A REVIEW OF BRANCH LINE DIRECTIONAL COUPLER WITH BROAD BANDWIDTH
Bhupesh Aneja Ish Tyagi Vaibhav Gautam Shivank Shukla Assistant Professor Student Student Student JSSATE, Noida(INDIA) JSSATE ,Noida(INDIA) JSSATE ,Noida(INDIA) JSSATE ,Noida(INDIA)
ABSTRACT
In this paper a review of branch-line coupler is proposed, that is able to operate at wider frequency range around centre frequency . In this novel coupler analytical readings are used which allow us to fully exploit the design flexibilty inherent in two arm branchline couplers with open stubs and arc stubs which avoids the low impedence lines at a centre frequency of 2.6GHz with conventional microstrip transmission lines.Moreover it is manufactured on FR4 substrate with probable miniaturized structure of a composite branch line structure.This review covers the study of branchline directional coupler & microstrip transmission line.
KEYWORDS
Branch line coupler, microstrip, characteristic impedence, design frequency, scattering parameters, open stubs .
1.INTRODUCTION
The branch-line couplers are mostly used in microwave and millimeter wave circuits. There are lots of applications of 90 degree hybrid and 180 degree hybrid branch-line tight couplers, such as 3-dB or 6-dB coupler in our modern microwave and millimeter wave communication systems [1].
Branch line couplers find various applications in wireless communication systems, such as the phase shifters, balanced amplifiers and mixers, and antenna/array feeding networks. It can only be operated in the odd multiples of the fundamental band, which prevents itself from the versatile applications in dual-band or multi-band systems [2]. Further, it avoids narrow line gaps and need for bond wires.
The disadvantages of branch line couplers are:
1. Inconvenient line impedance
2. Narrow operating bandwidth [3]
They offer equal magnitude and quadrature phase outputs at the operating frequency band. They have been used extensively in the design of power dividers and image rejection mixers.
A hybrid branch line is a special case of directional coupler with a coupling factor of 3dB and phase difference of 90° in the outputs of the through and couple arms. It has a property that when all the ports are matched, the power entering from one port will be divided into two other ports and the fourth port is isolated.
1.1 Proposed Approach
Even the conventional branch line couplers suffer from a too narrow bandwidth for many applications, therefore in order to increase the operating frequency range it becomes important to find some realizable circuits. Some work has been done on broad banding hybrid couplers in the past. Based on the past work, a novel broad banding procedure will be developed in the following coupler structure using stubs. As for the stubs, a structure consisting of two sections of high and low impedance transmission lines is presented in order to connect a low impedance stub to a signal line [23].
Since the input impedance of the coupler is real at the centre frequency f0, the reflection coefficient S11(f0) is real also for a defined mismatch at the centre frequency, the corresponding input impedance Z1 can be calculated [3]. The following figure shows a conventional branch line directional coupler with four ports. Here the port 1 is input port, 3dB output is obtained at port 2 and port 4 and the port 3 is isolated.
Port 1 2Ө1 Port 2
ZZZZZZzzhh
2Ө2 Z2
Port 3 Port 4
Z1
Fig1: Composite branch line coupler with 3dB gain
where Ө1 = Ө2 = 450
where Ө1 is electrical length. If Z0 is characteristic impedance, then
Z1 /Zo = ( 1+S11(fo))/(1-S11(fo)) -(1)
Now we can calculate the impedance level of inner coupler. If Z2 is input impedance of shunt transmission line then, Z1/Z2 = Z2/(Z0(2)1/2) -(2)
1.2 Microstrip transmission line
Microstrip is a special type of electrical transmission line which can be fabricated using printed circuit board technology and is used to convey microwave frequency signals .It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate.Microstrip is thus much less expensive when compared to traditionalwaveguide technology and is also well very lighter and more compact.
The disadvantages of microstrip when compared with waveguide are the generally lower power handling capacity and higher losses. Also unlike waveguide, a microstrip is not enclosed and is therefore susceptible to cross-talk and unintentional radiation.
