Coupled-line Sections Research Articles

Differential Bi-Level Microstrip Directional Coupler with Equalized Coupling Coefficients for Directivity Improvement

In this paper, a bi-level microstrip differential directional coupler has been investigated. It has been shown that the equalization of coupling coefficients can be successfully made with the use of appropriate dielectric stack-up and conductor geometry. The application of additional top dielectric layer can ensure proper equalization of coupling coefficients by lowering the value of capacitive coupling coefficient to the value of the inductive one. The theoretically investigated coupled-line section has been used for the design of a 3-dB differential directional coupler. The measurement results are compared with the theoretical ones.

Monolithic Miniaturized Differentially-Fed Branch-Line Directional Coupler in GaAs Monolithic Technology

In this paper, a design of a miniaturized branch-line directional coupler is presented. The coupler is designed with balanced coupled-line sections, which are electrically shortened by the application of lumped capacitors. To measure the parameters of the coupler, appropriate baluns have been designed. The coupler has been designed in a GaAs PH25 UMS (united monolithic semiconductor) technology with the center frequency of 24 GHz. The measured power split equals 3 dB with the transmission/coupling imbalance not exceeding 0.6 dB. The measured return losses equal 17 dB at the center frequency, whereas the isolation reaches 17 dB. The fabricated coupler‘s size equals 630 um × 487 um, which is 0.19 of the full size of the directional coupler in the chosen technology (1191 um × 1170 um).

A Class-D Tri-Phasing CMOS Power Amplifier With an Extended Marchand-Balun Power Combiner

This article presents a power amplifier (PA) design, which consists of eight class-D PA units on a single 28-nm CMOS die and a coupled-line power combiner on printed circuit board. The PA utilizes tri-phasing modulation, which combines polar and outphasing components in a way that eliminates linearity-degrading effects of multilevel outphasing while maintaining the back off efficiency.Each PA unit contains a cascoded output stage with a 3.6-V supply voltage, and multilevel operation is enabled by ON/OFF logic circuitry. Our analysis shows that the choice of power-combiner type is vital for reducing PA supply and ground ripple and thus ensuring reliable operation.Accordingly, the power combiner is implemented with extended Marchand baluns, which consist of input transmission lines and coupled-line sections. Unlike the original Marchand balun, our new topology is feasible for implementation under the layout restrictions caused by the multiple-unit PA on a single die.Measurement results show the PA achieving a peak output power of 29.7 dBm with a 34.7% efficiency, and operation with aggregated Long Term Evolution (LTE) signals at 1.7-GHz carrier frequency is verified with bandwidths up to 100 MHz.

Miniaturisation of branch line couplers with a compact transmission line topology based on coupled line section

This study presents a compact transmission line topology to reduce the circuit size of single- and two-section branch line couplers (BLCs). The proposed transmission line is composed of a high impedance series line and one coupled line section. The coupled line section consists of two high impedance series lines and one coupled line. The proposed line is analysed with even-odd mode analysis to derive closed-form design equations. To validate the equations, a compact single- and two-section BLCs are designed at 0.85 GHz based on the proposed line. The physical size occupied by the proposed single- and two-section BLCs is 23 and 19.5%, respectively, compared to conventional BLCs. Moreover, the proposed transmission line demonstrates the harmonic suppression characteristic due to the coupled line section. As a result of which, the second- and third-order harmonic of the proposed BLCs are suppressed. The proposed BLCs are designed, fabricated, and tested.

Multiple design approach of dual‐band Wilkinson power divider with arbitrary transmission line ratio

A dual-band (DB) Wilkinson power divider with multiple design approach is proposed in this article, which consists of two-section transmission lines (TLs) with arbitrary length ratio, one parallel LC circuit, and one resistor. Compared with the former works with equal physical lengths, the total physical size of the two-section TLs can be decreased effectively. For a given DB frequency ratio, the maximum size reduction can be newly summarized as (n-1)/(2n), where n indicates the length ratio of two-section TLs. From the close-formed design equations, multiple solutions of circuit parameters are newly summarized for DB operation, thus circuit design could be much more flexible and more efficient. Furthermore, in order to compact the proposed circuit size, compensation technology for two coupled-line sections is also considered in circuit fabrication. Finally, design charts and an experimental circuit show good agreement with the theoretical simulation. Compared with the former work under the same design conditions: f2/f1 = 3.2, f1 = 1 GHz and f2 = 3.2 GHz, the proposed work provides compact circuit size and the size reduction is 25% with n = 2, m = 1.

