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INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,
A Novel Design of A Microstrip 3dB Coupler
A. SARDI1, J. ZBITOU2, A. ERRKIK1, L. EL ABDELLAOUI1, A.
TAJMOUATI1, M. LATRACH3
1 LITEN Laboratory, FST of Settat, Hassan 1st University of
Settat-Morocco 2FPK LITEN Laboratory, Hassan 1st University of
Settat- Morocco
3Microwave 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 combinational-model (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”[6-9]. This
kind of coupler is commonly designed in microstrip technology , and
is one of the couplers called “phase quadrature”:
Port1 Port2
Port4 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:
)(103
1
P
PLogCdB
)(104
3
P
PLogDdB
)(104
1
P
PLogIdB
Z Z
ZZ
2/Z
2/Z
Z Z4/gL
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INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,
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 COUPLERSTRUCTURE
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
:
Port 1 Port 2
Fig.2. The quarter wave-length transmission line
Port 1 Port 2
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.
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 :
Where A=D=1+Za/Zb, B=Za(2+Za/Zb) and C=1/Zb.
Where [a,b] are the electrical lengths of T-model transmission
line.
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):
(1)
(2)
B. Using combinational-model(T- andπ-model) as equivalent of the
quarter wavelength line:
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 samecharacteristics (characteristic
impedance Zc and electrical length c) as shown in fig.4:
a
aZ
Z
tan
aa
bb
ZY
tan
2tan
,Z
4/gL
aaZ , aaZ ,
bbZ ,
0
0
0
0
JY
JZ
DC
BA
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Port 1 Port 2
Fig.4. Equivalent quarter wavelength transmission line of
combinational-model (T and π)
The fig.5 shows the equivalent of the quarter-wavelength
transmission line:
Port Port
ZAin ZB
in
Z, Equivalence Za,a
YCin
Z1,1 Z2,2 …… Zn,n
Fig.5. Equivalent circuit of the open-stub line
If ZAin= ZB
in, the result can be obtained as:
(3)
According to the fig.4 and the equation (3), we deduce the
following equations for the combinational model :
(4)
(5)
(6)
II. 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:
Fig.6. The layout of the final coupler structure
The S parameters of the coupler are given in fig.7:
Fig.7. S Parameters versus frequency
2)2tan(
4sintan
aa
aabb
Z
ZZY
a
aaa
ZZ
2sin2
)2cos42cos( 2
a
aacc
ZZ
ZZY
2cottan
aaZ , aaZ , aaZ , aaZ ,
bbZ ,ccZ , ccZ ,
nn
aaa
aaYYY
ZZZ
ZZ
tan....tantan
tantan
tantan2211
0
0
2
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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 .
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.9. The 3D coupler structure in CST Studio
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.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. 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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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:
(a)
(b)
(c)
(d)
Fig.17. Comparison of S-parameters between simulations results
and measurements
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INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,
.
According to the figures above, after have compared simulated
results of CST and measurement, we can deduce that we have a good
agreement.
IV. 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.
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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IJMOT-2014-8-622 © 2014 IAMOT
A. Using T-model as equivalent of the quarter-length of
transmission line:B. Using combinational-model(T- andπ-model) as
equivalent of the quarter wavelength line:II. SIMULATION RESULTS OF
THE COUPLERA. ADS results:B. CST Microwave Studio results:
III. ACHIEVEMENT AND MEASUREMENT IV.
CONCLUSIONACKNOWLEDGEMENT