DESIGN METHODOLOGY OF BRANCH LINE COUPLER USING FILTER AND CMOS FABRICATION TECHNIQUES
Deepak Ahirwar, Research Scholar Ruhee Matolya, Guide
Abstract - A branch-line coupler based on low pass microstrip filters instead of transmission lines is presented in this paper. The design is performed using a low pass filter of 5th, 7th and 9th order. This has been done to achieve a compact circuit layout in microstrip implementation. Prototypes with these proposed structures, operating at 2.1 GHz frequency, have been designed and implemented using microstrip technology. The proposed architecture is based on a low-isolation device and an adaptive cancellation unit. Some low- isolation microstrip devices have been considered dual-port filter systems are good low- design effort choices. Filters or microstrip devices might provide a better solution since they provide some additional level of selectivity against other interfering signals.
Keywords: Advanced System Design (ADS), Defected ground structures (DGS), dielectric resonant antenna arrays (DRA).
1 INTRODUCTION
A branch-line coupler, or quadrature hybrid, is a passive device, which is widely applied in power distributing and combining systems [1]. Other applications exploit their ability to provide a high degree of port-to-port isolation even while the ports are mismatched.
With the purpose of investigating characteristics of branch-line couplers, this project aims to design and fabricate a branch-line coupler fulfilling the following requirements:
• Center frequency: 2.4GHz
• Low coupling factor: -10dB High directivity
• Test substrate RO4350B
The coupler is designed on software Advanced System Design (ADS). The design is fabricated and afterwards measured. Finally, a comparison between the experimental results and the simulation results is performed to qualify the product. Filters can be divided into two different main types, lumped or distributed. Lumped elements consist of discrete elements, such as inductors and capacitors, while distributed elements use the lengths and widths of transmission lines to create their inductive or capacitive values.
The microstrip line is a good candidate for filter design due to its advantages of low cost, compact size, lightweight, a planar structure and easy integration with other components on a single board. To achieve better performance for the microwave filters, such as increasing the steepness of
the cut-off slop, and increasing the stop band range of the microwave filters, defected ground structures (DGS) are used. This technique is realized by etching slots in the ground plane of the microwave circuit.
2 CMOS FABRICATION PROCESS IN MICROSTRIP
With the advent of technology, researchers and designers use various technologies-aided software, machines and techniques for fabricating a microstrip effectively. The modern-day technology helps in the simulation and designing of a microstrip as per requirement. The fabrication process consists of the following steps:
1. A mask or transparency is prepared with the help of the available software.
2. The next step in the fabrication process consists of creating a photo- resist pattern for which a photo- resist material (consisting of photo- resist solution and plastic tray) and photo-resist equipment consisting of an Ultra Violet light compartment and temperature-controlled hot plate.
3. The next step involved in the process of fabrication consists of removing or etching the unwanted copper using some common solution so as to get the required copper as a conductor pattern.
4. The next step associated with the fabrication process involves
preparing a Micro strip housing i.e.
the microstrip base using good conductor materials like aluminium or copper.
5. The final step involves soldering the connectors into the circuit of the microstrip after the same are properly screwed with the ground plane.
3 FABRICATION TECHNOLOGIES
The researchers and designers embarked on the development of printed circuits which were planar in nature and also the
efficacy of lumped components in the domain of microwave frequencies subsequent to the development of transmission lines which were planar in nature like the microstrip lines and also the strip lines. The microwave industry witnessed a sea change with the miniaturisation as well as the batch fabrication of a large number of microwave functions in respect of bulk volume production. Preliminary research on printed circuits which were planar in nature
Figure 1 Microstrip Microstrip transmission lines consist of a
conductive strip of width "W" and thickness "T" and a wider ground plane, separated by a dielectric layer (the
"substrate") of thickness "H" as shown in the figure 1. The microstrip has most of its field lines in the dielectric region, concentrated between the strip conductor and the ground plane, and some fraction in the air region above the substrate as shown in fig. Microstrip is by far the most popular microwave transmission line, especially for microwave integrated circuits and MMICs. The major advantage of microstrip is that all active components can be mounted on top of the board.
