Simulation Tool: AWR Microwave Office •This project is an implementation of an IEEE paper "Design of Dual Band Cross-Coupled Branch Line Coupler" by Myun-joo Park and Byungje Lee. • Designed and simulated Dual Band Cross-Coupled Branch Line Coupler for the operating frequency of 1.5Ghz and 3Ghz. •The cross coupling in the branches introduce more design freedom.

1. EERF 6311 – Final Design Project, Siddharth Harshe
Dual Band, Cross-Coupled Branch Line Coupler
Myun-joo Park and Byungje Lee, Member, IEEE
Review paper summary: The main objective of the paper is to design
a Branch Line Coupler, having cross-coupled branches to obtain the
Dual band operation of the coupler. The coupler is designed with
operating frequencies of 1 GHz and 2 GHz. The material used in the
proposed structure is Teflon with a dielectric substrate thickness of
0.8mm and relative Permittivity of 2.5. In this paper, the author has used
Even-Odd Decomposition method and [ABCD] matrix method to analyze
the components. The author has made use of Microstrip line(MLIN) for
designing the structure in ADS (Advanced Design System) tool. The
Date of publication of the paper is 26th
September 2005. The component
in the paper is improvising Pozar’s conventional branch line coupler with
the cross- coupled branches to introduce more design freedom in the
branch coupler.
Conventional Design Details: Branch line couplers are the most basic
microwave component with branches having a quarter wavelength [1].
They are also called as Quadrature Hybrid Couplers with 90° phase
difference between the output of through and coupled arms. Any port in
the Coupler can be used as input port. The coupled and through port will
be on the opposite side of input port. Four quadrature wavelength
branches have impedance of Zo and Zo/√2.
Branch line couplers are used to implement High Power, tight coupling
and air Dielectric formats. The decomposition of Branch line coupler into
Even-Odd mode analysis produces a line of Symmetry and
antisymmetry having current(I)=0, voltage(V)=Max. and current(I)=Max.,
voltage(V)=0 respectively [3]. Figure 1 shows a Conventional Branch
Line coupler with all the 8 ports matched at 50Ω impedance. The Band
ratio of the circuit is f1/f2=2 with operating frequencies at 1GHz and
2GHz.
Paper Design Details: The schematic representation of the coupler is
shown in Figure 3. It has two additional cross coupled branches to design
more freedom in the coupler. Even-odd mode decomposition method and
ABCD matrix method are applied to the structure to obtain an even mode
and odd mode circuit. For Even mode, Z3 impedance is connected in
parallel to the horizontal impedance Z1 and for Odd mode, Z3 is
connected in parallel to the vertical impedance Z2. For different coupler
designs same coupling but different performance characteristics cab be
obtained which gives a free choice of selecting the Electrical Length θ.
Simulation: All the simulations are performed using AWR Microwave
Office Tool. For the Conventional branch line coupler, an ideal
Transmission line (TLIN) is used and for the paper design microstrip
Transmission line (MLIN) is used with a Microstrip Substrate (MSUB).
TxLine is used to calculate the Physical dimensions of the microstrip
transmission line. Table 1 shows the widths and physical length
dimensions of the MLIN. The proposed structure in Figure 1 is designed
with operating frequencies of 1GHz and 2GHz.
The schematics in Figure 3 and Figure 5 have additional cross-coupled
branches which are made using MLIN and are used to introduce more
design freedom in the proposed structure. Design 1 and Design 2 has a
characteristic impedance of 50Ω, whereas in the Design 3 a characteristic
impedance of 50.945Ω is used. In Design 3 additional microstrip
transmission lines of electrical length 20° are used to make the response
much better. To observe the performance of the design, Magnitude of S-
parameters are plotted against frequency and are compared with the
conventional coupler design.
Table 1: Dual Band Branch Line Coupler Design Details
Results and Discussion: The conventional design performance of
the ideal branch line coupler is shown in Figure 2 with perfect
isolation and return loss obtained at port 2 and port 4 respectively,
for the operating frequency of 1GHz and 2Ghz.
For the proposed design in Figure 3, the magnitude of S-parameter
is plotted to obtain a 39.92dB Return loss. 3.076dB Transmission,
3.151dB coupling and 42.38dB Isolation is obtained at the operating
frequency of 0.969GHz, similarly for the dual band performance
around 1.931GHz the magnitude of S-parameter is plotted to obtain
a 37.51dB Return loss. 3.097dB Transmission, 3.199dB Coupling
and 30.83dB Isolation is obtained. The Phase difference between
the isolated and coupled ports of -110.1° and 113.5° is obtained at
0.969GHz and 1.931GHz, respectively, which is shown in Figure 4.
For the design in Figure 5, the magnitude of S-parameter is plotted
to obtain 29.35dB Return loss, 3.708dB Transmission, 3.657dB
coupling and 31.39dB Isolation at the operating frequency of
1.5GHz, similarly for the dual band performance around 3GHz the
magnitude of S-parameter is plotted to obtain 25.06dB Return loss,
4.272dB Transmission, 4.333dB Coupling and 26.68dB Isolation.
The Phase difference between the isolated and coupled ports of -
132.8° and 127.8° is obtained at 1.5GHz and 3GHz, respectively,
which is shown in Figure 6.
Conclusion: The Dual Band, Cross-Coupled Branch Line coupler is
simulated successfully for the design frequency of 1.5GHz and
3GHz. The response obtained for the design frequency is
satisfactory. The use of cross-coupled branches has introduced
more design freedom to the circuit. This course project has provided
me with the deep understanding knowledge of AWR Microwave
Office along with the concepts of the 3dB Branch Line Coupler.
References:
[1] Myun-Joo Park and Byungje Lee, “Dual-Band, Cross Coupled
Branch Line Coupler,” in IEEE Microwave and wireless components
letters, Vol. 15, No. 10, October 2005.
[2] I.H. Lin, C. Caloz and T. Itoh, “A branch line coupler with two
arbitrary operating frequencies using Left handed transmission
lines,” in IEEE MTT-S Int. Dig., vol. 1, Jun.2003, pp. 325-328.
[3] David M. Pozar, Microwave Engineering, 4th
Ed., Wiley, 2011.
Design
frequency
(GHz)
Impedance
(ohm)
Width (mm) Electrical
length
(degrees)
Physical
length (mm)
1 – 2 Z1=35.35 W=2.02 EL=60 L=36.524
Z2=86.60 W=0.81 EL=60 L=37.674
Z3=43.3 W=2.72 EL=60 L=33.52
1.5 – 3 Z1=53.59 W=2.02 EL=60 L=23.21
Z2=88.23 W=0.81 EL=60 L=23.88
Z3=44.11 W=2.71 EL=60 L=22.98

2. Figure 1: Schematic diagram of conventional Branch Line Coupler
Figure 2: Measured Response of the designed Conventional Coupler Magnitude and
Output phase difference
Figure 3: Schematic diagram cross-coupled branch line coupler with operating
frequencies (1GHZ and 2GHz)
Figure 4: Measured Response of the designed Cross couple branch line coupler
Magnitude and Output phase difference for (1GHz and 2GHz)
Figure 5: Schematic diagram cross-coupled branch line coupler with operating
frequencies (1.5GHz and 3GHz)
Figure 6: Measured Response of the designed Cross couple branch line coupler
Magnitude and Output phase difference for (1.5GHz and 3GHz)