Simple Coupled-Line Tunable Bandpass Filter With Wide Tuning Range

In this paper, a tunable bandpass filter using coupled line with a wide tuning range is proposed. The proposed filter has a simple structure which is composed of a coupled-line section and two pairs of varactors. Meanwhile, simultaneously tuning on bandwidth and center frequency are introduced by varactors. The tunability is mainly realized by varactors connected to one end of the coupled line and varactors between ports and coupled line provide an impedance matching. A theoretical analysis is presented to illustrate the tunability by using even- and odd-mode theory. Additionally, the detailed effect of the two types of varactors is researched, respectively. To verify the concept, a prototype is designed, fabricated, and measured. The transmission zero exhibited in measurement is introduced by bias circuits. The measured results show that the tunable bandpass filter achieves a tunable center frequency of 0.494-1.257 GHz (87.15%) and a 3-dB bandwidth of 0.282-0.943 GHz (334%).

A Novel Compact Isolation Circuit Suitable for Ultracompact and Wideband Marchand Baluns

A novel compact isolation circuit (IC) is proposed for the ultracompact and wideband UHF Marchand balun topology, and exact design formulas are presented for arbitrary coupling coefficients and impedance-transforming termination impedances. The wideband Marchand balun mainly consists of two identical coupled transmission-line sections which are far shorter than 90° long, connecting segment, two or three lumped elements and the proposed IC. The novel IC features −180° phase delay between two balanced ports, and makes the ultracompact implementation possible, which is quite different from the conventional ones with 180° phase delays between two balanced ports. In addition, due to the −180° phase delays, excellent isolation performance and excellent phase responses can be performed. For the proof-of-concept demonstrations, one prototype with each coupled-line section being only 20° long and the half IC being only 20° long is tested at 700 MHz. The measured frequency responses are in good agreement with the predicted ones with the performance of the 10-dB return loss bandwidth of 40% and −15 dB isolation bandwidth of 121.4%. In terms of sizes versus bandwidths, the fabricated Marchand balun can be regarded as the best and the smallest ever recorded, based on the low-cost microstrip technology.

Miniaturized Single-Ended-to-Balanced Arbitrary Four-Section Coupled-Line Coupler With Inherent Impedance Matching

In this brief, a single-ended-to-balanced arbitrary four-section coupled-line coupler with inherent impedance matching is proposed. The proposed coupler consists of four coupled-line sections which achieves a compact circuit layout. Besides, the even- and odd-mode equivalent circuits combining with the standard and mixed-mode ${S}$ -parameters are adopted to derive the analytical equations. For demonstration, two prototypes operating at 2 GHz with different power division ratios ( ${k}=1$ and 3) and different input/output impedances of 50/ $50~{\Omega }$ and 50/ $100~{\Omega }$ , have been fabricated and measured. The sizes of two prototypes are 0.21 ${\lambda }_{\mathrm{ g}}^{2}$ and 0.14 ${\lambda }_{\mathrm{ g}}^{2}$ . A good agreement between the simulated and measured results is observed, which demonstrates the validity of the proposed design method and the circuit model.

A Coupled Line-Based Coupler With Simultaneously Tunable Phase and Frequency

The tunability in phase characteristics becomes more and more important in the modern wireless communication system. However, the existing phase tunable coupler structures suffer from the narrow bandwidth and limited tunable range due to the much more complex design requirements. The further deterioration in performance occurs when additional tunability in operating frequency is considered. In this paper, a new simultaneously phase- and frequency- tunable coupler is proposed with the enhancement in both bandwidth and phase tunable range. Two tunable capacitors are loaded on the two terminals of a coupled line section to realize a novel tunable transmission line. This tunable element is introduced between two quarter-wavelength coupled line sections to implement the simultaneous tunability in both phase and operating frequency. For demonstration purpose, a highly flexible 3 dB coupler was designed, fabricated and measured. For a fixed operating frequency of 2.4 GHz, the coupler can provide a continuously tunable phase difference from 30° to 150° within a 10% bandwidth. For a standard phase difference of 90°, the operating frequency of the same coupler can be varied from 2.0 GHz to 2.75 GHz. Good performance in return loss, isolation, and output amplitude imbalance is achieved for all these states.