Having a finite thickness of metal for the conductor strips tends to increase the capacitance of the lines, which affect the effective dielectric constant and the characteristic impendence.
In the previous years, wireless communication systems have developed tremendously, there was a prompt development in ultra-wideband systems, wireless internet like Wifi and Wimax, broadband personal communication systems and 3G (third generation), 4G (fourth generation) technologies. Due to this rapid development, there was a need for more rigid microwave components.
And now day’s satellite systems changed their path from static telecommunications systems to mobile, remote sensing and navigation applications. Microwave components play an important role in satellite systems. Microwave components include microwave resonant components such as microwave filters, dielectric resonant antenna arrays (DRA), and duplexers. Because of the rapid growth in the wireless communication area, it created more challenging requirements that enforce challenges on various novel designs, optimization and understanding of components. In microwave filters, challenges are to be faced in miniaturization, bandwidth, phase linearity, and selectivity of the filters.
4 BRANCH-LINE COUPLERS
Couplers play an important r in most antenna systems applications, such as modern radar, test equipment, and RF mixers. The quadrature branch-line coupler has been well-the known since the 1950s. In conventional structures, quarter wavelength transmission lines with an impedance of 50 and 35 Ohms are used as the basic building block. This frequently results in significant circuit size. A typical branch-line coupler has
quadrature outputs, which means that it divides the input signal between two ports having a 90° phase difference and the same power while the last one is isolated.
The working bandwidth of the branch-line hybrid is constrained by the isolation factor.
Up to day present day, many investigators have been working on memorizing the conventional transmission lines using various techniques. The use of interdigital capacitors instead of transmission lines is shown in [1]. Another realization is based on implementing open-circuit shunt stubs [2]. The usage of shunt capacitors, fractal geometry and equivalent miniaturized stubs is described in [3]. The widest range of the miniaturization techniques of microstrip couplers is observed in [4] and [5]. The device's area size reduction varies from 10 to 90 per cent but some of the designs are inconvenient for fabrication and most of them have much worse characteristics than the conventional
ones. The general idea of this paper is in the assumption that the quarter wavelength transmission lines of a conventional coupler can be replaced with microstrip low pass filters having the same electrical length, but at the same time, less physical length. To obtain a reduced-size coupler with remaining characteristics we will analyze its performance while replacing quarter wavelength transmission lines with low pass filters of different orders. To compare the results correctly it is necessary to fulfill the following conditions:
● The same design procedure for filters of different orders;
● The same EDA system for electromagnetic structures modelling and analyzing;
● Central frequency and substrate parameters are the same for all designs;
● Gaps between lines and line widths are not less than 0.4 mm, which can be easily implemented.
5 BRANCH-LINE COUPLER
Figure 2 Structure of a branch-line coupler The geometry of a branch-line coupler is
shown in Figure. A signal applied to port 1 is split into ports 2 and 3 with one of the outputs exhibiting a relative 90 phase shift.
5.1 Microstrip Directional Coupler Design
There are several methods to equalize or compensate for velocity inequality in even and odd modes of microstrip directional coupler designs. The dielectric overlay is one method, where the effective dielectric constant of the odd mode is increased to equalize the phase velocities. The wiggly line coupler and re-entrant mode couplers are recommended to provide high directivity performance and tight
coupling. The capacitively and inductively compensated directional couplers exhibit equalized phase velocities.
In a capacitively compensated microstrip directional coupler, the capacitive compensation is used with parallel-coupled microstrip lines, and the additional capacitor is fabricated using a dielectric substrate. The lumped capacitor is responsible for reducing the difference between the phase velocities in a microstrip directional coupler’s odd and even modes. This design offers tight coupling between the coupled lines as well.
The effectiveness of microstrip directional coupler designs can be analyzed by measuring the directivity and
tight coupling in dB and comparing it with a conventional microstrip coupler.
Cadence's PCB Design and Analysis tools can support you in calculating the design parameters and phase velocities of microstrip directional couplers.
