Ⅰ. Introduction
Couplers are fundamental passive components that play a pivotal role in the realm of microwave and radio frequency (RF) engineering. These devices are purpose-built to efficiently distribute, combine, or segregate electromagnetic signals across diverse applications, encompassing telecommunications, radar systems, wireless networks, and test equipment(Chen et al., 2006;Ravelo, 2006;Shie et al., 2009).
The primary function of a coupler is to facilitate the controlled transfer of electromagnetic energy between multiple transmission lines while preserving signal integrity. This capability is indispensable in various scenarios, such as power sharing between antennas, signal division in duplexers, and signal monitoring in testing setups. Couplers can be tailored to manifest specific attributes, including coupling ratios, bandwidths, and impedance matching, designed to meet the requisites of the intended application (Shi et al., 2016;Zheng et al., 2014;Wu et al., 2014). In response to the escalating demand for high-speed data transmission and low-latency communication, the millimeter-wave frequencies have drawn considerable attention. This segment of the electromagnetic spectrum offers expansive bandwidths for data transmission, rendering it ideal for accommodating the increasing data requirements of contemporary society. Millimeter-wave couplers play a pivotal role within these systems, ensuring efficient power division, combination, and signal routing. These couplers are meticulously engineered to address the specific demands of higher-frequency operation, delivering low loss, high isolation, and precise coupling ratios while preserving a compact form factor(Chi and Kim, 2020;Yang et al., 2020) . These compact yet potent devices are instrumental across a wide spectrum of applications, including wireless communication, automotive radar, satellite links, and emerging technologies such as 5G and beyond. In recent years, millimeter-wave technology has experienced substantial advancements to meet the requisites of emerging applications and to harness the vast potential of the millimeter-wave spectrum. The development and utilization of advanced materials have elevated the performance of millimeter-wave devices(Chi and Kim, 2020;Yang et al., 2020;Chirala and Floyd, 2006). These artificially engineered materials possess unique properties that enable the precise manipulation of electromagnetic waves, thereby facilitating innovative coupler designs with superior performance and miniaturization. Couplers devised for integration into waveguide structures have gained increased prominence, offering robustness and low loss (Sajin et al., 2010;Zhang and Arbabian, 2021). These couplers are indispensable for various millimeter-wave systems, including radar and imaging applications. Substrate integrated waveguide (SIW)-based couplers have gained popularity owing to their compact size and ease of integration into printed circuit boards (PCBs) (Guan et al., 2014;Doghri et al., 2014). Hybrid couplers are widely adopted in millimeter-wave applications for their ability to provide balanced outputs and excellent isolation(Chi and Kim, 2020;Abdel-Wahab and Safavi-Naeini, 2011). Advanced hybrid coupler designs have been developed to offer enhanced bandwidth and performance. Traditional millimeter-wave coupler technologies, such as multi-layered coupler(Ali et al., 2019), waveguide-based coupler (Deng et al., 2016;Almeshehe et al., 2022;Zarifi et al., 2022;Shen et al., 2018; Foneseca and Angevain, 2021), are complex and with high-cost. With the increasing prominence of millimeter-wave technology, the demand for efficient, compact, and high-performance components has reached unprecedented levels.
Coplanar waveguide with ground (CPWG) has emerged as a promising transmission line technology for millimeter-wave circuits. Compared with conventional microstrip and stripline configurations, the CPWG structure, characterized by a central signal conductor flanked by two ground planes and a ground plane covering the entire back-face of the substrate, offers several advantages that are ideally suited for high-frequency applications, particularly in the millimeter-wave frequency range. The presence of ground planes on both sides of the signal conductor confines the electromagnetic field, reducing radiation loss and ensuring efficient energy transfer. This low-loss characteristic is crucial for preserving signal integrity in high-frequency systems. CPWG is highly amenable to miniaturization and planar circuit integration, rendering it a valuable choice for creating compact and high-performance microwave components and circuits (Foneseca and Angevain, 2021;Zhou et al., 2012).
