RF Transmission Line Research Articles

Arduino-based investigation of transmission lines and impedance matching

Abstract Transmission lines play a crucial role in radio-frequency energy transmission between source and load. These systems connect essential physical phenomena like the formation of standing waves and the attenuation of electromagnetic waves in material media with practical considerations such as impedance matching and signal integrity. Understanding these systems requires knowledge of fundamental concepts like inductance, capacitance, and impedance. The absence of impedance matching between the line and load leads to a modulated amplitude in the resulting signal owing to interference between the injected and reflected signals. Here, we present an experiment utilizing an Arduino platform as a signal generator and a data acquisition interface to analyze a radio frequency transmission line. By employing a few-meter-long coaxial cable as the transmission line and a digital oscilloscope with a high sampling rate (several giga-samples per second), we demonstrate how to determine the reflection coefficient as a function of the load impedance. The reflection coefficient enables determination of the return loss, underscoring the significance of impedance matching for optimal performance among the source, transmission line, and the load.

Anisotropic Electron Mobility and Contact Resistance of β-Ga2O3 Obtained via Radio Frequency Transmission Line Methods on Schottky Devices.

Monoclinic semiconducting β-Ga2O3 has drawn attention, particularly because its thin film could be achieved via mechanical exfoliation from bulk crystals, which is analogous to van der Waals materials' behavior. For the transistor devices with exfoliated β-Ga2O3, the channel direction becomes [010] for in-plane electron transport, which changes to vertical [100] near the source/drain (S/D) contact. Hence, anisotropic transport behavior is certainly worth to study but rarely reported. Here we achieve the vertical [100] direction electron mobility of 4.18 cm2/(V s) from Pt/β-Ga2O3 Schottky diodes with various thickness via radio frequency-transmission line method (RF-TLM), which is recently developed. The specific contact resistivity (ρc) could also be estimated from RF-TLM, to be 4.72 × 10-5 Ω cm2, which is quite similar to the value (5.25 × 10-5 Ω cm2) from conventional TLM proving the validity of RF-TLM. We also fabricate metal-semiconductor field-effect transistors (MESFETs) to study anisotropic transport behavior and contact resistance (RC). RC-free [010] in-plane mobility appears as high as maximum ∼67 cm2/(V s), extracted from total resistance in MESFETs.

High-Precision Localization of Passive Intermodulation Source in Radio Frequency Transmission Lines Based on Dual-Frequency Signals

With the development of communication technology, the interference from passive intermodulation to the communication system has become increasingly severe. Locating the source of passive intermodulation and then repairing or replacing the device experiencing passive intermodulation is a reliable method. Therefore, this paper proposes a dual-frequency signal localization method based on the principle of phase ranging to accurately locate the passive intermodulation sources in RF cables. This method switches the test signal frequency to obtain the phase of passive intermodulation signals at different frequencies to localize the passive intermodulation source. This paper analyzes the principle of the passive intermodulation localization method based on dual-frequency signals, establishes a localization system for experiments, and finally, analyzes the experimental error to propose an optimization strategy. Through experimental verification, this method can stabilize the positioning error of a passive intermodulation source in a transmission line less than 0.06 m. Meanwhile, this method has high positioning accuracy and the advantages of slight computational complexity, fast positioning speed, and only a small number of requirements on hardware resources. It can work in communication scenarios like satellite and base stations, providing a new technical solution for localizing passive intermodulation interference sources in 5G and 6G communication systems.

Design and Fabrication of Stretchable Microwave Transmission Lines Based on a Quasi-Microstrip Structure.

Radio frequency (RF) electronics are vital components of stretchable electronics that require wireless capabilities, ranging from skin-interfaced wearable systems to implantable devices to soft robotics. One of the key challenges in stretchable electronics is achieving near-lossless transmission line technology that can carry high-frequency electrical signals between various RF components. Almost all existing stretchable interconnection strategies only demonstrate direct current or low-frequency electrical properties, limiting their use in high frequencies, especially in the MHz to GHz range. Here, we describe the design and fabrication of a simple stretchable RF transmission line strategy that integrates a quasi-microstrip structure into a stretchable serpentine microscale interconnection. We show the effects of quasi-microstrip structural dimensions on the RF performance based on detailed quantitative analysis and experimentally demonstrate the optimized device capable of carrying RF signals with frequencies of up to 40 GHz with near-lossless characteristics. To show the potential application of our transmission line in stretchable microwave electronics, we designed a single-stage power amplifier system with a gain of 9.8 dB at 9 GHz that fully utilizes our quasi-microstrip transmission line technology.

