A Branch Line Coupler with improved stopband and miniaturized size for L-band applications

doi:10.1016/j.aeue.2020.153208

AEU - International Journal of Electronics and Communications

Volume 120, June 2020, 153208

https://doi.org/10.1016/j.aeue.2020.153208 Get rights and content

Abstract

This paper offers a compact microstrip branch line coupler with amended stopband performance using stepped impedance resonators. The operational frequency is adjusted at 1.174 GHz for L-band applications. The proposed coupler has a 75% decrement of size compared with a typical branch line coupler at the operational frequency. Embedded stepped impedance resonators make attenuation (more than 12 dB) on 2nd to 17th harmonics. The insertion loss and coupling are −3.1 dB and −3.1 dB, respectively. The proposed branch line coupler has been simulated, fabricated and tested. Good accord has been attained between simulation and test results.

Introduction

Branch line couplers (BLCs) are one of the important parts of wireless and communication systems. It can be used in balanced amplifiers, as shown in Fig. 1(a). The balanced amplifiers are used to handle double power without distortion.

A microwave integrated circuits (MIC) is a mixture of the microwave circuits, which is consists of discrete active and passive components. These components are placed to an etched circuit on a common substrate. The etched circuit also contains passive components such as filters and couplers [1]. There are many researches and designs for BLC in recent years. In [2], a compact BLC has been developed using meandered transmission lines for the decrement in size. It has functional insertion loss. However, it has a bad coupling; besides that, harmonics are not attenuated. In [3], a branch line coupler at the same time with rat race coupler with series of the branch lines have been presented to achieve size reduction. They have appropriate return loss and isolation, but they do not have harmonics suppression. In [3], two BLCs and rat race couplers have been used to realize a balanced Doppler radar. The presented circuit in [4], repeals the Tx leakage and couplings among antenna. In [5], a dual-band BLC with interdigital structures has been presented. There are unwanted harmonics between two bands that should be repressed. In [6], a BLC has been realized by combining open stubs to the transmission lines of the typical 3 dB BLC to get miniaturization of size. It does not have stopband region. In [7], a BLC with folded transmission lines has been presented to realize compact size and a dual-band region. The proposed coupler in [8] is based on combinational models of meander line, to improve the performances. The design of a BLC with folded shunt stubs cells with a %54 size reduction and three suppressed harmonics has been presented in [9]. In [10], a modified BLC using odd and even mode analysis has been designed. In [11], using open stubs and resonators, smaller dimensions compared to the typical design has been obtained. It has a dual frequency band and well suppression between them, but it does not have upper stopband region. In [12], new design with curved microstrip lines to achieve compact overall size have been presented. Due to the eight series resonators, it has eleven suppressed harmonics. In [13], a BLC using interdigital structures with two passbands has been introduced; it has unequal power division in the passbands. The design of a BLC with multiple open stubs instead of a 35 Ω transmission lines has been introduced in [14]. In [15], a balanced amplifier using BLCs has been presented. A hybrid coupler with four suppressed harmonics using stepped impedance resonators has been presented in [16]. In [17], two BLCs using meandered microstrip lines and stepped impedance open stub cells have been designed. They have wide passband width with a fractional bandwidth (FBW) equal to 60.4% and 55.4%. To achieve decrement of size, typical microstrip lines in BLC have been replaced by transmission lines with open stubs in [18]. In [19], semi-circular microstrip cell and stepped impedance resonators are applied instead of typical BLC transmission lines, which results in 78% size decrement and harmonics suppression from 2nd to 11th. In [20], a BLC with three branch lines using meandered T-shaped microstrip cells has been introduced. It does not have rejection for unwanted harmonics. In [21], very well Miniaturized BLC using hairpin resonator cells and folded transmission lines has been designed. The absence of harmonics attenuation can be considered as its disadvantage. Ref. [22] presents a simple shape BLC which has an open stub in each of the transmission lines. The weakness that can be mentioned for it, is a large size, which is just a 29% decrement of typical size. A dual band BLC with folded microstrip lines for using in a dual band butler matrix has been designed in [23].