The proposed coupled line coupler was designed with the following characteristics: an operating frequency range of 27 –32 GHz, an insertion loss of -1.0 to -1.5 dB, a coupling coefficient of -20 to -20.5 dB, and an isolation below -30 dB. This research is centered around the theoretical analysis, design, and simulation of the Ka-band coupled line coupler, which was built on a single-layered substrate by CPWG technology. Crucial design parameters, including the length, width, and gap of the coupled line, the length and width of the compensatory transmission line, and the gap between the conductor and flanked ground plane, were methodically optimized to achieve the desired coupling ratio and operational frequency. Utilizing simulation tools, the performance of the coupler was thoroughly assessed in terms of insertion loss, return loss, coupling coefficient, and isolation. Additionally, the study accounts for the impact of manufacturing tolerances on the performance of the coupler. The proposed coupler exhibited exceptional coupling characteristics, positioning it as a viable choice for 5G systems. Its compact dimensions, easy of implementation, low-cost, minimal signal loss, and substantial isolation enable applications in integrated millimeter-wave circuitry. The presented design provides a promising avenue for the further development of high-frequency systems.
Ⅱ. Analysis and Design
Coupled line couplers are fundamental passive components with broad applications in microwave and RF circuits (Caloz et al, 2004;Ha et al., 2017;Gao et al., 2018). Their primary function is to facilitate the controlled transfer of electromagnetic energy between transmission lines while maintaining impedance characteristics and minimizing signal loss. This capability enables efficient signal and power exchange between various components or subsystems within a circuit. Coupled line couplers can be customized to accommodate a range of coupling ratios, offering flexibility to manage signal distribution in accordance with specific circuit requirements. Over time, coupled line coupler technology has evolved significantly to meet the increasing demands of emerging communication systems operating at higher frequencies and handling greater data rates. The adaptability and reliability of coupled line couplers establish them as essential foundational elements within contemporary communication systems. Consequently, researchers and engineers have consistently pushed the boundaries of design methodologies and materials to create couplers capable of operating at millimeter-wave and terahertz frequencies.
A coupler consists of a coupled line and four compensatory transmission lines (with an impedance of Z1 and a length of θ1), as depicted in <Fig. 1>. For a coupled line unit (indicated by a red dashed line in <Fig. 1>), the relationship between the coupling coefficient (C) and the even- and odd-mode impedances (Z0e and Z0o) can be derived as follows (Pozar, 2011):
where Z0 is the characteristic impedance of the coupled line unit. The coupling coefficient of the proposed coupler was set to -20 dB, and its characteristic impedance was 50 Ω. Consequently, Eqs. 1(b) and 1(c) allow for the calculation of the even- and odd-mode impedance of the coupled line, yielding values of 55.28 Ω and 45.23 Ω, respectively. According to the principles outlined in the coupled line coupler theory (Pozar, 2011), the electronic length (θ0) of the coupled line corresponds to λ/4.
This study presents a coupler designed for application in a 5G semiconductor system, specifically tailored for operation within the frequency range of 27 to 32 GHz. Given the limitations of conventional microstrip lines, which tend to exhibit elevated radiation losses and limited isolation in the millimeter-wave spectrum, the adoption of an alternative technology becomes imperative. A traditional coplanar waveguide (CPW) is a flat transmission line configuration characterized by a central signal conductor flanked on both sides by semi-infinite ground planes, all positioned on a dielectric substrate. In comparison with the microstrip counterparts, CPW interconnects offer various advantages, including simplified fabrication processes, reduced radiation losses, and seamless integration of both active and passive components directly onto the transmission lines. However, CPWs typically incur higher losses and are more susceptible to the influence of nearby structures. To facilitate the integration of hybrid circuits combining coplanar and microstrip elements, an additional lower ground plane is introduced, creating a conductor-backed coplanar waveguide, or CPWG. This lower ground plane serves a dual purpose: it can be employed for heat dissipation, offer mechanical support, and concurrently prevent electromagnetic fields from interacting with interconnects on lower circuit layers. The presence of ground planes both above and below the signal conductor in CPWG results in superior isolation, rendering it an excellent choice for applications where isolation is of paramount importance. The inclusion of ground planes within CPWG aids in achieving precise impedance control and matching, ultimately leading to simpler design and enhanced performance. The persistent presence of ground planes in CPWG contributes to minimizing radiation losses, establishing it as a transmission line with reduced signal loss, particularly at high frequencies.