The designed filters and their phase responses equivalent to a quarter wavelength line for 2000 MHz are shown in Fig. 5.1.
Figure 3 Phase responses of investigated structures in degrees The phase responses of the line and the filters are the same at the design frequency and begin to diverge only at about 3000 MHz. In addition, it can be seen that the higher-order filter can provide a wider range of operating frequencies due to the higher cutoff frequency of its design. Couplers contain lowpass microstrip filters, which were synthesized in the EDA system NI AWR Design Environment. The substrate was chosen to be FR-4, 1 mm height, with ε = 4.2 and tg δ = 0.02. Microstrip branch- line couplers based on the lowpass filters of different orders are shown in Fig. The area occupied by the coupler that contains fifth-order filters is 14.5 mm
×13.5 mm = 195.5 mm2. This design consists of four lowpass filters providing a 90° phase difference and having corresponding wave impedances.
(a)
(b)
(c)
Figure 4 Computational Domain (a), (b) and (c)
S-parameters of the proposed branch-line coupler are also shown in Fig. The usable bandwidth (defined by isolation -20 dB) for the first design is 166.7 MHz.
Simulation results indicate that the input port matching parameter (S11) is below – 35 dB at the central frequency of 2 GHz.
The magnitude imbalance between transmitted and coupled ports is 0.23 dB.
S21 and S41 are coupling factors with an 89.5° phase difference.
Figure 5 Microstrip couplers based on the low pass filters of different orders and their characteristics
The layout of the compact coupler based on the low pass filters of the seventh order occupies an area of 15.4 mm × 13.6 mm = 209.44 mm2 that is 6.6% larger than the design with the LPF of the fifth order. The central frequency of the input port matching and the maximum of the isolation is the same. The usable bandwidth (defined by isolation -20 dB) is 156 MHz. At the same time magnitude imbalance between transmitted and coupled ports is not more than 0.1 dB.
The phase difference is 89.5°, so the phase error is 0.33% of the phase difference of the ideal coupler and could be neglected.
The coupler based on the low pass filter of the ninth order occupies an area, which is slightly less than the coupler with filters of the seventh order. The simulated imbalance is 0.06 dB and could
be neglected. The phase difference between the output ports is 90.9°.
All the simulated results are summarized in Table 1. All of the proposed designs are realizable.
Implemented prototypes of all of the designs mentioned above. It is well seen that size reduction is significant and that all the designs have almost the same size.
The design procedure becomes more complicated for higher-order LPFs because of the increasing number of elements in filters and decreasing unoccupied area inside the coupler due to the bigger number of gaps between the elements. It leads to the necessity of coupling estimation and more iterations of miniaturization (moving the microstrip capacities in the way so they will occupy a mostly internal area of the coupler).
According to the results, a conclusion could be made that the most effective is the coupler based on the lowpass filters of the lower order. In this design, there are no extremely narrow microstrip lines and gaps, so it could be easily reproduced within simple PCB manufacturing.
The coupler with lowpass filters of the seventh order occupies the largest area due to strong coupling between different microstrip elements, which leads to the degradation of the parameters.
Overall, the size of the device increases with the increase in the order of the filters.
Fabricated couplers were tested using vector network analyzer R&S ZVA- 24 with the calibration kit ZV-Z52. All frequency responses were measured for all three fabricated couplers. The measured and simulated parameters agree very well.
6 RESULT DISCUSSION
The proposed architecture is based on a low-isolation device and an adaptive cancellation unit. Some low isolating microstrip devices have been considered dual-port filter systems are good low- design effort choices
Table 1 Comparison of compact branch-line couplers using lowpass filters of different orders
Type of design Area, mm2
Usable bandwidth, MHz
Phase difference,
degrees
Transmission coefficients S21 S41
Conventional 621.2 220 90 -3.2 -3.2
Based on 5th order LPF
195.5 166.7 89.5 -3.43 -3.66
Based on 7th order LPF
209.44 156 89.7 -3.588 -
3.495 Based on 9th order
LPF
205.6 154.8 90.9 -3.586 -
3.521
Figure 6 Size reduced coupler deploying discontinuous microstrip line 7 CONCLUSION
This work proposes the usage of low-pass microstrip filters of different orders for the miniaturization of branch-line couplers.