Power dividers anddirectional couplersare passive devices which areused in the field of radio technology. They couple a defined amount of the electromagnetic power in atransmission lineto a portenabling the signal to be used in another circuit. An important feature of directional couplers is that they only couple power flowing in one direction. Power entering the output port is coupled to the isolated port but not to the coupled port.
Directional couplers are most frequently constructed from two coupled transmission lines set close enough together such that energy passing through one is coupled to the other. This technique is favored by the devices which operate at microwave frequencies. However lumpedcomponent devices are also possible at lower frequencies. Also at microwave frequencies, particularly for the higher bands, waveguide designs can be used. Many of these waveguide couplers correspond to one of the conducting transmission line designs, but there are also certain types that are unique to waveguide.
Good approximation for phase velocity, propagation constant and characteristic impedance can be obtained from static or quasi static solutions. The phase velocity can be expressed as
Vp = c/√∈e - (3)
where c is speed of light, Ԑe is the effective dielectric constant of the microstrip line. Propagation constant can be expressed as,
β = k0√∈e - (4)
where k0 is wave number when air is used as dielectric. Since some of the field lines are in the dielectric medium and some are in the air, the effective dielectric constant satisfies the relation
1 ∈e ∈r - (5)
and is dependent on the substrate thickness d and conductor width W. Here Ԑr is the relative dielectric constant.
1.3 Formulas for effective dielectric constant, characteristic impedance and attenuation:
The effective dielectric constant(∈e) of a microstrip line is given by
∈e = (∈r+1)/2 + (∈r-1)/2 *1/√(1+12d/W) -(6)
If the dimensions of the microstrip line are known, then the characteristic impedance Z0 can be calculated as
Z0= {60/ ∈e log(8d/W+W/4d) } for W/d≤1
Z0={120π/√∈e[W/d+1.393+0.667log(W/d+1.44) ] for W/d ≥1} -(7)
Z0 , ϴ=β *l ZL Z
Zin
Fig2: Transmission lines without load Fig3: Transmission lines with load
1.4 For perfect mismatch in stubs
Open circuit ZL= ∞ , Zin = -j Z0 cotβl (8)
Closed circuit ZL = 0, Zin = j Z0 tanβl
(9)
1.5 Coupling factor
The coupling factor represents the primary property of a directional coupler. Coupling factor is a negative quantity, it cannot exceed0dBfor a passive device, and in practice does not exceed−3 dBsince more than this would result in more power output from the coupled port than power from the transmitted port.
The coupling factor is given as:
C3,1 = 10 log ( P3/P1) dB -(10)
where P1is the input power at port 1 and P3is the output power from the coupled port.
The main lineinsertion lossfrom port 1 to port 2 (P1– P2) is:
I4,1 = -10log( P4/P1) dB - (11)
1.6 Isolation
Isolation of a directional coupler can be defined as the difference in signal levels in dB between the input port and the isolated port when the two other ports are terminated by matched loads,
So Isolation is given by
Li2,1 = -10log(P2/P1) dB - (12)
1.7 Directivity
Directivity is directly related to isolation. It is defined as:
D3,4 = -10log(P4/P3)
= -10log(P4/P1)+10log(P3/P1) dB - (13)
where P3is the output power from the coupled port and P4is the power output from the isolated port.
The directivity should be as high as possible.
Substrate FR4 = ∈r = 2.2
h w Fig 4: conventional BLC with substrate height h ,width w
2. ABOUT THE SIMULATION SOFTWARE
PUFF is a scattering parameter and layout calculator for microwave circuits
Every element in the parts window is assigned a part with its parameters.
The two most important parameters of the simulation software are the characteristic impedance Z0 and the design frequency f0.To calculate the scattering parameters the characteristic impedance is used. The design frequency is used to calculate the electrical length of the transmission line. Different commands are used to obtain the smith chart and the frequency plot. The various types of window which appears on screen while using this software are,
2.1.The parts window -The program begins in the parts window. The initial parts list is taken from the setup file. The parts list can be edited by using the arrow key, the backspace key, enter key, insert key and delete key.
2.2 The layout window- This window is in the upper portion of the screen. The square represents the substrate, and the numbers on the side represents connectors. Typing an arrow key will draw the selected parts window in the direction of the arrow. The change in the x and y coordinates can be seen in the message box.