Self-Packaged Balanced Bandpass Filters With Impedance Transformation Characteristic

In this article, the co-design of novel self-packaged balanced bandpass filters (BPFs) integrated with the functionality of impedance transformation is presented for the first time. The proposed design is based on a modified branch-line structure, which is composed of two different branch lines and two identical coupled-line sections. Analysis results show that the considered filters can not only achieve Chebyshev equal-ripple responses with impedance transformation properties for the differential mode (DM) but also can efficiently improve the bandwidth of the common-mode (CM) rejection with resorting to this modified branch-line structure. Furthermore, according to the specified equal-ripple fractional bandwidth (FBW), in-band return loss (RL) level, and impedance transformation ratio ${m}$ , the DM frequency response can be directly determined. Two design prototypes, the second-order balanced BPF with a 50–75- $\Omega $ ( $m= 1.5$ ) impedance transformation and the third-order one with a 50–200- $\Omega $ ( $m= 4$ ) impedance transformation, are implemented in self-packaged forms by using the multilayer liquid crystal polymer (LCP)-bonded printed circuit board (PCB) technology. Theoretical, simulated, and measured results are recorded in good agreement, well verifying the design concept.

High‐performance differentially fed coupled‐line directional couplers realised in inhomogeneous medium

In this study, a generalised approach for the design of high-performance differentially fed coupled-line directional couplers in inhomogeneous medium is presented for the first time. Conditions for the realisation of ideal coupled-line section under differential excitation are provided and it is shown that generally in such medium couplers feature different capacitive and inductive coupling coefficients, which significantly deteriorate circuits' performance. Therefore, the compensation technique is proposed allowing for equalisation of the differential coupling coefficients by adding appropriate compensating elements to the per-unit-length capacitance and inductance matrices of the coupled-line section. The proposed approach allows for realisation of quasi-ideal differential coupled-line couplers designed in arbitrarily chosen dielectric stack-up, and almost for arbitrarily chosen coupling. The provided theoretical analysis and resulting design methodology are verified experimentally with a 3-dB differential coupler operating at f 0 = 1.5 GHz and realised in suspended microstrip technique. Moreover, a combination of laminate and additive manufacturing technologies was used for higher design flexibility and low-cost manufacturing.

Multi-Functional Balanced-to-Unbalanced Filtering Power Dividers With Extended Upper Stopband

Two new types of multi-functional RF wide-band differential-mode-to-single-ended filtering power dividers are reported in this brief. Their first configurations exploit half-wavelength ring resonators and coupled-line sections to attain common-mode suppression throughout a broad spectral range. Furthermore, asymmetrical coupled-line stages are incorporated in more-advanced schemes to enlarge upper-stopband bandwidth. Two demonstrative microstrip prototypes for the different structures of this RF tri-functional circuit concept that operate at the 1.7–2.7-GHz 4/5-generation long-term-evolution band are built and tested.

Microwave system with sensor utilizing GO-based gas-sensitive layer and its application to acetone detection

A novel microwave-based gas sensor realized as a coupled-line section covered with a graphene oxide (GO) film as a gas-sensitive layer is proposed and verified. It is shown that the developed sensor’s reflection coefficient, measured at 2.4 GHz with the aid of a dedicated five-port reflectometer featuring decreased measurement uncertainty, can be used for estimating the gas concentration with a high level of accuracy. The developed microwave-based sensors were tested at room temperature under exposure to various volatile organic compounds: ethanol, acetone, methanol and isopropanol in the 0–200 ppm concentration range, with the highest response for acetone. Moreover, the simple circuitry of the proposed system together with its high performance enables its application in portable devices.

