structure

III. RESULT

Analysis: The method of momentum (MoM) to model the balun [7], through electromagnetic numerical fit is applied. Design procedure for balun [8] using an equivalent circuit model for the two coupled line sections. Marchand balun [9] is implemented in RF mixer experimental design and full wave spectral domain Galerkin method [10] to analyze the electromagnetic performances of the designed broadside quarter wavelength coupled lines balun are applied in this paper.

Measurement : The layout (Fig. 1) of unbalanced input with metal (w1=3mm), gap spacing (g=0.4mm), coupling spacing (s=0.15mm), balanced output with metal wide (w2=3mm), quarter wavelength (L2=16.8mm) and FR4 substrate high (h1=1.6mm) at center frequency 2.45GHz of microstrip edge coupled-lines balun are design, as shown in Fig. 3. And the proposed balun layout (Fig. 2) of unbalanced input with metal (w1=1mm, L1=34.4mm), gap spacing (g=0.4mm), broadside coupling spacing (h1=0.4mm), balanced output with metal wide (w2=1.8mm), quarter wavelength (L2=16.8mm) and FR4 substrate high (h2=1.6mm) at center frequency 2.45GHz of multi-layer broadside coupled-lines balun are design as shown in Fig. 4. Fig. 3 and Fig. 4 show balun of microstrip coupled lines balun and broadside coupled line balun and balun structures with full wave electromagnetic simulation and the measured data of return loss and balanced output are shown in Fig.5, Fig.6, Fig. 7 and Fig. 8, respectively. PNA E8362B network analyzer with full two ports TRL

calibration is applied for planar microstrip and broadside coupled-balun measurement. Fig. 5 and Fig. 6 shows the return loss bandwidth of planar balun is about 730MHz

referenced –10dB . Insertion loss of balanced output is about 0.75dB different offset. The return loss bandwidth is about 2004MHz

referenced –10dB and balanced output bandwidth of the broadside coupled line balun increased the effectively and insertion loss of balanced output is about 0.55dB different offset, as shown in Fig. 7 and Fig. 8.

IV. CONCLUSION

This paper presents an improved bandwidth of

a broadside structure balun. The design methodology shows an effective and simple way

to improve the narrow spacing and bandwidth of the conventional planar coupled-line balun. Full wave analysis with spectral domain Galerkin method was applied for the design and the results show expected agreements for realization.

ACKNOWLEDGEMENT

The authors acknowledge the Antenna and Wireless System Integration Department of High Tech Computer, Corp (HTC), Taiwan for sport the wireless technique and measurement environment.

REFERENCES

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Figure 3. The planar Marchand microstrip coupled-line balun w1=3mm, w2=3mm, s=0.15mm, L1=34.4mm, L2=16.8mm, L3=10.8mm, h1=1.6mm and r

ε=4.4

Figure 4. The proposed broadside coupled-line balun

w1=1mm, w2=1.8mm, L1=34.4mm ,L2=16.8mm,

L3=10.8mm ,g=0.4mm, h1=0.4mm, h2=1.6mm and

r

ε=4.4

Figure 5. The measured return loss of planar microstrip balun

Figure 6. The measured balance outputs of planar microstrip

balun

balun

Figure 8. The measured balance outputs of broadside

coupled-line balun