Radio Frequency Input Power Research Articles

Flexible and Wearable Hybrid RF and Solar Energy Harvesting System

In this article, we demonstrate a flexible and wearable hybrid radio frequency (RF) and solar energy harvesting system for powering wearable electronic devices. The system consists of a flexible transparent antenna, a flexible transparent rectifying circuit, and an amorphous silicon solar cell. By utilizing transparent stacked structure, the rectenna and the solar cell can provide more hybrid output power to accommodate more application scenarios. The flexible transparent antenna shows two impedance matching bandwidths of 3.5–3.578 and 4.79–5.09 GHz, covering n78/n79 fifth-generation (5G) communication frequency band and 5-GHz WiFi frequency band. At the frequency of 3.5 GHz, the flexible transparent rectifying circuit achieves a high RF-to-dc conversion efficiency of 54.67% at 13 dBm RF input power. In the room with a light intensity of 210 lux, compared with a single solar cell, the hybrid energy harvesting system can obtain an additional 35.6%–769.5% output power when the RF source power is varied from 4 to 10 dBm. These results demonstrate that the proposed flexible and wearable RF-solar energy harvester can provide reliable electric power supply, which can be a potential candidate for powering wearable electronic devices, distributed sensors and IOT devices.

Linearization Schemes for Radio Over Fiber Systems Based on Machine Learning Algorithms

This work is regarding the concept and implementation of pre- and post-distortion schemes idealized for radio-over-fiber (RoF) systems. Analog RoF systems have been considered potential to increase the capillarity of future mobile networks. However, the integration of the radiofrequency (RF) and optical systems might introduce nonlinearities that increase the in-band and out-band interferences. The proposed schemes employ a multi-layer perceptron (MLP) artificial neural network (ANN) linearization for reducing the signal distortions. We have applied our linearization schemes to orthogonal frequency division multiplexing (OFDM) signals for investigating its performance, in terms of root mean square error vector magnitude (EVM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RMS</sub> ), normalized mean square error (NMSE) and adjacent channel leakage ratio (ACLR). Numerical results demonstrate promising linearization performance, since the EVM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RMS</sub> has been kept as low as 3%, attaining NMSE and ACLR lower than below −30 dB and −35 dB, for input RF power up to 23 dBm.

DEVELOPMENT OF CASCADED VOLTAGE DOUBLER RECTIFIER FOR RF ENERGY HARVESTING

Radio Frequency (RF) energy harvesting is a process where RF energy from the ambient source is collected and converted into an electrical energy by using a rectifier circuit. However, the collected RF energy only supplies very low input power. Therefore, it is important to design a circuit that not only rectified the RF signal, but also with amplified characteristic to obtain a higher output voltage from a low input power. Driven by the increasing use of Internet of Things (IoT) devices operating in the 2.4 GHz Industrial, Scientific, and Medical (ISM) band, the presented rectifier circuit in this paper is designed in the same band as well. Initially, the voltage doubler circuit is chosen as the primary rectifier circuit, afterward cascaded into several stages until the most optimized result is obtained. The optimization is investigated across -30 dBm to 0 dBm of RF input power by varying the value of capacitor and resistor at a single stage. Based on the topology analysis, Dickson topology yields slightly higher voltage compared to Villard. In turn, the optimized number of stages is 6 because higher stages resulted to less output power. The measured reflection coefficient of the fabricated prototype is better than 40 dB at the center frequency with 240 MHz bandwidth. The rectified voltage is 3.4 V with 0 dBm input power. When it is supplied by 5 dBm input power, the green LED that connected to rectifier circuit output is light-up, confirming the RF energy harvesting application.

Design of a Highly Efficient Wideband Multi-Frequency Ambient RF Energy Harvester.

For low input radio frequency (RF) power from −35 to 5 dBm, a novel quad-band RF energy harvester (RFEH) with an improved impedance matching network (IMN) is proposed to overcome the poor conversion efficiency and limited RF power range of the ambient environment. In this research, an RF spectral survey was performed in the semi-urban region of Malaysia, and using these results, a multi-frequency highly sensitive RF energy harvester was designed to harvest energy from available frequency bands within the 0.8 GHz to 2.6 GHz frequency range. Firstly, a new IMN is implemented to improve the rectifying circuit’s efficiency in ambient conditions. Secondly, a self-complementary log-periodic higher bandwidth antenna is proposed. Finally, the design and manufacture of the proposed RF harvester’s prototype are carried out and tested to realize its output in the desired frequency bands. For an accumulative −15 dBm input RF power that is uniformly universal across the four radio frequency bands, the harvester’s calculated dc rectification efficiency is about 35 percent and reaches 52 percent at −20 dBm. Measurement in an ambient RF setting shows that the proposed harvester is able to harvest dc energy at −20 dBm up to 0.678 V.

