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INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY, VOL.9, NO.6, NOVEMBER 2014

A Novel Design of A Microstrip 3dB Coupler

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A. SARDI , J. ZBITOU , A. ERRKIK , L. EL ABDELLAOUI , A. TAJMOUATI , M. LATRACH 1

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LITEN Laboratory, FST of Settat, Hassan 1st University of Settat-Morocco 2 FPK LITEN Laboratory, Hassan 1st University of Settat- Morocco 3

Microwave Group, ESEO France E-mail: [email protected]

Abstract— This paper presents a novel design of branch

3dB coupler, it is designed and simulated by using FR4 substrate at the operating frequency 2.45 GHz in the ISM “Industrial Scientific Medical” band. The design is based on two different models, the T-model and combinationalmodel (T- and π-model) in order to reduce the size and improve the performances. After a theoretical study on the use of open stubs, we present the simulation results of this branch line coupler by using ADS from Agilent technologies and CST Microwave Studio. Good agreement is found between simulated and measured results. Index Terms- Branch-line coupler, ADS, CST microwave Studio.

I. INTRODUCTION The couplers are from of the most passive components used in modern communication systems [1]. These hybrid couplers are the key elements in the design of microwave devices such as power amplifiers, mixers and antenna systems due to their simplicity, wide bandwidth power distribution, and high isolation between ports [2-6]. This work is unscrewed into two parts. The first part devoted to the theoretical study of directional coupler with a detailed development of the 3 dB coupler. In the second part, we will discuss the conception, optimization and the achievement of a new coupler (3dB, 90°) structure which is build by using softwares ADS [7] and CST Microwave Studio [8]. II. THEORICAL STUDY OF COUPLERS Couplers called “Branch-Line” as shown in fig.1,directional couplers are generally used for distribution to 3dB of energy, with a phase difference of

90° between the way “direct” and the way “coupled”[69]. This kind of coupler is commonly designed in microstrip technology , and is one of the couplers called “phase quadrature”:

Port1

Z /

Z

2

Z

Port2

 

Z  L   g / 4  Z  Port4

Z

Z /

2

Z

Port3

Fig.1. Branch Line Microstrip Structure

According to fig.1 above, the power between the port 1 will be divided between the port 2 (direct path), and port 3 (channel coupled) with a phase difference of 90° between the outputs. No energy is transmitted to port 4 (isolated port). The directional coupler is characterized by three parameters:  Coupling:

 Directivity:

 Isolation:

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P1 ) P3 P3 D dB  10 Log ( ) P4 P1 I dB  10 Log ( ) P4 C dB  10 Log (

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We can observe that the coupler has a high level of symmetry. Each port may be used as an input. This symmetry is reflected by examining the S-matrix, since each line can be obtained by transposing the first. III. NOVEL MICROSTRIP COUPLER STRUCTURE A. Using T-model as equivalent of the quarter-length of

transmission line:

In this model, we have two transmission lines which have the same characteristics (characteristic impedance Za and electrical length a) between these two transmission lines we have an open stub[10-11] which has the following characteristics (characteristic impedance Zb and electrical length b) as shown in fig.2 and fig.3 :

Z , Port 2

Port 1



L  g / 4



T-model approach is adopted individually to reduce dimensions of the quarter-wavelength transmission lines which tend to miniaturize the microstrip branch-line couplers. Defining the sets Zo ≡[Za,Zb], and  ≡[a,b] ,the equivalent T-model is shown in fig3. In order to relate the models in fig.2 and fig.3, we obtain the ABCD matrice (L=g/4 ,=l=/2) equation given by :

A B  0 C D    JY 0    Where C=1/Zb.

A=D=1+Za/Zb,

Za,a Port 2

Zb,b

and

By using the expression for T-model in [13-15], we can express the equations for Equivalent quarter wave-length transmission line by using T-model as shown in equation (1) and equation (2):

Fig.2. The quarter wave-length transmission line

Port 1

B=Za(2+Za/Zb)

Where [a,b] are the electrical lengths of Tmodel transmission line.

Z tan a

(1)

2 Za tan a

(2)

Za 

Za,a

JZ 0  0 

Yb tan b 

B. Using combinational-model(T- andπ-model) as

equivalent of the quarter wavelength line:

Fig.3. Equivalent quarter wave-length transmission line by using T-model

The terms Zo and  are respectively the characteristic impedance and electrical length of conventional branch line arms of the coupler. In order to reduce the quarter-wavelength transmission line, equivalent T-model of the transmission line is employed as illustrated in fig.3.

In this model, we have four transmission lines which have the same characteristics (characteristic impedance Za and electrical length a), and three open stubs [12], the one of these stubs has the following characteristics (Zb and electrical length b), for the two others stubs we have the same characteristics (characteristic impedance Zc and electrical length c) as shown in fig.4:

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INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY, VOL.9, NO.6, NOVEMBER 2014 II.