Numerical evaluation of bandwidth and optical loss in InP-organic hybrid optical modulator with doping optimization

We examine the influence of doping profile optimization on the trade-off relationship between modulation bandwidth and optical loss in an InP-organic hybrid (IOH) optical modulator, comparing it with a Si-organic hybrid (SOH) optical modulator. By incorporating the RF transmission line model, which enables a more precise modulation bandwidth analysis than the RC constant model, we demonstrate that the IOH modulator can achieve a modulation bandwidth of over 500 GHz with a 2 dB loss, capitalizing on the higher electron mobility of InP. In contrast, the SOH modulator cannot attain a 200 GHz modulation bandwidth with acceptable optical loss. Furthermore, we explore the potential for further enhancing the modulation bandwidth of the IOH modulator by shortening its length, making the IOH modulator a promising candidate for future ultra-high-speed optical modulation.

TRIPLE-BAND CIRCULAR PATCH MICROSTRIP ANTENNA FOR WIRELESS COMMUNICATION

In this article, a triple-band patch antenna using a circular patch microstrip antenna is constructed with a low-profile patch antenna using a 1.5 mm thick FR4 substrate (εr= 4.3). As mobile devices like smartphones and laptops become more common, the usage of tiny, triple-band antennas has also become more common. The objective of this research is to design a circular patch antenna with a 3–11 GHz dipole-like radiation pattern, which may be used for industrial, scientific, and medical reasons. For the suggested triple-band antenna design, the slotted radiator with six arches and the reduction of the ground plane are among the components. In addition, the standard 50 Ω RF transmission line stimulates the patch antenna; this transmission line is attached to the microstrip line by an impedance-compliant SMA connection. Data from the experiments are obtained, evaluated, and compared; to the patch's surface current distributions, gain, and radiation patterns. According to the measurements, S11 has an impedance bandwidth in the 5.5 GHz band of less than -10 dB, 6.6 GHz band, and 10.4 GHz band at all frequencies. The superb radiation pattern performance, relatively consistent gain throughout the bands, and practical bandwidths of this antenna make it a good choice for C-band and dual X-band applications.

Three-dimensional modeling and simulation of RF cavity for a transmitter with bandwidth of 40–100 MHz at 100-kilowatt level

A 1.5 MW final power amplifier is to be developed for ion cyclotron resonance heating (ICRH) system for the China fusion engineering testing reactor (CFETR) project. First, the feasibility of driver power amplifier with input matching circuit and output resonator cavity and a newly developed tetrode was cross-verified through three-dimensional (3D) simulations and experiments. A single coaxial cavity in parallel with the tetrode covered the frequency range of 40 to 100 MHz at a 100-kilowatt level. In contrast to the previous method of deducing matching parameters using RF circuit and transmission line theory, the use of 3D transmitter simulations provided more accurate matching parameters of tuning components at different operating frequencies. A new phenomenon of high frequency oscillation between the short strip inductor and the shield cavity was found in the input simulation results, which was subsequently verified by experiments. Specific reasons for dividing the output into two parts of high and low frequency were found through output circuit simulations. One is that the length of short stub decreases with the increase of operating frequency, thus decreasing the frequency resolution if all frequencies were using low frequency port. The other is that the coupling capacitor increases sharply with the increase of frequency due to parasitic effect in the low output. Further, the simulation facilitated the study of parasitic effects on matching parameters caused by strip-inductance, inter electrode capacitances of tetrode, and RF cavity. Thus, facilitating the design of final power amplifier in the future. Finally, the amplifier installed with the new tetrode DB968 achieved the design objectives in the test experiment with 150 and 110 kW power outputs at 48 and 100 MHz, respectively.