The coupler was designed using the Nelco NY 9220 substrate characterized by a relative dielectric constant of 2.2, a height of 0.254 mm, a copper thickness of 0.018 mm, and a loss tangent of 0.0009. The initial design, simulation, and tuning of the coupler were conducted within the advanced design system (ADS). While ADS allows for rapid optimization, there is a significant deviation between circuit simulation and real-world performance. To obtain more precise simulation results, a three-dimensional (3D) model of the CPWG-based coupled line coupler was created and simulated using the high frequency structure simulator (HFSS). In comparison with traditional microstrip and stripline configurations, CPWG technology offers several advantages that are better suited for millimeter-wave circuitry. <Fig. 2>(a) presents the schematic of coupler A, comprising a single coupled line unit with four compensatory transmission lines based on the CPWG technology.
<Fig. 2>
Proposed coupled line coupler. (a) Schematic of the CPWG-based coupled line coupler (Coupler A). S-parameters for varying geometric parameters in the coupled line coupler: (b) length of coupled line L1; (c) width of coupled line W1; (de) gap in coupled line S1; (e) length of the compensatory transmission line L2; (f) width of the compensatory transmission line W2; and (g) gap between the conductor and ground plane S2.
This study explores the effects of key geometric parameters of the coupler, including the length, width, and gap of the coupled line (L1, W1, and S1), the length and width of the compensatory transmission line (L2 and W2), and the gap between the conductor and the flanked ground plane (S2). The variations in these parameters were explored experimentally. For example, L1 ranged from 1.0 to 1.8 mm with 0.2 mm increments, W1 varied from 0.59 to 0.71 mm with 0.03 mm increments, and S1 varied from 0.28 to 0.40 mm at 0.03 mm intervals. Similarly, L2 ranged from 6.56 to 7.36 mm with 0.2 mm increments, W2 varied from 0.59 to 0.71 mm with 0.03 mm increments, and S2 varied from 0.6 to 1 mm at 0.1 mm intervals. <Fig. 2>(b) –(g) present the S-parameters for these variations in the geometric parameters (L1, W1, S1, L2, W2, and S2) in the coupled line coupler. For instance, <Fig. 2>(b) shows that as L1 increases from 1.0 to 1.8 mm, there is a frequency shift of approximately 0.5 GHz, with noticeable changes in the magnitude of S31 and S41, while S21 remains relatively constant. <Fig. 2>(c) reveals that an increase in W1 from 0.59 to 0.71 results in significant changes in both the frequency and magnitude of S11. This change leads to a narrowing of the bandwidth and a decrease in magnitude, with S21 and S31 experiencing minimal changes. In <Fig. 2>(d), variations in S1 from 0.28 to 0.40 mm cause changes in the magnitude of S31 from approximately -21 to -18 dB, as well as in S11. However, these changes have almost no impact on S21. <Fig. 2>(e) indicates that L2 significantly affects the frequency, with the third poles of S11 shifting from 33.1 to 30.4 GHz as L2 increases from 6.56 to 7.36 mm. The magnitude of S11 also undergoes slight changes, while the magnitudes of S21 and S31 remain nearly unchanged. <Fig. 2>(f) demonstrates that W2 variations slightly affect the magnitude of S31 and S41, while S21 remains unchanged. Finally, <Fig. 2>(g) shows that the variations in S2 from 0.6 to 1.0 mm influence the frequency of the first pole of S11, which increases from 25.8 to 28.1 GHz. The performances of both S21 and S31 improve as S2 increases. Due to the extensive number of S-parameter curves, certain parameters that have minimal impact are not displayed for clarity and comparison. For example, S11 in <Figs. 2>(b) and 2(f) and S41 in <Figs. 2>(c) –(e) and 2(g) are not shown. Based on the analysis and simulation results, it is possible to adjust the dimensions to meet specific requirements effectively.
