APEMC 2015
Mixed Type Branch Line Coupler Designs
Ssu-Jung Wu, Jan-Dong Tseng, Yu-Hui Shih, Kuang-Hao Lin
Department of Electronic Engineering, National Chin Yi University of Technology
No. 57, Sec. 2 Chun Shan road, Taiping Dist., Taichung City, Taiwan, R.O.C.
happycat690@gmail.com
jdtseng@ncut.edu.tw
sugar@ncut.edu.tw
Department of Electrical Engineering, National Formosa University
No.64, Wenhua Rd., Huwei Township, Yunlin County, Taiwan, R.O.C.
khlin@nfu.edu.tw
Abstract— A mixed type branch line coupler combining T-type
and pi-type equivalent circuits and keeping main lines
characteristic impedance at 5ȍ LV Ueported. The new branch
line coupler contains only capacitors and open stubs. Two
prototypes of two and three branches are designed, simulated
and measured. The size reduction reached 40% comparing with
the original circuits. The simulated and experimental results
show a good agreement within the frequency of interest.
I.
INTRODUCTION
Wireless communication overcomes the space limitation and
through message exchanging connects peoples. This feature
let wireless communication devices becomes a necessary part
in modern life. In the recent year, 3G, 4G, Wi-Fi systems are
widespread in our live environment, help people rapidly share
the message and provide various real-time services. For more
adequate to use these wireless devices, compact size, low cost,
easy production, and high performance becomes the basic and
the necessary requirements.
in order to keep the main transmission line DW ȍ OHvel and
eliminate the discontinuity occurred at the junctions of main
line and the branch. The pi-type equivalent circuit converts
the transmission line section of characteristic impedance Z
and electrical length ș into one higher impedance level
transmission line, Z A and ș A , and two shunted capacitors C A ,
shown in Fig. 2(a), (b). After setting the original transmission
line’s parameters Z, ș, and giving ș A , the values of Z A and C A
can be determined by (1) and (2). The capacitors can be
further replaced by open stub, shown in Fig. 2(c), for easy
fabrication. Giving the electrical length ș B , the open stub’s
characteristic impedance Z B can obtained from (3). The Ttype equivalent circuit consists of two transmission line
sections with high impedance level and a shunted capacitor. In
the similar way, after giving the transmission line parameters,
Z and ș, and setting the electrical length, ș C , the values of Z C
and C B can be determined by (4) and (5).
The most commonly used couplers in microwave and RF
subsystems for RF signal processing are branch lines [1] and
rat-race couplers [2]. Normally, branch line has the features
that outputs have 90° phase difference and equal power output.
It is often used in power amplifiers, mixers, phase shifters,
and phase array antenna system. Because conventional branch
line is constructed by quarter wavelength transmission lines, it
often occupies large area when operating frequency is below
1GHz. In the past, some techniques for size reduction were
reported. They used pi-type equivalent circuit [3], T-type Fig. 1 The schematic diagram of two section mixed type branch line coupler
equivalent circuit [4], coupled line [5] or serial connected
high/low impedance transmission lines [6-7] to achieve the
size reduction [8-10].
In this paper, the technique combining pi- and T-type
equivalent circuits to reach size reduction and, in the same
time, removing the discontinuities occurred at the junctions of
main lines and branches is reported. All the main lines
characteristic impedance could keep at ȍOHYel.
II. CIRCUIT DESIGN AND ANALYSIS
The mixed type branch line, shown in Fig. 1, uses T- and pitype equivalent circuit replacing the transmission line sections
978-1-4799-6670-7/15/$31.00 Copyright 2015 IEEE
(a)transmission line section
(b)Pi-type with capacitors
(c) Pi-type with open stubs
(d) T-type with capacitors
Fig. 2 The T- and pi-type equivalent circuits
APEMC 2015
(1)
are 47.20 and -40.80, respectively, at 2.45GHz. The phase
difference is 880. It is very well within 20 comparing with the
theoretical value 900.
(2)
(3)
(4)
(5)
Two prototype of mixed type branch line circuit were
designed at 2.45GHz and 915MHz.
(a)Two section branch line coupler
The schematic diagram is shown in Fig. 1. The branches
between port 1 and 4, and port 2 and 3 are replaced by pi-type
equivalent circuit. The branches originally are characteristic
impedance = ȍ and electrical length ș ƒ. Giving the
electrical lengths ș 1 =60° and ș 2 =50°, and applying (1) and (3),
we have the branches and the open stubs characteristic
impedance as Z 1 ȍ and Z 2 ȍ, respectively. The
original transmission line sections between port 1 and 2, and
port 3 and 4 have characteristic impedance = ȍ and
electrical length ș ƒ. After giving characteristic impedance
Z 3 ȍ, and applying (5) and (6), the electrical length
ș 3 =35.3° and the capacitance C 1 =0.9pF is determined. By
using Line Gauge, the subprogram of IE3D, the electrical
parameters converted into the physical parameters as
L 1 =13.3mm, L 2 =11.4mm, W 1 =2.42mm, W 2 =3.11mm,
Length(O.S.)=9.4mm, Width(O.S.)=2.42mm, where O.S.
stands for open stub. Fig. 3 shows the circuit layout. The
capacitors are placed at the central positions of the main
transmission lines, the open stubs are connected at the inner
corner of the junction. Fig. 4 is the fabricated circuit layout.