Investigation of Electromechanical Reliability and Radio Frequency Performance of a Highly Stretchable Liquid Metal Conductor for Wearable Electronics

Abstract Liquid metal-based gallium conductors exhibit unique physical and electromechanical properties, which make them excellent candidates for the next generation of wearable electronics. In this study, a novel fluid phase-based gallium conductor was stencil printed on thermoplastic polyurethane (TPU) to fabricate a stretchable conductor as well as a stretchable radio frequency (RF) transmission line. The electromechanical reliability of the conductor during high elongation as well as cyclic tension and bend fatigue was evaluated and compared with commercially available stretchable silver-filled polymer paste. The microstructure of the liquid metal conductor and the silver paste was investigated via scanning electron microscopy (SEM) before and after the samples were subjected to high elongation (>100%). Unlike the silver paste, the liquid metal conductor maintained its microstructural integrity while its resistance showed a linear response to changes in length. A cyclic tension fatigue test confirmed the fatigue-free performance of the liquid metal conductor during 8000 stretching cycles at a strain amplitude of 30%. The electromagnetic structure of the RF transmission line was simulated and then compared to the measured data. The measurements for insertion loss showed that U-bending, 90 deg twisting, and 1000 stretching cycles at a strain amplitude of 100% did not have a significant impact on the RF performance. Details of the DC tests and RF measurements, including the microstructural analysis and simulation results, will be discussed in this article.

Waveguide Analysis of Radiofrequency Transmission in Well Tubulars for Electromagnetic Heating of Heavy Oil

This paper presents the numerical modelling and simulation of a 2D vertical well tubular model excited by a coaxial radiofrequency transmission line placed in its annular space for the purpose of downhole RF energy transmission. The study identifies the mechanism of energy losses due to coupling between the transmission line and steel tubing and casing using resonance mode analysis. The design of a left-handed artificial power line is proposed to minimize the losses due to the right-handed properties of the coaxial transmission line. The design utilizes ideal lumped element method to determine the left-handedness of the right-handed coaxial transmission line over the industrial heating radio-frequency band. The model is developed and simulated using COMSOL multi-physics commercial simulation software. The effect of transmission line position within the annular space on the tangential magnetic field and resulting power losses along the tubing is discussed. It is observed that the resulting artificial power line improves RF power transmission over 60 m depths of the vertical oil well within the industrial heating radiofrequency band.

Considerations of aerosol-jet printing for the fabrication of printed hybrid electronic circuits

Additive manufacturing (AM) methods are coming of age and are being used to not only fabricate non-functional prototype parts but also to fabricate high-quality functional parts such as electronic circuits. Direct-write (DW) printing is emerging as one of the more promising AM methods for the fabrication of electronic circuits. Of the various DW methods, aerosol-jet (AJ) printing offers several advantages such as providing ink stream widths as small as 10–20 μm and nozzle-to-build plate stand-off heights of 2–5 mm. These capabilities have enabled AJ printing to be used for fabricating high-quality RF components, transmission lines, and antennas, along with fully additively fabricated passive components such as resistors, capacitors, and inductors. However, in order to fabricate such components, structures with precise geometries need to be printed. Therefore, AJ printing needs to be elevated to a technique that is beyond a simple fabrication method capable of printing individual traces with low enough resistance values to be used as circuit traces. With this in mind, the steps that will enable a more mature AJ printing methodology, namely a careful control of the printed trace width, ink steam deposition rate, ink scale factor, and print speed, are presented and described in detail.

Imaging vibrations of electromechanical few layer graphene resonators with a moving vacuum enclosure

Imaging the vibrations of nanomechanical resonators means measuring their flexural mode shapes from the dependence of their frequency response on in-plane position. Applied to two-dimensional resonators, this technique provides a wealth of information on the mechanical properties of atomically-thin membranes. We present a simple and robust system to image the vibrations of few layer graphene (FLG) resonators at room temperature and in vacuum with an in-plane displacement precision of ≈0.20 μm. It consists of a sturdy vacuum enclosure mounted on a three-axis micropositioning stage and designed for free space optical measurements of vibrations. The system is equipped with ultraflexible radio frequency transmission lines to electrically actuate resonators. With it we characterize the lowest frequency mode of FLG resonators by measuring its frequency response as a function of position on the membrane and by extracting its effective mass. We use the background noise of the undriven vibrational spectrum to calibrate in-plane displacement. Finally, we measure the first three vibration modes of a resonator whose membrane is partially folded and find that folds locally suppress vibrations.

