Low RF Power Harvesting Enabled Wireless Sensor Node With Long-Distance Communication Capability

We have studied the influence of radio frequency (RF) power on material properties of hydrogenated amorphous carbon (a-C:H) films prepared by RF plasma-enhanced chemical vapor deposition (CVD). The RF power has a significant impact on film properties such as electron spin resonance (ESR) spin density, the optical band gap, chemical bonding and photoluminescence (PL). Raman spectroscopy, infrared (IR) absorption and ESR measurements reveal that the residual defect density (ESR spin density) increases with increasing RF power due to the increase of sp2 C–Hx components. We obtained an ESR spin density as small as 8 ×1016 spins/cm3 at a low RF power, which is comparable to that of high-quality amorphous silicon films. Plasma optical emission spectroscopy indicates that the intensity ratio (IC–H/IH) between hydrogen peaks (IH) and hydrocarbon peaks (IC–H) significantly decreases with increasing RF power. The optical band gap, obtained from UV-visible spectroscopy, varies from 3.0 to 1.9 eV with increasing RF power. The broad emission band centered around 2.4 eV was observed in the PL spectra of the samples deposited at lower RF powers. We found a 0.5–0.3 eV Stokes shift by comparing the PL emission and the absorption coefficient spectrum from the films deposited at ≤100 W RF powers, whereas, we failed to observe a clear Stokes shift from the films deposited at >100 W RF powers. A weak correlation between the PL peak and the optical band gap is demonstrated.

Harvesting of the ambient radio-frequency (RF) energy is of great interest to address the increasing energy demand for emerging applications, e.g., the internet of things, wearable devices, various sensors and implants, and 3D integrated circuits. To use the weak ambient RF power density for such applications, we need rectifiers capable of operating much below 1 μW RF power with high efficiencies. However, the development of such technologies is severely limited by the conventional semiconductor rectifiers [1], which fail to operate in the weak RF limit. We propose a new rectifier concept [2], simultaneously utilizing the spin-orbit torque (SOT) and the Hall effect, that can provide ~200 μV DC voltage from a 500 nW of radio-frequency (RF) power using existing materials, with a power conversion efficiency as high as 71%. The DC voltage strength can be efficiently enhanced to ~300 mV from the same RF power with a series array of such devices while matching the low impedance of the receiver antenna. We estimate a very high curvature coefficient of the proposed rectifier, in the order of 104 V-1, where the theoretical limit of conventional Schottky diodes is determined by the inverse of the thermal voltage q/kBT ~ 40 V-1 [3].Various SOT materials are being extensively studied for efficient current-induced switching of scaled ferromagnets for memory applications (see, e.g., Ref. [4] and references therein). Here, we propose a new application of SOT in the rectification of weak RF by coupling it to conventional Hall devices. The basic idea can be explained using a Hall bar and a solenoid connected in parallel, see figure. A current-carrying conductor placed in a magnetic field (B) exhibits a voltage drop in the direction orthogonal to both the current and the B-field, known as the Hall effect. If a fraction of the current flows through a solenoid underneath the Hall bar to generate the B-field, the Hall voltage will be unidirectional irrespective of the current direction, leading to a rectification. We propose a structure where such a solenoid is replaced with a bilayer consisting of a material exhibiting SOT and a ferromagnet (FM) with a low anisotropy energy barrier. We design the device such that the current equally divide between the Hall and the SOT layers. The FM magnetization, on average, follows the fraction of the current flowing in the SOT layer, provided that the current can produce sufficiently strong SOT. The FM applies an effective B-field to the Hall layer and thus leads to a similar rectification case as described in the solenoid case. The Hall effect and spin-orbit-torque are both proportional to current density, which improves inversely with device cross-sectional area, providing the largest signals at the nanoscale.To make the proposed rectifier sensitive to weak RF current, we need a material with a high SOT factor, an FM with low anisotropy energy, and a low Gilbert damping constant. An FM with low anisotropy energy is typically achieved by lowering the total magnetic moment or by tuning the FM thickness to optimize near the transition point between in-plane and perpendicular anisotropies or by using isotropic geometries. We consider a soft ferrite with a very low anisotropy field and a low total magnetic moment, that is expected to easily follow a SOT induced by a weak RF current. Such a low anisotropy energy magnets can achieve a wide frequency bandwidth of operation, depending on the total magnetic moment in the FM and the angular momentum conservation [5]. We simulate the magnetization dynamics using the stochastic-Landau Lifshitz Gilbert equation, which considers the stochastic nature of such low anisotropy energy magnets due to the presence of thermal noise. We can further optimize the device structure and use emerging materials to enhance the figure-of-merits at even lower RF power. The proposed device can lead to important new technologies addressing the alarming energy issues in the era of the internet of things, wearable devices, and densely integrated 3D circuits.This work is supported by the Center for Energy Efficient Electronics Science (E3S), NSF Award 0939514. **