Given the high operating frequency of the proposed coupled line coupler, a wave launch connector designed for frequencies up to 50 GHz was employed. However, the width of this connector, approximately 11.9 mm, posed a challenge as it did not fit within the limited distance between the input port and the coupled port of coupler A. To address this issue and ensure a smooth measurement process, modifications were made to coupler A, resulting in the modified coupler B, as shown in <Fig. 3>(a). In this modification, the 50 Ω port transmission lines were extended at the coupling and isolation ports. This extension is solely for measurement convenience and will be removed when applying the coupler within an actual circuit system. A comparison of the S-parameters between couplers A and B is presented in <Fig. 3>(b), showing that there is a highly coincidence in S21 between the two structures, with only minor disparities in S11, S31, and S41.
Ⅲ. Fabrication and Measurement
The proposed coupled line coupler was simulated and optimized in both ADS and HFSS, as outlined in Section 2. The study explored the relationships between the key parameters, specifically the dimensions of the proposed coupled line coupler (comprising line width, gap distance, and coupling length) and its performance metrics, including insertion loss, return loss, coupling ratio, and isolation. These relationships are illustrated in <Fig. 2>(b) –(g). By employing simulation and thorough analysis, the desired performance characteristics can be readily obtained by adjusting the dimensions of the proposed coupler. The proposed coupler was specifically designed for use on the Nelco NY 9220 substrate, characterized by a relative dielectric constant of 2.2, a height of 0.254 mm, a copper thickness of 0.018 mm, and a loss tangent of 0.0009. To facilitate the measurement process well, wave launch connectors were employed in this study. For practical measurement purposes, coupler B was fabricated and assessed, even though coupler A remains a viable option within the context of 5G semiconductor systems. Additionally, in the millimeter-wave frequency spectrum, the inclusion of via holes is crucial when considering the employment of CPWG technology.
Via holes, which are vertical conductive pathways linking different layers of a PCB or substrate, constitute an indispensable element of CPWG technology and serve several pivotal roles in this context. Via holes establish a robust electrical connection between the signal trace and the ground plane within CPWG, which is instrumental in minimizing signal loss, reducing radiation, and ensuring effective isolation between the signal line and the ground. Strategically positioned via holes are used to exert precise control over the characteristic impedance of CPWG lines. Via holes can also be employed to isolate or shield particular regions of the CPWG circuit, which proves invaluable in applications where unwanted coupling or electromagnetic interference must be minimized. Furthermore, via holes serve as reliable mounting points for components, such as surface-mount devices or chip capacitors. They facilitate straightforward and secure attachment, particularly in compact CPWG designs where available space is limited. In certain scenarios, via holes are used to transition signals from CPWG to other transmission lines, such as microstrip and stripline, which may coexist on the same PCB or substrate. This enables effective interconnection between different types of transmission lines. Via holes can also be organized into arrays, enhancing electrical connections and aiding in thermal dissipation in CPWG designs. Overall, via holes within CPWG technology serve a multifaceted role in facilitating signal transmission, maintaining impedance, connecting to ground planes, isolating sensitive regions, enabling component mounting, and facilitating interconnections with other transmission lines. Appropriately designed and strategically placed via holes are of paramount importance in achieving desired CPWG performance, particularly in high-frequency and millimeter-wave applications, where precision is essential.
Coupler B based on CPWG technology and featuring via holes is simulated in HFSS. The coupler’s layout is illustrated in <Fig. 4>(a). Via holes are strategically incorporated to enhance electrical connections and facilitate heat dissipation within CPWG designs, effectively reducing voltage drops and preserving signal integrity. Notably, the eight larger holes are designated for port mounting. By leveraging the insights gained from the experimental analysis presented in the second section, the optimal dimensions of the coupler were determined. Specifically, the length, width, and gap of the coupled line (L1, W1, and S1) measure 0.34, 1.4, and 0.65 mm, respectively. The compensatory transmission line’s length and width (L2 and W2) were established at 6.96 and 0.65 mm, while the gap between the conductor and flanked ground plane (S2) was set at 0.8mm. Additionally, the 50 Ω transmission line’s length and width (L3 and W3) were determined as 6.0 and 0.77 mm, with a modified length (L4) of 9.0 mm. The via hole was specified with a diameter (D1) of 0.3 mm, and the connector hole was assigned a diameter (D2) of 1.8 mm. The proposed coupler was physically fabricated on a Nelco NY 9220 substrate, and the layout of the constructed coupler is shown in <Fig. 4>(b). The measurement of the coupler’s S-parameters was conducted using an Anritsu MS4647B vector network analyzer (VNA), with the four ports connected to the analyzer via wave launch connectors. The measured S-parameters of the proposed coupler are depicted in <Fig. 4>(c).