Opportunities and challenges of 2D materials in back-end-of-line interconnect scaling

As the challenges in continued scaling of the integrated circuit technology escalate every generation, there is an urgent need to find viable solutions for both the front-end-of-line (transistors) and the back-end-of-line (interconnects). For the interconnect technology, it is crucial to replace the conventional barrier and liner with much thinner alternatives so that the current driving capability of the interconnects can be maintained or even improved. Due to the inherent atomically thin body thicknesses, 2D materials have recently been proposed and explored as Cu diffusion barrier alternatives. In this Perspective article, a variety of 2D materials that have been studied, ranging from graphene, h-BN, MoS2, WSe2 to TaS2, will be reviewed. Their potentials will be evaluated based on several criteria, including fundamental material properties as well as the feasibility for technology integration. Using TaS2 as an example, we demonstrate a large set of promising properties and point out that there remain challenges in the integration aspects with a few possible solutions waiting for validation. Applications of 2D materials for other functions in Cu interconnects and for different metal types will also be introduced, including electromigration, cobalt interconnects, and radio-frequency transmission lines.

A miniaturized ECR plasma flood gun for wafer charge neutralization.

In modern ion implanters, a plasma flood gun (PFG) is used to neutralize wafer charge during the doping process, preventing the breakdown of floating wafers caused by the space charge accumulation. Typically, there are two kinds of PFGs, namely, dc arc discharge with filament and RF discharge. As a PFG, the filament one has limited lifetime and cannot avoid metallic contamination because of the thermal emitting filament. RF discharge PFG has been developed to solve these problems, including prolonging the source lifetime and avoiding metal pollution. Recently, a 2.45 GHz electron cyclotron resonance (ECR) ion source is also regarded as a potential choice for PFG. However, the dimension of the 2.45 GHz ECR source system including the size of the source itself and its meter's length RF subsidiary limits its application within an ion implanter. At Peking University, a miniaturized 2.45 GHz permanent magnet electron cyclotron resonance plasma flood gun with a coaxial RF transmission line has been built and tested. The dimensions of the ECR source body are Φ60 mm × Φ88 mm with a Φ30 mm × Φ40 mm plasma chamber. Its RF transmission line consists of a 200 W microwave generator, a 30 cm coaxial line, a 7 cm coaxial-to-waveguide transducer, and a microwave window that also serves as a vacuum seal. In continuous wave experiments, the electron extraction currents can be as high as 8.8 mA at an input RF power of 22 W with argon gas. The gas flow is less than 1.0 SCCM for this test.

The Mechanical Effects Influencing on the Design of RF MEMS Switches

Radio-frequency switches manufactured by microelectromechanical systems technology are now widely used in aerospace systems and other mobile installations for various purposes. In these operating conditions, these devices are often exposed to intense mechanical environmental influences that have a strong impact on their operation. These negative effects can lead to unwanted short-circuit or open-circuit in the radio-frequency transmission line or to irreversible damage to structural elements. Such a violation in the operation of radio-frequency microelectromechanical switches leads to errors and improper functioning of the electronic equipment in which they are integrated. Thus, this review is devoted to the analysis of the origin of these negative intense mechanical effects of the environment, their classification, and analysis, as well as a review of methods to reduce or prevent their negative impact on the design of radio-frequency microelectromechanical switches.