2.3 The board window- The relative dimensions of the circuit board in the layout window are specified from the board window. These dimensions set the scale which is used to draw the distributed components on the screen.
2.4. The plot window- After an analysis has been completed the plot window is used to list the values of the scattering coefficients at design frequency fd.
S(dB)
Fcy
Frequency(GHz)
Fig4: Frequency range of wide band directional coupler
Table 1. Comparison Of Various Proposed Approaches
REPRESENTATIVE / SETUP / RESULTSBernd Mayer and Reinhard Knochel[3] / Branch line coupler with improved design flexibility and broad bandwidth / Open stubs can be used to avoid low impedance lines .
M. Y. O. Elhiwaris, S. K. A. Rahim, U. A. K. Okonkwo
and N. M. Jizat[11] / Miniaturized Size Branch Line Coupler Using Open Stubs With High Low Impedances / The size reduction of the proposed design is 64.21% with comparable performance as that of the conventional branch line coupler.
Tamasi Moyra, Arabinda Roy, Susanta Kumar Parui, Santanu Das[1] / Design Of 10 dB Branch line Coupler By Using DGS / Adding DGS introduces some inductance and increases the effective electric length of the microstrip line and provides slow wave characteristics.
Tse Yu Chen , Pei Lin and
Ting[2] / Miniaturized Branch line Coupler with Coupling dependent dual frequency Operation / Compared to the conventional branch line coupler operating at 0.9GHz, the proposed coupler shows a size reduction by 56%.
Iwata Sakagmi, Ryo Teraoka and Takatsugu Munehir.[23] / A Reduced Branch line Coupler With Eight Stubs / Using low impedance transmission line for the stubs, The proposed coupler reduced to 25% in the ratio of area.
Hui-Yong Zeng, Guang-ming Wang, Zhong-wu Yu and Xiao-kuan Zhang[24] / Miniaturization of Branch–Line Coupler Using Composite Right/Left–Handed Trans- mission Lines / A coupler without Lumped Components can be fabricated easily with a standard printed circuit board process
Abdulkadir Yilmaz 1, Emine Rumeysa Cetiner 1, and Moamer Hasanovic [25] / Sonnet Modelling Simulation of Broadband Branchline Coupler / A cascaded two-section broadband branchline coupler has been designed and simulated using SONNET to have low input VSWR and high isolation over a wider bandwidth. The results of the simulation provide satisfactory results with a deviation of 5% in the frequency band.
Kimberley W. Eccleston, Member, IEEE, and Sebastian H. M. Ong, Student Member, IEEE[12] / Compact Planar Microstripline Branch-Line and Rat-Race Couplers / The development and design method for compact branch-line and rat-race couplers can
be realized in microstripline with one layer of metal without additional lumped elements.
N. Zheng1, L. Zhou1 and W.-Y. Yinz[13] / A Novel Dual-Band Shaped Branch-Line Coupler With Stepped-Impedance Stubs / A novel 2.4/5.2 GHz compact dual-band branch-line coupler using stepped-impedance stub is demonstrated and analyzed with closed form formulas obtained
K. P. Ray, V. Sevani, and S. Kakatkar Sameer[18] / Compact Broadband Gap-Coupled Rectangular Micropstrip Antennas / Multiple numbers of parasitic
elements formed by splitting a single RMSA into numbers of
equal, smaller strips along the width has been carried out
3. CONCLUSION
The analysis in this paper shows that with the help of varying parameters of shunt and series transmission lines and by varying the parameters of open or short circuit stubs around the design frequency, the operating bandwidth of the branch line coupler can be increased efficiently.
Besides this, the performance of the branch line coupler can be increased with the help of different improved designs using PUFF and DosBox . Further the open stubs help in designing high impedance lines, which help in miniaturization and enhanced output.
The plot obtained gives all the scattering parameters implicitly on the specified design parameters.
REFERENCES
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[3] Bernd Mayer and Reinhard Knochel ,1997’Branch line coupler with improved design flexibility and broad bandwidth’ Asia Pacific Microwave Confernce .
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