<Fig. 4>
Proposed CPWG –based coupler B with via holes. (a) Simulated module in HFSS; (b) Fabricated layout with four wave launch connectors; (c) Measurement results of S-parameters.
In the 27 –32 GHz operating frequency band, the insertion loss S21 is around -1.0 to -1.5 dB, the coupling ratio S31 is around -20 to -20.5 dB, and the isolation S41 is below -30 dB. Owing to the inherent losses in the substrate and connectors and the errors in fabrication and measurement, the simulated and measured results differ slightly; nonetheless, these errors are within acceptable limits. <Table 1> presents a comparison of the proposed CPWG millimeter-wave coupler with previous works.In previous works, millimeter-wave couplers often used 3D waveguide or multi-layer structures, which were difficult to process and costly. The CPWG-based coupler proposed in this paper adopts a single-layer structure, which is easy to implement, low cost, and has a good performance in the passband. Specially with comparison of the results in Zarifi et al.(2022) and Almeshehe et al.(2022), this CPWG-based coupler has better insertion loss and isolation characteristics, and smaller size.
<Table 1>
Comparison with previous works
| Ref. | Technology | Freq.(GHz) | S21(dB) | S11(dB) | Coupling(dB) | Isolation(dB) | Size |
|---|---|---|---|---|---|---|---|
| Chi.(2020) | MIMcap. Multilayer | 28 | -3.86 | - | -3.94 | - | 1.15 0.98mm (0.027 ) |
| Ali.(2019) | PRGW Multilayer | 26~34 | -3~-6 | <-10 | -3~-6 | <-10 | 1.3×1.3 |
| Deng.(2016) | Rectangular Waveguide | 26.3~40 | -0.35 | <-28 | -19.79±0.56 | <-30 | 60.03×23.22×26.48 mm3 |
| Almeshehe. (2022) | Waveguide Cavity | 28 | -2.9 | -25 | -3.1 | -25 | 78.2×28.22× - mm3 |
| Zarifi.(2022) | GapWave guide | 30 | - | -18 | -0.5 | -18 | - |
| Shen.(2018) | SIGW Multilayer | 23.59~29 | -3 | <-15 | -3 | -20 | 1×1 |
| Ali(2022) | PRGW Multilayer | 30 | -3.4 | <-15 | -3.4 | <-15 | 1.1×0.97 |
| Cayron (2024) | IPD3-D | 22.1~27.5 | -1.3~-1.7 | -17.5 | -10 | -12 | 0.84 mm2 |
| This work | CPWG Single layer | 27~32 | -1.0~-1.5 | -8 | -20~-20.5 | <-30 | 28.5×24.8 mm2 |
Ⅳ. Conclusion
This study introduces a CPWG-based coupler designed for 5G semiconductor systems. The research systematically optimizes key design parameters, including line width, length, and gap distance, to achieve the desired S-parameters and operating frequency. The designed and tested coupled line coupler, operating within the 27 to 32 GHz frequency range, has produced highly promising results. With a low insertion loss ranging from -1.0 to -1.5 dB, a consistently high flatness of the coupling coefficient between -20 and -20.5 dB, and an impressive isolation level below -30 dB, this coupler exhibits exceptional coupling characteristics. These findings affirm its suitability for seamless integration into 5G semiconductor systems. Moreover, the coupler’s compact design, minimal signal loss, and substantial isolation capabilities render it an appealing choice for the integration of millimeter-wave circuitry. This research serves as a foundation for the development of high-frequency communication systems, presenting a promising solution that addresses the unique demands of 5G technology and beyond. The proposed design not only enhances the performance of millimeter-wave systems but also contributes to the ongoing advancement of telecommunications and semiconductor technologies in the high-frequency domain.