In this paper, the transmission lines of the conventional BLC are replaced by modified resonators, which result in achieving a BLC with miniaturized size and wide stopband, as shown in Fig. 1(b). There are many types of resonators such as radial, hairpin, ring, tapered, rhomboid, stepped impedance, trapezoid shaped resonators. Among them, the stepped impedance resonators have more standard and formula designs. In this design, a BLC with wide stopband and compact size using stepped impedance resonators is presented. Advanced design system (ADS) is used for simulation results. RT/duroid 5880 material with dielectric constant equal to 2.2 and a thickness of 31 mil is used for substrate.

Section snippets

The 35 Ω microstrip line design

At the first step, an LC lumped circuit with five order, as shown in Fig. 2, is presented. The lengths of the equivalent microstrip lines of the presented LC lumped circuit are calculated based on the equivalent values which are presented in [1]. Fig. 2(b) and (c) show the layout and frequency response of the presented five-order Elliptic response filter.

To increase attenuation in stopband a shunt resonant circuit is added to the circuit, as demonstrated in Fig. 3(a). Fig. 3(b) shows, its

The 50 Ω microstrip line design

The proposed 50 Ω transmission line is designed in a similar way for a 35 Ω transmission line. So after designing and bending the microstrip lines, the proposed layout for 50 Ω transmission line is obtained as demonstrated in Fig. 5(a) and its responses are displayed in Fig. 5(b).

The proposed BLC

After embedding the proposed 35 Ω and 50 Ω transmission lines into the BLC structure, the proposed BLC is created as illustrated in Fig. 6(a). Its frequency responses are shown in Fig. 6(b). As can be seen, the proposed BLC has 1.174 GHz operational frequency with a 75% size reduction. It has harmonic suppression from 2nd to 17th harmonics. Fig. 7 shows the phase difference between S₂₁ and S₃₁. It can be seen that there is −273° or 87° phase difference, which is the proper value for a BLC.

Odd and even mode analysis

The coupling effect between resonators is investigated by means of the odd and even mode analysis. Fig. 8 shows the equivalent circuit of the presented BLC in which L_th₁, L_th₂ and L_tv₁, L_tv₂ introduce the equivalent inductances of the horizontal and vertical transmission lines, respectively. L_rh₁ − C_rh₁, L_rh₂ − C_rh₂ and L_rv₁ − C_rv₁ represent the LC resonance circuit of the resonators in the horizontal and vertical microstrip lines. C_ch₁, C_ch₂ and C_cv depute capacitance effects between

Measured results

The proposed BLC has been fabricated. Fig. 11(a) shows a photograph of the fabricated BLC. Measured results are shown in Figs. 11(b) and 11(c). To show clear responses at the operating frequency, S₂₁ and S₃₁ are magnified as shown in Fig. 11(d). The test results indicate insertion loss, coupling factor, return loss and isolation at working frequency are −3.1, −3.1, −21.1 and −20 dB, respectively. Table 1 shows the amount of suppression levels of the harmonics.

Table 2 shows a comparison between

Conclusion

A BLC with proper specifications such as compact size, proper insertion loss, return loss, coupling factor and isolation with improved stopband performance which guarantees unwanted harmonics will be removed, was presented in this paper. The operational frequency of the proposed BLC is 1.174 GHz, which can be used in communication systems in the L- band frequency range. It has a 75% size reduction. So, it can be used as an embedded part in the balanced amplifiers circuits easily.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors would like to thank the Kermanshah Branch, Islamic Azad University, Kermanshah, Iran; for the financial support of this research project.

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Citation Excerpt :
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Regular paperA Branch Line Coupler with improved stopband and miniaturized size for L-band applications

Abstract

Introduction

Section snippets

The 35 Ω microstrip line design

The 50 Ω microstrip line design

The proposed BLC

Odd and even mode analysis

Measured results

Conclusion

Declaration of Competing Interest

Acknowledgment

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AEU-Int J Electron C

AEU-Int J Electron C

AEU-Int J Electron C

AEU-Int J Electron C

Microstrip Filters for RF/Microwave Applications

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IEEE Trans Microw Theory Tech

24 GHz balanced doppler radar front-end with Tx leakage canceller for antenna impedance variation and mutual coupling

IEEE Trans Antennas Propag

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IEEE Microw Wirel Compon Lett

Regular paper
A Branch Line Coupler with improved stopband and miniaturized size for L-band applications