The circuit is fabricated on FR-4 substrate, thickness 1.6mm,
İ r =4.3, and have the dimensions 38mm x 25.5mm.
Fig. 4 The fabricated circuit layout of the two sections branch line coupler
(substrate FR-4, thickness=PPİ r =4.3, dimensions 38 mm x 25.5mm)
(a)
magnitudes of |S 11 |, |S 21 |, |S 31 | and |S 41 |
Fig. 3 The circuit layout of the two section branch line coupler
(L 1 =13.3mm, L 2 =11.4mm, W 1 =2.42mm, W 2 =3.11mm,
Length(O.S.)=9.4mm, Width(O.S.)=2.42mm)
The simulated and the experimental frequency responses of
the scattering parameters are shown in Fig. 5.
In Fig. 5(a), at frequency 2.45GHz, the measured return loss is
|S 11 |=-23.66dB, the insertion loss are |S 21 |=-3.36dB and
|S 31 |=-4.01dB. Fig. 5(b) shows the phases of ğS 21 and ğS 31
(b)phases of ğS 21 and ğS 31
Fig. 5 The simulated and experimental results
APEMC 2015
(b) Three section branch line coupler
Fig. 6 shows the schematic diagram of three section branch
line coupler. In the similar way, the branches between port 1
and 4, and port 2 and 3 have the characteristic impedance
= ȍ and electrical length ș ƒ. After giving the
electrical lengths ș 4 =65° and ș 5 =25°, and applying (1), (3),
the characteristic impedances Z 4 = Z 5 =1ȍ are determined.
The central branch originally has the characteristic impedance
= ȍ and electrical length ș ƒ. Setting the electrical
lengths ș 6 =70° and ș 7 =50°, the characteristic impedances of
transmission line and the open stub in the Pi-type equivalent
circuit are calculated as Z 6 ȍ and Z 7 ȍ. The
transmission lines between port 1 and 2, and port 3 and 4 are
replaced by the T-type equivalent circuit. All the original
transmission lines have characteristic impedance = ȍ
and electrical length ș ƒ. Setting the value ș 8 =35.35° and
applying (5) and (6), the characteristic impedance Z 8 and the
capacitor value C 2 are determined as ȍ and 2.44pF,
respectively.
Fig. 8 The final layout of three section branch line coupler, substrate is FR-4,
WKLFNQHVVLVPPİ r =4.3, dimensions 117.9mm x 75.4mm
Fig. 9 shows the simulated and experimental frequency
responses of the magnitudes of the scattering parameters, |S 11 |,
|S 21 |, |S 31 | and |S 41 |, and the phases of ğS 21 and ğS 31 . At
frequency 915MHz, shown in Fig. 9(a), the measured return
loss is |S 11 |=-16.65dB, the insertion loss are |S 21 |=-3.93dB and
|S 31 |=-3.91dB, and the isolation is |S 41 |=-17.27dB. Fig. 9(b)
shows the phases of ğS 21 and ğS 31 are -28.98° and -116.47°,
respectively. The phase difference is 87.49°. It is within
2.51° comparing with the theoretical value 90°. These two
branch lines also have size reduction of 39% and 41%
comparing with the original designs.
Fig. 6 The schematic diagram of three section branch line coupler
By using Line Gauge, the subprogram of IE3D, the electrical
parameters converted into the physical parameters as
L 3 =80.97mm, L 4 =34.86mm, W 3 =6.22mm, W 4 =0.58mm,
W 5 =9.69mm, length(O.S. 1 )=13.41mm, width(O.S. 1 )=0.58mm,
length(O.S. 2 )=26.7mm, width(O.S. 2 )=0.76mm, where O.S.
stands for open stub. Fig. 7 shows the circuit layout. The
capacitors are placed at the central positions of the main
transmission lines, the open stubs are connected at the inner
corner of the junction. Fig. 8 is the fabricated circuit layout.
The circuit is fabricated on FR-4 substrate, two substrate are
stacked to achieve double thickness, thickness 3.2mm, İ r =4.3,
and has the dimensions 117.9mm x 75.4mm.
(a)
Fig. 7 The circuit layout of three section branch line coupler
(L 3 =80.97mm, L 4 =34.86mm, W 3 =6.22mm, W 4 =0.58mm, W 5 =9.69mm,
Length(O.S. 1 )=13.41mm, Width(O.S. 1 )=0.58mm, Length(O.S. 2 )=26.7mm,
Width(O.S. 2 )=0.76mm)
magnitudes of |S 11 |, |S 21 |, |S 31 | and |S 41 |
APEMC 2015
ACKNOWLEDGMENT
This work was supported in part by the National Science
Ministry, Taiwan, R.O.C., under Grant NSC-102-2622-E-167023-CC3 and NSC-103-2815-C-167-014-E.
REFERENCES
(b) phases of ğS 21 and ğS 31
Fig. 9 The simulated and measured frequency responses of three section
branch line coupler
III. CONCLUSIONS
Two mixed type branch line coupler designs with constant
impedance level in main transmission line sections are
reported. The T- and pi-type equivalent circuits were used to
keep the main transmission lines remain at ȍ and to avoid
the discontinuity at the junctions of main transmission and
branches. Two prototypes designed at 2.45GHz and 915MHz
were simulated, fabricated on FR-4 substrate and measured.
The simulated and experimental results show that the
frequency responses of the scattering parameters are in a good
agreement within the frequency of interest.
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