Three compact couplers with filters of the fifth, seventh and ninth order on the central frequency of 2 GHz were designed and fabricated. All of the design shows a significant reduction in the size of the coupler. The most compact and simple design is realized with the fifth order as its size is 13.5 mm × 14.5 mm = 195.5 mm2 which is 68.5% smaller than the conventional coupler. The proposed architecture is based on a low-isolation device and an adaptive cancellation unit.
Some low-isolation microstrip devices have been considered dual-port filter systems are good low-design effort choices.
REFERENCES
1. Kai-Yu Tsai, Hao-Shun Yang, Jau-Horng Chen, and Yi-Jan Emery Chen, “A miniaturized 2 dB Branch-Line Hybrid Coupler With Harmonics Suppression,” IEEE Microw. Wireless Compon. Lett., vol. 21, no.
10, pp. 537-539, Oct. 2021.
2. Kimberley W. Eccleston, and Sebastian H.M.
Ong, “Compact Planar Microstripline Branch- Line and Rat-Race Couplers,” IEEE Trans.
Microw. Theory Tech., vol. 51, no. 10, pp.
2119-2125, Oct. 2013.
3. J. Zhu, Y. Zhou, and J. Liu, “Miniaturization of Broadband 3-Db Branch-Line Coupler,”
Progress in Electromagnetics Research Letters, vol. 24, pp. 169-176, 2021.
4. Xiaohua Wang and Xianzhi Meng, “ A Novel Neural Networks-Based Approach for Designing FIR Filters” Proceedings of the 6th World Congress on Intelligent Control and Automation, June 21 - 23, 2006 Dalian, China
5. Amanpreet Kaur, “Design of FIR Filter Using Particle Swarm Optimization Algorithm for Audio Processing “ International Journal of Computer Science and Network (IJCSN) Volume 1, Issue 4, August 2012
6. Lo-Chyuan Su,“ Neural Least-Squares Design of FIR Digital Filters with Trigonometric
Properties. “WHAMPOA An
Interdisciplinary Journal 54(2008) (103- 110) 7. S. M. Shamsul Alam, “Performance Analysis of FIR Filter Design by Using Optimal, Blackman Window and Frequency Sampling Method “International Journal Electrical &
Computer Sciences ” IJECS-IJENS Vol:10, No: 01 (13-18)
8. Ricardo A. Losada. “Practical FIR Filter Design in MATLAB “The Math Works Inc.
Apple Hill Dr Natick, MA 01760, USA March 31, 2003 ”
9. Sheenu Thapar, “A Low Pass FIR Filter Design Using Genetic Algorithm Based
Artificial Neural Network “International Journal of Computer Technology and Electronics Engineering (IJCTEE) Volume 2, Issue 4, August 2012.
10. Yong Ching Lim, “A Weighted Least Square Algorithm for Quasi Equiripple FIR and IIR Digital Filter Design “Transactions On Signal Processing Vol.40. NO. 3. March 1992,(551- 558)
11. Graham. Goodvin and Kwai Sang Sin, Applied To Problems In Prediction Filtering And Control Study of A Particular Adaptive Paraketw Estimator “Department of Electrical and Computer Engineering The University of Newcastle New South Wales, 2308,” Australia.
12. V. Ralph Algazi “On the Frequency Weighted LeastSquare Design of Finite Duration Filters.
“IEEE Transactions on circuits and systems, VOL. CAS-22,
13. Nasir Mahmood Asif, “A Unified Approach Using Neural Networks efficient Algorithms in Audio Signal Processing” Engineering College National University of Science and Technology.
14. David Hermann, “window based prototype filter design for highly oversampled filter bank in audio application” IEEE (ICASSP) April 16- 20, 2007 Honolulu.