Low-Temperature and High-Speed Fabrication of Nanocrystalline Ge Films on Cu Substrates Using Sub-Torr-Pressure Plasma Sputtering

We fabricated nanocrystalline Ge films using radio-frequency (RF) magnetron plasma sputtering deposition under a high Ar-gas pressure. The Ge nanograins changed from amorphous to crystalline when the distance between the Ge sputtering target and the substrate was decreased to 5 mm and the RF input power was 11.8 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (60 W), where the deposition rate was as high as 660 nm/min. In addition, the size of the nanocrystalline grains increased from 100 to 307 nm when the RF input power for plasma production was increased from 11.8 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (60 W) to 17.7 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (90 W). In the developed narrow-gap plasma process at sub-Torr pressures, nanocrystalline Ge films were successfully fabricated on Cu substrates at low temperatures, without the substrate being heated. However, when annealing was conducted under an N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> atmosphere, which is the conventional method to induce solid-phase crystallization, the amorphous Ge layer on a Cu substrate changed to a Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Ge crystal layer through interdiffusion of Ge and Cu atoms at 400–500 °C.

Numerical simulation and experimental research on an inductively coupled RF plasma cathode

In this study, numerical simulation and discharge current tests were conducted on an inductively coupled radio frequency (RF) plasma cathode. Numerical simulations and experimental measurements were performed to study the factors influencing the electron extraction characteristics, including the gas type, gas flow, input power and extracting voltage. The simulation results were approximately consistent with the experimental results. We experimentally found that the RF input power mainly determines the extracted electron current. An electron current greater than 1 A was acquired at 270 W (RF input power), 2.766 sccm (xenon gas). Our results prove that an inductively coupled RF plasma cathode can be reasonable and feasible, particularly for low power electric propulsion devices.

Quad-Band Rectenna for Ambient Radio Frequency (RF) Energy Harvesting.

RF power is broadly available in both urban and semi-urban areas and thus exhibits as a promising candidate for ambient energy scavenging sources. In this research, a high-efficiency quad-band rectenna is designed for ambient RF wireless energy scavenging over the frequency range from 0.8 to 2.5 GHz. Firstly, the detailed characteristics (i.e., available frequency bands and associated power density levels) of the ambient RF power are studied and analyzed. The data (i.e., RF survey results) are then applied to aid the design of a new quad-band RF harvester. A newly designed impedance matching network (IMN) with an additional L-network in a third-branch of dual-port rectifier circuit is familiarized to increase the performance and RF-to-DC conversion efficiency of the harvester with comparatively very low input RF power density levels. A dual-polarized multi-frequency bow-tie antenna is designed, which has a wide bandwidth (BW) and is miniature in size. The dual cross planer structure internal triangular shape and co-axial feeding are used to decrease the size and enhance the antenna performance. Consequently, the suggested RF harvester is designed to cover all available frequency bands, including part of most mobile phone and wireless local area network (WLAN) bands in Malaysia, while the optimum resistance value for maximum dc rectification efficiency (up to 48%) is from 1 to 10 kΩ. The measurement result in the ambient environment (i.e., both indoor and outdoor) depicts that the new harvester is able to harvest dc voltage of 124.3 and 191.0 mV, respectively, which can be used for low power sensors and wireless applications.

A Miniaturized Single-Wall Carbon Nanotubes Field Emission Cathode With RF Excited by Coaxial Resonant Cavity

A miniaturized radio frequency (RF) excited grid-free field emission cathode made by single-wall carbon nanotubes (SWCNTs) is suggested and studied. The λ/4 coaxial resonant cavity is used as field-emitted electron source with length of 26.5 mm and diameter of 12.8 mm. Electrons can be pulled out by a small electrostatic field and an RF electric field without interception. The RF electric field of inner conductor surface at the open end, which pasted SWCNTs, can reach 3 MV/m when the input power is 30 W, and the field is larger than the turn-on field of SWCNTs. The results of simulation and experiment both show that the maximum emission current under input RF power of 30 W is 30 times larger than without RF excitation, which can reach 24 mA at the anode voltage of 950 V. This miniaturized RF excited grid-free field emission cathode electron source can be used as electron source, in a high repetition frequency vacuum electronic devices, e.g., terahertz (THz) devices, and X-ray tubes.