Za,a

Port 1

Za,a Za,a

Zc,c Zb,b

Za,a

Port 2

Zc,c

Fig.4. Equivalent quarter wavelength transmission line of combinational-model (T and π)

The fig.5 shows the equivalent of the quarterwavelength transmission line:

SIMULATION RESULTS OF THE COUPLER

The new miniaturized 3dB coupler is simulated by using an FR4 substrate with relative permittivity 4.4, loss tangent 0.025 and thickness h=1.58 mm. This coupler is designed at a frequency of 2.45 GHz. It is designed and simulated by using Momentum integrated into ADS and CST Microwave Studio electromagnetic simulators. A. ADS results:

After many series of optimizations, the final circuit is presented in fig.6; the coupler has as dimensions 24.5x 21 mm2:

Port

Port ZAin

ZBin

Z,

Equivalence Za,a YCin Z1,1

Z2,2

……

Zn,n Fig.6. The layout of the final coupler structure

Fig.5. Equivalent circuit of the open-stub line The S parameters of the coupler are given in fig.7:

If ZAin= ZBin, the result can be obtained as: Za tan  Z0 tana  Y1 tan1  Y 2 tan 2  .... Yn tann Za 2 tana tan  Z0Za

(3)

According to the fig.4 and the equation (3), we deduce the following equations for the combinational model : Yb tan b 

Za sin 4a  Z ( Za tan 2a ) 2

Yc tan c 

(4)

Z cot 2a  Za ZZa

(5)

Z( cos2 2a  4  cos2a) Za  2sin2a

(6)

Fig.7. S Parameters versus frequency

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As shown in fig.7, we have good isolation and good matching input impedance less than -20dB in the frequency band [2.20GHz,2.73GHz], with an insertion loss around -3dB .

S parameters results are presented in fig.10:

Fig.10. S Parameters versus frequency

As shown in fig.10, we have good performances of the simulated coupler. Fig.8. Phase difference versus frequency for Port 2 and Port 3

The phase difference between the output ports of the coupler is depicted in fig.8. The phase difference is 91.73° at the resonance frequency 2.45 GHz. Such value is acceptable for all receivers since ± 5° error is negligible and indicates good transmission percentage. B. CST Microwave Studio results:

After the validation of the 3dB coupler in ADS we have simulated this circuit by using CST Microwave Studio that is 3D electromagnetic software, the novel structure of the branch line couple is shown in fig.9:

Fig.11. S-parameter phase in degrees versus frequency

The phase difference is about 87° at the resonance frequency 2.45 GHz. According to the figures above, we can deduce that we have the same results between ADS and CST results. III.

Fig.9. The 3D coupler structure in CST Studio

ACHIEVEMENT AND MEASUREMENT

After the comparison of simulation results on CST Microwave Studio and ADS, the coupler structure is achieved by using LPKF machine as presented in fig.16:

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INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY, VOL.9, NO.6, NOVEMBER 2014

(b) Fig.16. The photograph of the proposed coupler

Measurement was performed with a vectorial network analyzer (HP 8719ES).The entire area of the fabricated coupler is 24.5 x 21 mm2. The bandwidth is 2.25-2.75 GHz. The simulation and measurement results are shown in Fig.17:

(c)

(a)

(d) Fig.17. Comparison of S-parameters between simulations results and measurements

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INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY, VOL.9, NO.6, NOVEMBER 2014 .

[6]

According to the figures above, after have compared simulated results of CST and measurement, we can deduce that we have a good agreement.

[7] [8] [9]

CONCLUSION This study permits to validate into simulation and fabrication a novel microstrip 3dB Branch-Line coupler structure, by conducting firstly a theoretical study by using the different equations giving the equivalence between a quarter wavelength transmission line and a new model based on open stubs. This new model is the key to develop a novel structure of 3dB coupler at any frequency, measurement and simulation results of this coupler are validated in the ISM band centered at 2.45 GHz, with good isolation, good matching input impedance, this structure has -3dB as a coupling coefficient and 90° for the phase between the output ports. The dimensions of this novel design are 24.5x21 mm2. IV.

[10]

[11]

[12]

[13] [14]

[15]

P .F. Combes.Circuits Passifs,Propagation,Antennes, 2nd ed. Dunod.Paris, 1997. http://www.home.agilent.com. https://www.cst.com. C.DALL’OMO,Contribution à l’étude d’antennes à pointageélectronique en millimétrique. Conception et réalisation de différentes topologies de Matrices de Butler, Electronique des Hautes Fréquences,Optoélectronique (13Novembre 2003). Sun, K. O., S. J. Ho, C. C. Yen, and D. Weide, "A compact branch-line coupler using discontinuous microstrip lines," IEEE Microw. and Wirel. Compon. Lett., Vol. 15, No. 8, 519-520, 2005. G.-Q. Liu, L.-S. Wu, and W.-Y. Yin, “A compact microstrip ratrace coupler with modified lange and T-shaped arms” Progress In Electromagnetics Research, vol. 115, pp. 509-523, April 2011 Tang, C. W. and M. G. Chen, "Sythesizing microstrip branchline couplers with predetermined compact size and bandwidth," IEEE Trans. Microwave Theory Tech., Vol. 55, No. 9, 1926-1933, 2007. Sun, K. O., S. J. Ho, C. C. Yen, and D. Weide," A compact branch-line coupler using discontinuous microstrip lines", IEEE Microw. and Wirel. Compon. Lett., Vol. 15, No. 8, 519-520, 2005 M. Y. O. Elhiwairis, S. K. B. A. Rahim, U. A. K. Okonkwo, N. B. M. Jizat, and M. F. B. Jamlos, "Miniaturized size branch line coupler using open stubs with high-low impedances," Progress In Electromagnetics Research Letters, Vol. 23, 65-74, 2011.

ACKNOWLEDGEMENT We thank Mr. Angel Mediavilla Sanchez Director of DICOM Laboratory in Santander in Spain and Mr. Mohamed Latrach Professor in ESEO France, for allowing us to use and to perform simulations by using softwares and measurement on the VNA.

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