RF Signal Multiplexer Embedded Into Multifunctional Composite Structure

This article describes the design and fabrication of an active four-port microwave switching network embedded into a multilayer aerospace composite structure. The multifunctional design is based on three broadband Integrated Device Technology (IDT) F2972 single-pole-double-throw (SP2T) RF switches and also includes a control and power supply circuit. The composite structure has been developed using a preimpregnated structural glass (S-Glass) material (Hexcel HexPly 914E). The conductive tracks of the switch and its ground plane have been fabricated using a nonwoven carbon–nickel–copper (CNC) veil material and a semiflexible conducting silver ink. This article contains an analysis of the electrical properties of both materials at microwave frequencies. The equivalent electrical conductivities of materials were determined using a curve fitting method that was then used to verify the computational modeling of the switching network. The RF switching network was developed based on a hybrid manufacturing approach. The RF transmission lines and a component of the control circuitry were transferred onto uncured pre-preg material using a laser to ablate select regions of the CNC veil material. Postcuring, flexible silver ink was additively printed onto the surface to create the remaining control circuits. The RF performance of the design has been evaluated, and it indicates that the overall performance was consistent with the measured S-parameters of the same circuit fabricated on Rogers 5880 substrate.

Flip chip bonding using ink-jet printing technology

In this paper, a bump-forming method for flip chip bonding using ink-jet printing technology is proposed. A flip chip bonded transmission line using ink-jet printed silver bumps consisting of conductive ink containing silver nanoparticles was fabricated to verify the electrical characteristics after the flip chip bonding process. The transmission line was designed according to microstrip theory for a frequency range of 300 kHz to 3 GHz, with the size of fabricated line being 35 (width) × 8.64 (length) × 0.5 (thickness) mm3. The electrical characteristics of a reference microstrip and the flip chip bonded line were compared with FEM simulation and measurement results. Direct current (DC) resistances of both the reference line and the flip chip bonded line were measured to be 3.1 Ω and 3.2 Ω, respectively. The discrepancy between measured insertion loss and the simulation result was only 0.04 dB at 3 GHz, and the return loss was greater than 15 dB in the measurement frequency range. As a result of the analysis, it was confirmed that the DC resistance of ink-jet printed bumps account for 0.56% of the total DC resistance, and the ink-jet printed bumps hardly affected the radio frequency (RF) characteristics of the RF transmission line at low frequencies. The results demonstrate that ink-jet printed silver bumps can be used for a simple and low-cost flip chip bonding process. We expect that the proposed method can be applied to the packaging of various electronics such as flexible, wearable devices and RF applications.

Fabrication and Characterization of CoFe2O4 and MnFe2O4 Nanomagnetic Thin Films for RF Applications

In order to have magnetic films with better radio frequency (RF) properties that can be utilized in a wide range of microwave devices and circuits, such as frequency selective limiters, tunable filters, and antennas, this paper presents the fundamental studies on ferromagnetic resonance (FMR) of magnetic nanoparticles, the fabrication process of the magnetic nanoparticle films, and the scattering parameters measurement results of the magnetic nanoparticle films on a microstrip RF line. We explore the FMR profiles of various magnetic nanoparticles and their corresponding magnetic susceptibilities. Two novel methods, the layer-by-layer (LbL) process and the solution cast (SC) method, are proposed to deposit cobalt ferrite (CoFe2O4) and manganese ferrite nanoparticles (MnFe2O4) on soda-lime glass substrates to fabricate magnetic films with various thicknesses (2.5– $125~\mu \text{m}$ ). In addition, a 3-D printed RF transmission line integrating the magnetic film is used to test the effect of the magnetic nanoparticles. By comparing the measured group delays of microstrip lines with different films, we can obtain some insights into the relationship of the magnetic film thickness, types of magnetic materials, deposition methods, and the amount of the group delay. Specifically, thicker films result in higher group delay, and the SC method is more efficient in fabricating thicker films than the LbL process. The fill factor of these two methods, however, shows that the LbL process generates higher particle loading in the films, while the SC films have considerably more organic binders inside to maintain them. Given the same film thickness, manganese ferrite nanoparticles bring higher group delay than the cobalt ferrite nanoparticles. This paper proposes new methods of fabricating thicker nanomagnetic films and explores the characterization of different magnetic materials with various film thicknesses. It, therefore, paves the way for fabricating monolithic microwave devices and circuits utilizing nanomagnetic materials.