Continuum modelling of an asymmetric CCRF argon plasma reactor: Influence of higher excited states and sensitivity to model parameters

AbstractPrecise control of capacitively coupled radiofrequency (CCRF) plasma reactors is required to achieve desired outcomes in surface functionalisation and material synthesis processes. This necessitates detailed mapping of the large process parameter space and a thorough understanding of spatial and temporal variations of the plasma throughout the reactor. These goals can only feasibly be achieved with accurate numerical modelling. Previous numerical studies of CCRF discharges have implemented a range of simplifying assumptions to improve numerical tractability, such as small electrode spacing, radial uniformity, fewer active species and simplified boundary conditions, while neglecting self‐bias formation. Although this approach is useful in developing the methodology for continuum plasma modelling, it poses challenges for direct comparison with experimental data and for understanding the behaviour of plasma processes employed in the surface treatment of large, complex objects, or the synthesis of nanoparticles. Here we report the development of a two‐dimensional axisymmetric continuum model for a CCRF reactor with a pure argon 13.56‐MHz discharge using the finite element method. The large electrode spacing and reactor design result in two distinct discharge regions and the formation of a strong DC self‐bias on the powered electrode. The plasma discharge is studied as the pressure is varied from 0.1 to 0.3 Torr, over the radiofrequency input power range of 25–100 W, which leads to consistent enhancements of the electron density and self‐bias. The impact of the electron energy distribution function (EEDF) on the discharge is assessed, with the assumption of a Druyvesteyn EEDF resulting in a bulk electron density and temperature of 3.4 × 1015 m−3 and 3.3 eV, respectively, compared with 8.1 × 1015 m−3 and 1.9 eV in the Maxwellian case. The asymmetric power distribution throughout the reactor is quantified to build a reduced domain model with a lower computational cost. The effect of an electrically floating parallel plate electrode is assessed, resulting in a 42% higher bulk plasma potential as compared with the grounded case. The inclusion of resonant and 2p excited states of argon is shown to have a major impact on the discharge dynamics, leading to an order of magnitude reduction in bulk electron density. This study proposes a robust numerical model of a CCRF argon plasma discharge to facilitate future simulations of more complex discharges with important implications in plasma surface engineering and synthesis of materials.

A microwave detector based on spoof surface plasmon polaritons structure

In this paper, a Schottky diode detector based on spoof surface plasmon polariton (SPP) structure is reported. The detector consists of an input matching network, a Schottky diode, and an output filtering network. The proposed detector is designed using a step impedance section and λ/4 radial stub integrated in the circuit. Compared with the conventional microwave detector, experimental results show that the proposed detector achieves 31% improvement of detection sensitivity with the high input radio frequency (RF) power and 95% improvement of the detection sensitivity with the low input RF power by the SPPs. The proposed detector is attractive for use in aircraft, vehicles, and wireless communication due to its miniaturization, low-cost, and easy fabrication with planar circuits.

An Energy-Efficient UWB Transmitter with Wireless Injection Locking for RF Energy-Harvesting Sensors.

An ultralow-power ultrawideband (UWB) transmitter with an energy-efficient injection-locked radio frequency (RF) clock harvester that generates a carrier from an RF signal is proposed for RF energy-harvesting Internet-of-Things (IoT) sensor applications. The energy-efficient RF clock harvester based on the injection-locked ring oscillator (ILRO) is proposed to achieve optimal locking range and minimum input sensitivity to obtain an injection-locked 450 MHz clock in ultralow-power operation. A current-starved inverter-based delay stage is adopted that allows delay adjustment by bias voltage to minimize dynamic current consumption while maintaining a constant delay regardless of changes in process, supply voltage, and temperature (PVT). To minimize static current consumption, a UWB transmitter based on a digital-based UWB pulse generator and a pulse-driven switching drive amplifier is proposed. The proposed injection-locked RF clock harvester achieves the best RF input sensitivity of −34 dBm at a power consumption of 2.03 μW, enabling energy-efficient clock harvesting from low RF input power. In ultralow-power operation, a 23.8% locking range is achieved at the RF injection power of −15 dBm to cope with frequency changes due to PVT variations. The proposed UWB transmitter with RF clock harvester achieves the lowest energy consumption per pulse with an average power consumption of 97.03 μW and an energy consumption of 19.41 pJ/pulse, enabling operation with the energy available in RF energy-harvesting applications.