Highly Reconfigurable Dual-Band Coupler With Independently Tunable Operating Frequencies

To support different standards simultaneously within a wireless communication system, research on dual-/multi-band components has been conducted extensively. However, the operating frequencies of existing works are usually fixed resulting in a low flexibility. To solve this problem, a highly reconfigurable coupler whose two operating frequencies can be controlled independently is proposed for the first time in this paper. Based on the analysis of a basic configuration, the conditions for realizing the desired independent frequency tuning can be obtained. To realize this unique property, varactors loaded coupled line sections are introduced on a conventional branch line coupler. Owing to the symmetrical structure of the proposed coupler, explicit theoretical analysis can be conducted. For demonstration, two reconfigurable couplers are designed, fabricated, and measured. The first coupler can provide frequency tunability from 3.4 to 4.2 GHz at the upper band with the lower band fixed at 2.4 GHz. The other one can not only provide a tunable frequency from 1.3 to 1.6 GHz at the lower band with the higher band fixed at 3.4 GHz, but also give a tunable higher band from 2.9 to 3.4 GHz with the lower band fixed at 1.55 GHz. The measured results agree well with the simulated results, validating the proposed design methodologies.

Wideband Filtering Phase Shifter Using Transversal Signal-Interference Techniques

A wideband filtering phase shifter is presented in this letter using transversal signal-interference techniques. The proposed structure creates two signal-propagation paths by introducing extra coupling between two short-ended stubs. Cascaded coupled-line sections and parallel short-ended stubs can generate a constant passband and multiple transmission zeros (TZs) in the stopband and meanwhile provide the arbitrary value of phase shift within the filtering band range. The positions of TZs and phase shift range can be easily controlled. Explicit relations between the objective filtering band and the related parameters are given, and cases with different in-band phase shift values are studied. To validate the design, a prototype is built, simulated, and tested using microstrip lines. The experimental results agree with the predicted ones, demonstrating that the device can realize any given in-band phase shift with a wideband filtering response.

Pseudo-elliptic narrow bandpass filter using shorted coupled-line of higher order design

This paper presents an implementation of quarter wavelength single-shorted coupled-lines for narrow bandpass filter application. It is shown as a new way of creating a single resonance bandpass filter by inter-connected of two <br /> single-shorted quarter wavelength coupled-line sections. By adding more single-shorted coupled-line into the configuration, the form of halfwavelength resonator can increase the degree of order of the filter. For the design of 4<sup>th</sup> order resonator, the coupledlines are arranged inter-connected to each other forming five-fingers lines layout. Due to the interconnection of the coupledlines, transmission zeros appear at the two stopbands which improves the selectivity of the filter response. Investigation on the parametric of the 4<sup>th</sup> order resonator is conducted to observe the controlling parameters and it’s realiability responses of the resonator. For compactness, five-fingers meandered lines is proposed. It is found that the size of the meandered lines resonator was successfully reduced by 33% compared to the five-fingers straight lines resonator of the same order. For validation of concept, the 4<sup>th</sup> order meandered lines resonator was designed at 1 GHz and fabricated on RO3210 microstrip substrate with characteristics given as h = 1.27 mm, Ɛr = 10.2 and tan δ = 3x10<sup>-3</sup>. The measurement results show good agreement with the simulation results.

Design of a wideband filtering power divider with good in‐band and out‐of‐band isolations

In this article, a wideband filtering power divider with good in-band and out-of-band isolations is designed based on a hybrid Wilkinson and Gysel structure. To achieve a good in-band response, two additional in-band transmission poles can be introduced by installing the coupled-line structures at each port. By mounting a stepped impedance open stub at the input port, two transmission zeros are generated near the passband to improve the passband skirt. Furthermore, the out-of-band rejection and isolation are achieved by the other two transmission zeros, which are produced by the open stub and the three coupled-line sections mentioned above. Additionally, a good in-band isolation is realized by the isolation resistor between output ports. For the demonstration, a wideband filtering power divider centered at 1.5 GHz with a 56% fractional bandwidth and 20-dB isolation is designed and fabricated. The simulated and measured results are in good agreement with each other.