Transition metal dichalcogenide (TMDC) film is a kind of 2D material that has recently attracted attention regarding its physical and electrical properties [1]. In TMDC film, a molybdenum disulfide (MoS2) has an appropriate bandgap and high mobility even in atomic layer thickness, which is expected to be applied for a next-generation gate-all-around (GAA) nanosheet (NS) 3D-stacked FET (3DSFET) [2, 3]. In addition, due to its flexibility and transparency, the MoS2 film is also promising as a human interface device [4]. Furthermore, the sputtering method is expected to be used for the industrial application of MoS2 film formation because of its affinity with conventional semiconductor processes and suitability for large-area deposition [5–7]. In this study, the relationship between plasma control in sputtering and MoS2 film quality has been investigated.A SiO2/Si substrate was used as the base substrate. After SPM cleaning (H2O2 : H2SO4 =1 : 4), MoS2 films were deposited by radio frequency (RF) magnetron sputtering systems, as shown in Figure 1 (a). The sputtering parameters were substrate temperature of 300oC, Ar pressure of 0.4 Pa, Ar flow rate of 7 sccm and target-to-substrate distance of 150 mm [8]. To improve the MoS2 crystallinity, annealing was performed in a sulfur vapor atmosphere (S-annealing), as shown in Figure 1 (b). The sulfur powder was placed in zone 1 heated at 250oC, and the sample in zone 2 heated at 700oC for 40 min [8]. After sample preparation, MoS2 films before and after S-annealing were evaluated by high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) and in-plane X-ray diffraction (XRD).Figure 2 shows the HAADF-STEM images of the 1ML-MoS2 film after sputtering and S-annealing. The MoS2 crystal structure can be seen in both samples. In addition, the fast Fourier transform (FFT) image after S-annealing shows the bright diffraction pattern derived from MoS2 crystal, which confirms that the crystallinity is recovered by S-annealing. Furthermore, it was also confirmed that the multilayer-MoS2 films are deposited, as shown in Figure 3.Although S-annealing improves the quality of MoS2 films, further improvement is needed to enhance the device performance for industrial applications. Therefore, the dependence of MoS2 film quality on RF power during sputtering is investigated. Figure 4 shows the grain sizes calculated from in-plane XRD patterns of MoS2 films at RF powers of 25, 50 and 100 W [8, 9]. The MoS2 film grain size is degraded at respectively low and high RF powers, which is considered to be related to the particle energy and number of particles (particle flux) reaching the substrate surface. The MoS2 target is sputtered by argon ions accelerated by the sheath electric field. Therefore, high-energy particles such as recoiled Ar are expected to reach the substrate during deposition. On the other hand at low RF power, the particle energy flux is insufficient for surface migration. In contrast, particle flux increases at high RF power conditions, which shortens the average particle travel length on the substrate surface. Thus, too low or too high RF power will reduce the surface migration of the incident MoSx, which degrades the MoS2 grain size. It indicates that optimization of the particle energy with middle RF power is essential for the high crystallization of MoS2 film. Furthermore, it is confirmed that the grain size after S-annealing is dependent on that after sputtering.In conclusion, the MoS2 grain size was enhanced by optimizing the particle energy and flux with middle RF power. In addition, the film quality after S-annealing depended on that after sputtering. Therefore, controlling the plasma condition during sputtering is important for further improving the final MoS2 film. These results are expected to lead to applications of MoS2 films as 3DSFETs and human interface devices. Acknowledgments This work is partly supported by Assistant Profs. T. Hoshii, T. Kawanago and I. Muneta, and Open Facility Center in Tokyo Tech, Collaborative Research Chair/Division Program founded by Sony Group Corp., MEXT Initiative to Establish Next-generation Novel Integrated Circuits Centers (X-NICS) Grant Number JPJ011438, and JSPS KAKENHI Grant Numbers 20H05880 and 22K04181.

The nonlinearity of power amplifiers is one of the principal reasons degrading the transmission quality in the wireless communication systems comprising the single-carrier frequency-division multiple access (SC-FDMA) utilised for the up-link in the LTE system. In this letter, we analysed and predicted the nonlinearity of SC-FDMA signals in spectrum respect. Owing to the nonlinearity of radio frequency (RF) power amplifiers, an analytic model is established on the basis of a third-order intercept point (), which is fit for weakly nonlinear systems and devices. The research results confirm that the spectrum regrowth with RF power amplifier is specified and measured efficiently. This sort of design inherits the advantages of orthogonal frequency division multiplexing (OFDM) and is of great help for both designing RF power amplifiers and further improving the spectrum regrowth with pre-distortion outside channels, extended to other wireless systems based on SC-FDMA modulation.