Simple Adaptive Rectifier with High Efficiency over a Range of 21 dBm Input Power for RF Energy Harvesting Applications

This paper presents a novel simple adaptive and efficient rectifier for Radio Frequency (RF) energy harvesting applications. Traditional rectifiers have maximum RF-DC Power Conversion Efficiency (PCE) over a narrow range of RF input power due to diode breakdown voltage restrictions. The proposed adaptive design helps to extend the PCE over a wider range of RF input power at 2.45GHz using a simple design. Two alternative paths arecontrolled depending on the RF input power level. Low input power levels activate the first path connected to a single rectifier; low power levels make the diode operate below its breakdown voltage and therefore avoiding PCE degradation. High input power levels activate the second path dividing it into three rectifiers. This keeps input power at each rectifier at a low power level to avoid exceeding the diode break down voltage. Simulated PCE of this work is kept above 50% over a range of 21.4 dBm input power from -0.8dBm to 20.6dBm.

Quad-Band Multiport Rectenna for RF Energy Harvesting in Ambient Environment

This article proposes a quad-band multiport harvester that can scavenge ambient radio frequency (RF) energy of available frequency bands (i.e. GSM-900, GSM-1800, 3G and Wi-Fi) efficiently. This design's novelty is the use of frequency-dependent multiple antenna ports that enables the harvester to fully exploit all available frequency bands, spatial diversity, and polarization for maximizing the harvested RF energy at a lower power density level in an ambient environment. Indistinctly, the proposed antenna consists of 8-port which maintain 0.33λ×0.33λ (i.e. wavelength λ calculated at 0.9 GHz in free space) average area per port and has achieved more than 35% relative bandwidth (BW) with a maximum gain of 6.5 dBi to cover the selected frequency bands. Besides, the proposed quad-band rectifier with a multi-stub impedance matching network is feasible for RF harvesting over the available ambient frequency bands. The proposed rectenna's measurement results show that the generated average dc output voltage is increased up to 0.750 V at a low RF input power density of -27 dBm for a load resistance of 2.11 kΩ. The overall dc rectification efficiency of the 8-port pixel harvester is also assessed and depicted to be 66.52% when the resultant RF input power density level is -27 dBm. Measurement in an ambient outdoor and controlled indoor environment is also conducted, demonstrating that the new 8-port pixel RF harvester can produce a dc output voltage of 0.797 V which is greater than some of the reference RF harvesters.

Multiband asymmetric radio frequency to direct current converter circuit for wireless power transfer

AbstractThis article presents the multiband asymmetric radio frequency (RF) to direct current (DC) converter circuit for the wireless power transfer (WPT) applications. The proposed circuit is composed of a power divider, novel asymmetric multiband matching circuit and RF Schottky diode quadrupler voltage rectifier circuit. Unlike the conventional narrow band quadrupler using two Greinacher cells with opposite polarities, the proposed circuit aims to cover the multifrequency bands, which are the cell phone 1.8–2.1 GHz, the industrial, scientific, and medical (ISM) bands 2.4–2.5 GHz and 5.725–5.875 GHz. The interested RF input power range of −10 to 20 dBm for the WPT application is focused. The measured RF‐to‐DC conversion efficiency (η) of 50–65% at 1.8–2.1 GHz, 65% at 2.45 GHz and 16% at 5.8 GHz are obtained at 10 dBm optimal input power.

Investigation Into Self Actuation Limitation and Current Carrying Capacity of Chalcogenide Phase Change GeTe-Based RF Switches

This article reports investigation of self-actuation in phase change material (PCM) germanium telluride (GeTe)-based radio frequency (RF) switches as a limitation to RF power handling. PCM switches are fabricated in-house with multiple design dimensions for evaluation and testing under various RF power levels. RF power handling capability and maximum dc current carrying capacity in the ON-state of the switches is studied for four different device layouts. Self-actuation in the OFF-state is reported at varying input RF power and dc voltage levels. A tradeoff between RF power handling capacity and device performance is discussed, establishing a correlation between design parameters and the power handling. The improved PCM switches exhibit a record high continuous wave (CW) RF power handling capability ranging from +35.5 to +39.2 dBm.