Response Diversity of Stub-Loaded Ring Bandpass Filter Based on Commensurate Line Element: Single- and Dual-Band Applications

Response diversity of a commensurate line bandpass filter is described and discussed. It is found that a stub-loaded ring topology can generate different bandpass responses in a single band or two symmetric bands, depending on the line admittances in the topology. For a single band, the response can be designed to be an equiripple delay, Chebyshev in magnitude, or elliptic in magnitude. For dual bands, the out-band attenuation curve can be adjusted with almost unchanged in-band matching. A generalized synthesis algorithm, supporting both the single- and dual- band applications, is given to calculate the admittance of each line element, according to the prescribed in-band specifications, e.g., ripple factor and cut-off frequencies. Further study reveals that the admittance of the line segment between the stubs is the key element dominating the response type in the single-band case. When the admittance is positive, zero, and negative, the response is an equiripple delay, Chebyshev in magnitude, and elliptic in magnitude, respectively. To validate the synthesis algorithm, four filter sets with different responses are given and simulated through a transmission line model. For circuit verification, a microstrip filter with a single-band elliptic response is designed, fabricated, and measured, of which the negative admittance line is realized by a coupled line section with shorted ends. The experimental results show that the filter achieves elliptic response successfully. The measured 3-dB fractional bandwidth is 86.6% centered at 0.97 GHz, within which the minimal insertion loss is 0.88 dB at 0.81 GHz. Furthermore, two transmission zeroes beside the passband are observed clearly at 0.31 and 1.59 GHz, offering a sharp rejection in the stopband.

Compact and Wideband General Coupled-Line Ring Hybrids (GCRHs) for Arbitrary Circumferences and Arbitrary Power-Division Ratios

Coupled-line ring hybrids (CRHs) are presented for arbitrary circumferences and for arbitrary power-division ratios. They each consist of three single transmission-line sections and a set of coupled transmission-line sections with or without two identical capacitances and contain most properties of the conventional ring hybrids. Therefore, they may be called general CRHs and can be designed for the frequency performance better than that of any other conventional one. By adopting the coupled-line sections, the 180° phase shift can be guaranteed, regardless of fabrication technologies, but three or four unknown variables more should be considered additionally, which results in a complicated derivation process. To make it simpler, two conditions of $S_{11}^{e}=S_{22}^{o}$ and $S_{22}^{e}=S_{11}^{o}$ , where $S_{11}$ and $S_{22}$ are scattering parameters, and the superscripts of ${e}$ and ${o}$ are meant as even- and odd-mode excitations, respectively, are newly employed. To verify the suggested theory, two prototypes were tested for the power-division ratios of 0 and −5 dB without and with capacitances, respectively. The measured results are in good agreement with the predicted ones, and the measured bandwidths with 15-dB return loss are 82.9 % and 97.65 % for the power-division ratios of 0 and −5 dB, respectively.

Single-Ended-to-Balanced Filtering Power Dividers With Wideband Common-Mode Suppression

Four novel architectures of wideband five-port single-ended-to-balanced power dividers with broadband common-mode suppression are proposed in this paper. The bandpass filtering capability with/without close-to-passband transmission zeros are also incorporated in some of them to obtain multifunctional single-ended-to-differential-mode filtering/power division operation. Two ideal 180° phase inverters are used to replace the conventional half-wavelength transmission line-based inverters in the basic power divider structure so that wideband common-mode rejection and a smaller circuit size are achieved. Quarter-wavelength transmission lines and coupled-line sections are exploited to simultaneously realize the bandpass filtering and wideband power division functionalities. Each five-port power divider exhibits broadband 3-dB single-ended-to-balanced power division, high in-band input–output power-matching levels, high in-band power isolation levels between the two balanced outputs and wideband common-mode rejection. The theoretical analysis, parametric study, and design procedure for the conceived single-ended-to-differential-mode power dividers are provided. Furthermore, for experimental and demonstration purposes, four microstrip prototypes designed at 1 GHz are manufactured and characterized.