Most existing behavioral models of the radio frequency (RF) power amplifiers assume that power amplifiers are memoryless devices. But in broadband communication systems, memory effects of the RF power amplifiers are significantly observed. The traditional memoryless models can no longer accurately depict the input-output relationship of power amplifiers. This paper investigates power amplifiers memory effects and a pre-distortion method is proposed for linearizing RF power amplifiers with memory effects. A valid behavioral model based on the Volterra series is suggested to treat memory effects. Since the characteristics of amplifiers change during transmission time, a recursive least squares algorithm with size- fixed observation matrices is developed to update the parameters of the proposed power amplifiers model. The identification algorithm can decrease computational complexity and data storage space and facilitate real-time online identification. Simultaneously, the pre-distortion model is constructed to simulate. Simulation results show that the proposed method can effectively compensate for the nonlinear distortion and memory effects of RF power amplifiers. The model's accuracy is used to evaluate power amplifiers memory effects by the enormalized mean square error (NMSE). In the end, simulation and evaluation of this pre-distortion system proceeded, and the power spectral density estimation was also applied to calculate the adjacent channel power ratio.

Peripheral electromagnetic energy available around us in the form of solar, wind, vibrational and Radiofrequency (RF) energy has the latent potential to be a substitute for traditional energy resources. RF power scavenging or harvesting is a process wherein the RF energy radiated by sources in the form of electromagnetic fields such as wireless radio, television signals, radio signals and cell phone areacquired and transformed into useful DC voltage via a power generating circuit connected to an antenna that receives the signals. This paper describes the design and operation of low-cost harvester utilizing average ambient RF power levels observed in urban areas. Two ultrahigh frequency (0.3-3GHz) harvesters comprising of an antenna to receive the signal, a network to match the impedances, an ac to dc converter and an energy storage element rectifier were constructed to capture two frequency bands in the frequency spectrum from the two bulky RF sources (GSM900 and Wi-Fi). Prototypes for each band were fabricated and tested for effectiveness.

In this work a design flow for class C radiofrequency (RF) power amplifiers (PA) with on-chip output networks in nanometric technologies is presented. This is a new parasitic-aware method intended to reduce time-consuming iterations which are normally required in fully-integrated designs. Unlike other methods it is based on actual transistors DC characteristics and inductors data both extracted by simulation. Starting from the output power specifications a design space map is generated showing the trade-offs between efficiency and components sizing, thus enabling the selection of the most appropriate design that satisfies the harmonic distortion requirements. As a proof of concept of the proposed method, a design example for an IEEE 802.15.4 2.4 GHz PA in a 90 nm CMOS technology is presented.

Radiofrequency (RF) power source during high power operation of a particle accelerator often exhibit saturation nonlinearity in amplitude and phase for sinusoidal input signal. This leads to an undesirable distorted RF signal as input to the RF cavity, which is the basic building block of an accelerator. Linearization of RF power source is an important necessity for an operational accelerator to improve and simplify RF cavity control performance, in addition to improving efficiency. The RF power source being a final control element of the RF cavity electric field control loop, its linear operation over a wider RF input amplitude range is often sought. Digital predistortion has been one of the preferred approaches for linearizing RF power source. This approach needs the determination of the input-output characteristics of the predistorter from those of an RF power source. Among many approaches, least squares estimation techniques are suitable for the design of digital predistorter. Among them, a recursive least squares estimation is most suitable and is elaborated in this paper. Amplifier nonlinearity and its handling using adaptive least squares estimation is explained, followed by modeling and simulation results. Input-output data acquired from an experimental 200 W, 30 MHz RF amplifier is used as input for simulations to demonstrate linearization capability by predistortion. After building up the basics, a technique for amplitude and phase linearization for data generated from the Saleh model of an RF power amplifier is presented. For an assessment of linearization capability, one needs to measure it. The effect of nonlinearity extent on linearity as a function of the number of model coefficients is analyzed. Experimental data is used to analyze the effect of nonlinearity extent and measurement noise. This paper is a comprehensive coverage of the design and analysis of a linearization scheme and is applicable for any general engineering application. A digital predistortion algorithm needs to be implemented in a control system for the RF cavity. This problem forms an important inverse problem of general nature under the purview of the modern control system community.

