Regular paperA generalized broadband coupled-line based rat-race coupler with arbitrary power division ratios and free terminated impedances
Introduction
As an indispensable part of RF and microwave components in wireless communication systems [1], [2], [3], [4], [5], the coupler is widely used in various applications, such as balanced amplifiers, balanced mixers, phase shifters and antenna arrays. However, due to the large size and the narrow bandwidth of the traditional rat-race couplers, they are not suitable for modern communication systems. To reduce the circuit size, as well as improve the integration for integrated circuits (ICs) and systems, there is a strong motivation in developing systems capable of pursuing extraordinary performance and multiple functionalities. In recent years, couplers with multiple functionalities such as arbitrary power-division ratios [6], [7], [8], [9], [10], dual-band [11] or multiband operation, size reduction [12], [13], free terminated impedances [9], [10], filtering integration [7], and widened bandwidth [14], [15], [16], [17], [18], [19], [20] have been reported recently.
In an ever-increasing wireless communication market, wider bandwidth is always required to achieve higher data rate. Therefore, extensive researches have been reported to achieve the broadband operation. The multi-branch technique [15], [16], as a well-known method to extend bandwidth, has been widely studied. But it is undesirable for reduced-size and low-cost system requirement. The coupled-line (CPL) [17], [18], [19], [20] is usually employed to achieve wide bandwidth but it generally exhibits a low coupling factor (C). To further increase coupling factor, patterned ground and vertically installed planar (VIP) structure [16], [18] were adopted. However, the production process is more complex and the circuits with these structures are restricted in some occasions. Therefore, the circuit size and system complexity should be considered when comparing the bandwidth between couplers.
In addition, compared to the conventional couplers, the couplers with free terminated impedances are getting a lot of attention since their ability of easy connection with other components without introducing impedance-transformation networks. Furthermore, the couplers with arbitrary power-division ratios could dramatically improve the integration for ICs and systems. However, most of them have a narrow bandwidth. In [13], a planar miniaturized arbitrary phase-difference coupler with arbitrary coupling coefficients was proposed, however, only 20% fractional bandwidth is available. The couplers with arbitrary power-division ratios and alternative input-output impedances were proposed in [9]. The design theory of the structure is supplemented in [10] but it was limited by a narrow bandwidth.
Overall, up to date, a planar and simple wideband coupler with compact size, arbitrary power-division ratios and alternative terminated impedances has not been proposed.
In this brief, for the first time, a simple and planar broadband coupled-line (SPBCL) based rat-race coupler with arbitrary power-division ratios and free terminated impedances is presented. One shorted coupled-line section and two identical open-circuited (SCPL-OC) stubs are introduced to further improve the bandwidth. Overall, the proposed SPBCL rat-race coupler has the following significant advantages: i) wider bandwidth; ii) arbitrary power-division ratios; iii) free terminated impedances; iv) compact size; v) planar and simple circuit model with closed-form analytical equations.
Section snippets
Analysis of the proposed coupler
The schematic of the proposed SPBCL rat-race coupler with arbitrary power-division ratios and free terminated impedances is shown in Fig. 1. The designed rat-race coupler comprises three transmission lines (TLs) with the characteristic impedances of Z1, Z3, Z4 and electrical lengths θ1, θ3, and θ4 between ports 1 and 2, ports 3 and 4, ports 1 and 4, respectively. In addition, between ports 2 and 3, the section (M) consists of two identical open-circuited stubs with the characteristic impedance
Discussion
In this section, the characteristic impedances are limited within the range from 20 to 140 Ω according to the practical consideration.
Based on the equations (14a), (14b), (14c), it can be observed that the coupling coefficients C and the electrical length θ5 are free variables in these analytical design equations and they exert no influence on the responses at the center frequency (f0). Therefore, the influences of C, θ5, and θ0 on the fractional bandwidth (FBW) should be studied. The
Full-wave simulated and measured results
For the purpose of feasibility verification, a wideband rat-race coupler with free terminated impedances at the center frequency f0 of 1 GHz is fabricated and characterized. For low-cost consideration, the prototype is fabricated on PCB, but the verification is still sufficient for integrated circuit implementation. The circuit is fabricated on a RO4350B substrate with a relative permittivity of 3.66, loss tangent of 0.0037, and thickness of 1.524 mm (60 mil). Fig. 6 shows the layout and
Conclusion
In this paper, a novel, simple, and planar structure composed of three TLs, one shorted CPL and two identical open-circuited stubs for wideband rat-race coupler is proposed and implemented. The analyses and circuit configuration are also provided. This proposed coupler has five main advantages over the conventional ones, such as wider bandwidth, compact size, planar and simple circuit model, arbitrary power division ratio and free terminated impedances. The above characteristics of the proposed
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.
Acknowledgement
This work was supported by the Beijing Natural Science Foundation under Grant JQ19018, in part by the National Special Support Program for High-Level Personnel Recruitment under Grant 2018RA2131, in part by the National Natural Science Foundation of China under Grant 61971052, and in part by Science and Technology Key Project of Guangdong Province, China (2019B010157001).
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