Design and Analysis of MEMS Shunt Capacitive Switch with Si3N4 Dielectric and Au Beam Material to Improve Actuation Voltage and RF Performance in Consideration With and Without Circular Perforations

This paper presents the design of a MEMS shunt capacitive switch with fixed–fixed beam for high frequency RF applications. The switch dimensions were very carefully chosen to obtain the optimum performance of the switch when actuated with RF transmission line. The designed switch uses two different structures: beam without perforations and beam with circular perforations with same switch dimensions. Spring constant of the switch with perforations shows reduction in requirement of actuation voltage due to significant reduction in spring constant. Actuation voltage analysis was also done for two structures and the actuation voltage for switch with perforations found to be 6.80 V which is 35.23% less compare to switch without perforations (10.5 V). RF characteristics in terms of Insertion loss, return loss and isolation found to be − 0.05 dB, − 43 dB and − 12 dB without perforations and it was − 0.03 dB, − 48 dB and − 15.5 dB for the switch with perforations at 62 GHz frequency. The switch designed with circular perforations shows improvement in insertion loss, return loss and isolation by 20%, 15% and 24% respectively compare to switch without perforations. The switch can be used for V-band applications like satellite communications and RF applications.

Radio Frequency Planar Coil-Based On-Chip Probe for Portable Nuclear Magnetic Resonance

Miniaturization of nuclear magnetic resonance (NMR) systems has seen a tremendous development during the last two decades. However, it has not gained popularity particularly, for biological investigations due to high manufacturing cost, sophisticated tools requirement, and mismatching problems arising due to connecting wire between radio frequency (RF) coil and transmission line. The aim of the research work presented in this paper is to develop cost effective, portable, and benchtop NMR system for biological investigation related to healthcare. In this paper, RF planar spiral coil is developed on double sided printed circuit board (PCB) using conventional PCB technology having high-quality factor of 29.265 at 21.287 MHz. Then, the same coil is used to develop on-chip NMR probe by extending the transmission line. This developed probe is a novel design, which improves the coupling between RF coil and transmission line; and hence, improves the sensitivity of NMR. A pair of permanent magnets having 0.5 T magnetic field strength with rectangular iron yoke has been used in the NMR system. Finally, the efficiency of this portable NMR was tested by recording the NMR spectrum of human blood cells. The spectrum obtained was compared to the commercial NMR (Bruker, ULTRASHIELD −500 PLUS) and found to be similar with remarkably small sample volume requirement. Moreover, in this research, we report a very low-cost coil and probe with novel probe design using CST tool, for the first time to the best of our knowledge.

Design and Implementation of a Compact 3-D Stacked RF Front-End Module for Micro Base Station

In the current 4G Long Term Evolution and the incoming 5G mobile communication technology, as the number of radio frequency (RF) devices in the RF front-end module is increasing fast, miniaturization is essential and significant. The 3-D RF system-in-package (SiP) technology is an excellent miniaturization solution. In this paper, a 700–2900-MHz 3-D stacked RF module with the size of 23 mm $\times$ 23 mm $\times$ 3.3 mm has been designed and implemented, which is used for a micro base station. It is more compact than other reported 3-D RF SiP products used for micro base stations. The 3-D RF module integrates four bare dies, one quad flat no-lead packaged chip, and more than 200 passive devices. Considering the electromagnetic isolation requirement, mechanical performance, and effective interconnection, the detailed structure design is presented in this paper. Simulations are performed for the electrical characterization and the thermal performance evaluation of the stacked module. The simulation results show that the insertion loss ( $S_{21}$ ) and return loss ( $S_{11}$ ) of the RF signal transmission lines are controlled below 0.42 dB and over 15 dB within 3 GHz, respectively. The electromagnetic isolation between different RF and/or intermediate frequency signal transmission lines is no less than 65 dB within 3 GHz. In thermal management simulation, the highest junction temperature of the chips in the 3-D structure is below 94 °C at the severe condition (50 °C ambient temperature and nature air convection). Finally, the assembly flow is presented, and the test results indicate the good electrical performance of the 3-D RF module.