In this paper, the current–voltage graphs of discharge in the chamber of capacitive coupled radio frequency (CCRF) at low pressure were presented for Langmuir probe. The Langmuir probe measurements for estimating the electron density and temperature in capacitive coupled discharges at low pressures were presented and the electron temperatures of the Ar–H2 mixture discharge generated at different conditions were reported using the Langmuir probe. The focus of this study is that the CCRF discharge can be determined and explained using the characteristics of plasma by means of Langmuir probe measurements for the different hydrogen rates in Ar–H2 mixture discharge. The measurement results of Langmuir probe gave values around 1015 m−3 for the electron density. The floating potential depended on the electronegative gas amount. It was found that the increase of hydrogen gas amount in the mixture discharge caused the decrease of the floating potential. Also, a decrease in the argon (Ar) metastable with the increase in hydrogen (H2) content was obtained. When the applied radio frequency (RF) power was increased, the thickness and collisionless sheath occurring at lower RF power could transform to thin sheath.

Broadband homonuclear mixing pulses with low radiofrequency power are essential for NMR spectroscopy of proteins and small molecules, especially for emerging applications in high field NMR. We have analytically designed a mixing pulse with high bandwidth-to-power ratio, using our recently developed multi-frame method. Here, we compare the new pulse, NF4 (mixing in the fourth nutating frame), to the best currently available sequence, focusing on the low-power regime. We use simulations and experiments to compare the two pulses' relaxation properties and bandwidth, and demonstrate that NF4 has approximately 1.35 times higher bandwidth, with similar effective relaxation. Therefore, NF4 is a good choice for broadband homonuclear mixing, particularly when the available radiofrequency power is limited.

A high density ‘blue’ mode has been observed when operating the Helicon Double Layer Thruster (HDLT) with xenon. Using a Langmuir probe and a retarding field energy analyser (RFEA), the plasma source and exhaust have been characterized at various radio frequency (RF) powers and operating pressures. When operating at low RF powers, the HDLT prototype is shown to be in a capacitively coupled mode. As the RF power is increased, a discrete mode transition occurs over a small RF power range (at about 625 W at 0.45 mTorr) and the plasma inside the source increases in density significantly and changes to a bright white/blue colour. This high density mode exhibits hysteresis, and radial measurements inside the source reveal a centrally peaked profile that is indicative of a helicon wave-sustained discharge. The quality, or Q, factor of the matching box is determined as a function of RF power and is shown to decrease in the high density mode, consistent with the increase in plasma density observed. The xenon exhaust of the HDLT prototype is investigated axially with the Langmuir probe and the RFEA and is shown to follow a Boltzmann expansion with an electron temperature of about 6 eV.

A theoretical model for non-coherent start-of-frame-delimiter (SFD) detection in IEEE 802.15.4a is presented and closed form solutions for SFD threshold and signal-to-noise ratio (SNR) estimation are derived. The derived results are implemented in an impulse radio ultra wide band (IR-UWB) demonstrator operating in the 6–10.6 GHz UWB band. The demonstrator is designed for low power operation (in the mW range for combined digital base band (DBB) & radio frequency (RF)). The transmitter (TX) RF is duty cycled at pulse level and the receiver (RX) RF is switched to pulse level duty cycling when possible. Moreover, digital TX and RX resources are only switched on when needed. Measurement results are in close agreement with the derived closed form solutions.

In this article, we present a novel decomposed vector rotation (DVR)-based recurrent neural network behavioral model for digital predistortion (DPD) of radio frequency (RF) power amplifiers (PAs) in wideband scenarios. By representing memory terms of DVR with recurrent states and redesigning the piecewise modeling, we propose a novel recurrent DVR scheme. To ensure stable operation and enhanced modeling accuracy, we integrate the recurrent DVR into the gated learning mechanism of the modified Just Another NETwork (JANET). Experimental results confirm that the proposed DVR-JANET model provides much improved linearization performance with significantly reduced model complexity compared with the recent existing models.

In this work a comparison of three different behavioral models for radio frequency (RF) power amplifier (PA) is done. The used models were a Volterra series model, the memory polynomial model (MPM) and the generalized memory polynomial model (GMPM). A special focus is put on their modeling capabilities in the adjacent and alternate band. It was found that the Volterra model gives the best modeling performance but also has a high amount of parameter. The GMPM has a significantly lower amount of parameter yet a comparable modeling performance. The MPM uses only very few parameter but still achieves a good modeling performance. The modeling performance for each model was highest in the inband, is less in the adjacent band and decrease further in the alternate band. It also is highly dependent on the used order of nonlinearity.KeywordsPower Amplifier (PA)behavioral modelingnonlinear filtersnormalized mean square error (NMSE)adjacent channel error power ratio (ACEPR)

A new dynamic behavioral modelling technique, based on support vector regression (SVR), is presented in this paper. As an advanced machine learning technique, the SVR method provides an effective option for behavioral modeling of radio frequency (RF) power amplifiers (PAs) including the nonlinear and memory effects. In this paper, the basic theory of the proposed SVR modelling method is provided, along with details on model implementation of RF PAs. Experimental test example for a LDMOS PA is presented, revealing that the new modelling methodology provides a very efficient and extremely accurate prediction.