Coupled-line co-directional coupler with inherent harmonic suppression and its wideband implementation
Introduction
Directional couplers are the key passive component used in communication systems [1], beamforming [2], balanced mixer or amplifiers [3] and so on. In general, there are three kinds of directional couplers including co-directional (COD) coupler, contra-directional (CTD) coupler and trans- directional (TRD) coupler [4]. The COD coupler is usually constructed by branch lines (BLs), named as BL-COD coupler. Since four λ/4 branch lines are used, it has the inherent characteristics of large size and narrow bandwidth.
Many efforts have been proposed for the size reduction of BL-COD coupler [5], [6], [7], [8], [9]. In [5], a miniaturized BL-COD coupler is presented. By using the meander high-impedance transmission lines, a size reduction of 70.9% is obtained. However, the structure is complicated and L-section matching network is needed for port matching. In [6], periodically grounded slow-wave transmission lines are utilized and the size is reduced by 50.3%. In [7], BL-COD coupler based on non-periodic step impedance shunt stub is presented. As compared to the conventional BL-COD coupler, the area is reduced by 67.7%. In [8], asymmetrical T-structure is adopted for realizing the BL-COD coupler. The area occupies 12.2% of the conventional structure. In [9], a compact BL-COD coupler is developed using π-equivalent shunt-stub-based artificial transmission lines. The coupler occupies about 8% of the conventional design. In [10], by substituting the λ/4 transmission line (TL) with the stub-loaded TL, a harmonic suppression rat-race coupler with a size reduction of 70% is achieved. Although the methods mentioned in [6], [7], [8], [9], [10] can realize miniaturization to some degree, the complexity in the design process and fabrication limits the applications.
To widen the bandwidth of the BL-COD coupler, the most effective method is to cascade multi sections of the couplers. The theory of multi-section cascaded BL-COD coupler is presented in [11]. Then, several improvements are reported. In [12], a broadband coupler is designed by using a four-section cascaded BL-COD coupler. The measured relative bandwidth is 43.3%. However, the defected ground is used for realizing high impedance transmission lines. In [13], a three-section cascaded broadband BL-COD coupler with equivalent L-section is proposed. More than 50% of the area is saved and larger than 50% of the bandwidth is achieved. Later, a two-section cascaded wideband BL-COD coupler using parallel-coupled three-line is demonstrated [14], which obtained a relative bandwidth of 55% by adopting the defected ground planes and the multilayer process. In [15], a modified three-section cascaded BL-COD coupler is proposed with ultralow amplitude imbalance. The measured fractional bandwidth is 65%. In [16] a miniaturized three-section cascaded BL-COD coupler is presented. The volume is reduced by 87.7% with more than 50% relative bandwidth. However, since high impedance transmission lines are used [14], [15], [16], the fabrication difficulty is increased.
Additionally, since the spurious and harmonic bands in the microwave devices may cause interferences and degrade system functionality, the suppression of such bands is a due in certain applications. Therefore, the researches of directional couplers with harmonic suppression have also become a hot spot in the research field of radio frequency devices. In [17], an optimization-driven framework is proposed for the design of wideband multi-section branch-lines, which can suppress odd harmonics in the proposed designs due to enforcing the desired electrical characteristics over a given frequency band. In [18], stub-loaded transmission lines are applied to replace the branches of the broadband BL-COD couplers, which achieve harmonic suppression and small size. Recently, a modified T-shaped transmission line is proposed to reduce the size and suppress unwanted harmonics [19]. The size is reduced by 77% and the eighth harmonic is suppressed. In [20], a compact transmission line topology to reduce the size of single- and two-section BL-COD couplers is presented. The physical size occupied by the proposed single- and two-section BL couplers is 23% and 19.5%. Moreover, at least 20 dB suppression of up to third harmonics is achieved.
In the reported literatures, the advances in COD couplers are almost concentrated on BL couplers, including the size reduction, wideband and harmonic suppression. In the paper, a novel implementation of the COD coupler is proposed for the first time, and the wideband realizations are also demonstrated. The coupler is constructed by coupled line (CL), which can be named as CL-COD coupler. It has the advantages of small size and inherent harmonics suppression compared with the traditional BL-COD coupler. Although studies on coupled-line based couplers have been reported [21], [22], [23], [24], [25], [26], which are named as the forward-wave directional couplers (FWDCs), they are all developed by periodical structures and implemented with multilayer of non-planar structures. While the proposed CL-COD can be fabricated on single layer substrate with compact structure. In Section 2, the theoretical analysis of the proposed CL-COD coupler and its wideband implementation are introduced. In Section 3, the parameters of the CL-COD coupler, as well as the parameters of the wideband CL-COD coupler with two- and three-section cascaded are analyzed. For demonstration, two prototypes are designed, fabricated and measured in Section 4, followed by a conclusion in Section 5.
Section snippets
Theory of the single-section CL-COD coupler
The schematic of the proposed single-section CL-COD coupler is shown in Fig. 1. It is composed of a parallel coupled line, a shunt inductor and two open-circuited shunt stubs. The parallel coupled line with an electrical length of θ11 owns the even- and odd-mode characteristic impedances of Zce and Zco, respectively. The inductor named as L1 is loaded between the center of the coupled line. The open-circuited stubs with the electrical length of θ12 and the characteristic impedance of Z1 are
Parameters discussions on the single-section CL-COD coupler
To verify the design equations of the single CL-COD coupler, a 10-dB coupler (k = 0.32) working at 1 GHz with harmonic suppression at 2 GHz is designed as an example. Firstly, according to (11a), (11b), the electrical lengths of the coupled line (θ11) and the open-circuited stubs (θ12) can be calculated as 90° and 45°, respectively. Secondly, according to (8), (9a), it is found that the characteristic impedance of the odd-mode coupled line (Zco) and the open-circuited stubs (Z1) are functions
Implementation and results
In this section, two CL-COD couplers are fabricated to validate the design theory, including a 10-dB single-section coupler and a 3-dB three-section cascaded coupler. The two couplers were implemented on the F4B substrate (εr = 3, tanδ = 0.003, h = 1.5 mm). Fig. 13 shows the layouts of the fabricated prototypes, and the photographs are shown in Fig. 14. For the 10-dB coupler, circuit parameters obtained in Section 3.1 are transformed into physical dimensions and carefully optimized. For the
Conclusion
A novel CL-COD coupler with small size, wide bandwidth and harmonics suppression is designed by inductor loaded parallel coupled line and a pair of open-circuited stubs. Besides, wideband CL-COD couplers with harmonics suppressions are formed by cascading multi-sections of the CL-COD couplers. For validation, simulation as well as measurement are carried out on two CL-COD couplers, including a 10-dB single-section coupler and a 3-dB three-section cascaded coupler. The measurement results agree
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.
Acknowledgments
This work was supported in part by the National Natural Science Foundation of China under Grant 51809030 and Grant 61871417, in part by the Natural Science Foundation of Liaoning Province under Grant 2020-MS-127 and Grant 2019-MS-024, in part by the Liaoning Revitalization Talents Program under Grant XLYC2007067, and in part by the Fundamental Research Funds for the Central Universities under Grant 3132021231.
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