Plasma-based isotropic etching polishing of synthetic quartz

Abstract To efficiently eliminate the surface defects and subsurface damage layer of synthetic quartz introduced by grinding, a plasma-based isotropic etching polishing (plasma-IEP) technique is proposed in this study. The smoothing of synthetic quartz by plasma-IEP is attributed to the formation, overlapping and merging of numerous and ultra-smooth etch pits formed by isotropic etching of SiO2 using inductively coupled plasma. Plasma diagnostics have revealed the existence of large amounts of etching radicals. The input radio frequency power and CF4 flow rate have been proved to be the two determinant factors of the material removal rate (MRR) of plasma-IEP. Under the optimized conditions, a maximum MRR of 5.62 μm/min of a 2-inch wafer has been achieved which is much higher than that of the conventional CMP process. After plasma-IEP for 30 min, the Sa roughness of the ground synthetic quartz decreases from 270.6 nm to 17.4 nm and the inner surface of the isotropic etch pits is smooth at the atomic level. The results presented in this paper demonstrate that plasma-IEP is a promising approach for the highly efficient and damage-free semi-finishing of synthetic quartz.

MPPT integrated DC–DC boost converter for RF energy harvester

This study proposes a design of a maximum power point tracking (MPPT)-based DC–DC boost converter for an radio frequency (RF) energy harvester. The MPPT technique has been implemented in the converter using an adaptable load tracking mechanism with variable input RF power to meet the supply voltage tolerance limits. The proposed MPPT scheme is implemented using a sample-and-hold circuit in the tracking loop of the DC–DC boost converter. The complete energy harvester system prototype has been realised in a 130 nm standard complementary metal-oxide-semiconductor (CMOS) process. The footprint of the proposed converter circuit is much smaller compared to similar designs reported in the literature making it applicable for wearable and implantable biomedical applications.

Compact Dual-Band Rectenna Based on Dual-Mode Metal-Rimmed Antenna

This paper proposes the design of a dual-band integrated rectenna. The rectenna has compact size of 0.4 × 0.3 × 0.25 cm3 and operates at 925 MHz and 2450 MHz bands. In general, the rectenna consists of two main parts, the metal-rimmed dual-band antenna used for harvesting the radio frequency (RF) signals from the environment and the rectifier circuit to convert these receiving powers to the direct current (DC). Because of the dual resonant structure of the antenna, the rectifier circuit can be optimized in terms of size and the frequency bandwidth, while the conversion efficiencies are always obtained 60% at the RF input power −2.5 dBm and −1 dBm for the lower band and the higher band, respectively. Measured results show that the metal-rimmed antenna exhibits −10 dB reflection coefficient in both desired frequency bands. Moreover, the antenna achieves 47% and 89% of total efficiency respectively at 925 MHz and 2450 MHz, which confirms that the proposed rectenna is well applicable in most of the miniaturized wireless sensor networks and IoT systems.

Dynamic range improvement of six-port receiver through analysis of output DC offset

In this paper, we improved the error vector magnitude (EVM) performance of six-port receiver using a low noise amplifier (LNA) bias control algorithm based on analyzing the correlation between normalized output DC offset voltage and EVM relative to input radio frequency (RF) power variation. In contrast to the other works published in the literature, we consider test setup through which the software impairments of six-port receiver blocks are considered. Therefore, in this work to substantiate the EVM improvement, a designed 26–30 GHz CMOS six-port receiver is used which consists of LNA, coupled line bandpass filter, six-port correlator, diode power detectors and two instrumentation amplifiers for recovering the baseband I/Q data. The EVM improvement process is assessed in terms of 64QAM modulation, and according to obtained results, EVM is reduced from 61% when using constant DC bias to 22% achieved with an LNA bias control algorithm for low input power (− 60 dBm) at the center frequency of 28 GHz. Moreover, the EVM is become less sensitive to input RF power, and also the dynamic range is improved more than 20 dB.

Tunable Matching Networks Based on Phase-Switched Impedance Modulation1

The ability to provide accurate, rapid, and dynamically controlled impedance matching offers significant advantages to a wide range of present and emerging radio-frequency (RF) power applications. This article develops a new type of tunable matching network (TMN) that enables a combination of much faster and more accurate impedance matching than is available with conventional techniques and is suitable for use at high power levels. This implementation is based on a narrow-band technique, termed here phase-switched impedance modulation (PSIM), which entails the switching of passive elements at the RF operating frequency, effectively modulating their impedances. The proposed approach provides absorption of device parasitics and zero-voltage switching (ZVS) of the active devices, and we introduce control techniques that enable ZVS operation to be maintained across operating conditions. A prototype PSIM-based TMN is developed that provides a 50-Ω match over a load impedance range suitable for inductively coupled plasma processes. The prototype TMN operates at frequencies centered around 13.56 MHz at input RF power levels of up to 200 W.