Low-frequency Loop Research Articles

The propagation characteristics of electromagnetic waves through plasma in the near-field region of low-frequency loop antenna

A high-speed vehicle flying through the atmosphere between 100 and 20 km may suffer from a “communication blackout.” In this paper, a low frequency system with an on-board loop antenna to receive signals is presented as a potential blackout mitigation method. Because the plasma sheath is in the near-field region of the loop antenna, the traditional scattering matrix method that is developed for the far-field region may overestimate the electromagnetic (EM) wave's attenuation. To estimate the EM wave's attenuation in the near-field region, EM interference (EMI) shielding theory is introduced. Experiments are conducted, and the results verify the EMI shielding theory's effectiveness. Simulations are also conducted with different plasma parameters, and the results obtained show that the EM wave's attenuation in the near-field region is far below than that in the far-field region. The EM wave's attenuation increases with the increase in electron density and decreases with the increase in collision frequency. The higher the frequency, the larger is the EM wave's attenuation. During the entire re-entry phase of a RAM-C module, the EM wave's attenuations are below 10 dB for EM waves with a frequency of 1 MHz and below 1 dB for EM waves with a frequency of 100 kHz. Therefore, the low frequency systems (e.g., Loran-C) may provide a way to transmit some key information to high-speed vehicles even during the communication “blackout” period.

Investigation of the difference between the low frequency limit of the impedance spectrum and the slope of the polarization curve

Analysis of both polarization curve and Electrochemical Impedance Spectroscopy (EIS) provides meaningful information about the electrochemical behavior of Proton Exchange Membrane Fuel Cells (PEMFC). Nevertheless, the interpretation of the electrochemical responses remains complex without the use of physics-based models. This paper investigates the experimentally observed and unexpected difference between the low frequency resistance of a cell and the slope of the polarization curve. A 0D stirred tank model is proposed to analyze the experimental data and a simple relation is obtained linking the two resistances difference with the air stoichiometry. Furthermore, it is shown that, until moderate current densities, the low frequency loop of the impedance spectra can be associated to oxygen transport limitation due to convective oxygen supply along the channel, without having to resort to diffusion limitations. Finally, the equivalent electrical circuit (EEC) corresponding to this model is built as a tool to process results from other tests.

Impact of Inhomogeneous Catalyst Layer Properties on Impedance Spectra of Polymer Electrolyte Membrane Fuel Cells

A physically-based 1D impedance model of the cathode catalyst layer that accounts for inhomogeneous catalyst layer (CL) properties is developed. The impact of locally changing proton conductivity, volumetric double-layer capacity, ORR rate and effective porosity on the characteristics of impedance spectra is analyzed. It was found that a non-constant conductivity and capacity profile within the CL results in a specific fingerprint in the high frequency range of the spectra where at homogeneous conditions a linear 45° branch would be expected. Furthermore, a well pronounced splitting of a high and low frequency loop can be provoked by introducing non-uniformly distributed CL properties. The simulation results show that electrochemical impedance spectroscopy is a valuable tool to investigate the inhomogeneity of the catalyst layer originated by the electrode preparation itself or by degradation processes.

Contribution to digital power factor correction controllers in high intensity discharge lamps electronic ballast applications

Utility voltage fluctuations not compensated by the low-frequency voltage loop of the power factor correction (PFC) stage cause a perceptible variation in the light emitted by lamps, unpleasant for the human eye and known as flicker effect. A novel extension of the digital control for PFC stages analyses the input voltage, detects the fluctuations in the range of human flicker sensitivity and modifies the PFC output voltage controller to avoid their propagation to the DC bus. The purpose of the controller is to make the PFC compatible with any second stage lamp driver, assuring the mitigation of the lamp light variation caused by utility disturbances in consistent with the human eye perception. The controller is implemented in a field programmable gate array. A constant lamp luminance is achieved with this digital controller with no dependence on the next ballast stage.

A proton exchange membrane fuel cell impedance model taking into account convection along the air channel: On the bias between the low frequency limit of the impedance and the slope of the polarization curve

The Nyquist impedance spectra of proton exchange membrane fuel cells fed with air exhibit generally two loops, the low frequency one being ordinarily associated with oxygen diffusion. However, as shown experimentally by Schneider et al. and numerically by Kramer et al., the convective motion of air along the flow field channels results in an additional impedance that contributes significantly to this low frequency loop. In this paper, we derive an analytical expression of a 2D convecto-diffusive impedance taking into account convection along the channel direction, which makes it possible to determine the extent to which the low frequency loop results from diffusion losses in the direction perpendicular to the MEA or from convective phenomena occurring along the channel direction. This work confirms that AC induced sinusoidal variations of the oxygen concentrations amplify significantly along the air channel, especially at low frequency. However, we show in addition that using constant air (and hydrogen) flow rates when performing EIS contributes also significantly to the low frequency loop: since the air flow rate is not oscillating in phase with the current, the oxygen stoichiometry is not constant and the operating conditions in the presence of the AC signal cannot converge to the steady state conditions when the frequency tends to zero. As a consequence, contrary to the common conception, the fuel cell low frequency resistance Rlf cannot be equal to the slope of the polarization curve RDC (in identical operating conditions). The bias between Rlf and RDC is all the more significant since the oxygen stoichiometry is low; locally, it is the most important near the air channel outlet. In order to assess the magnitude of these effects, it is possible to use a non-dimensional number, function of the characteristic times associated with convection along the channel and with diffusion in the direction perpendicular to the membrane.

A low power CMOS 3.3 Gbps continuous-time adaptive equalizer for serial link

This paper describes using a high-speed continuous-time analog adaptive equalizer as the front-end of a receiver for a high-speed serial interface, which is compliant with many serial communication specifications such as USB2.0, PCI-E2.0 and Rapid IO. The low and high frequency loops are merged to decrease the effect of delay between the two paths, in addition, the infinite input impedance facilitates the cascade stages in order to improve the high frequency boosting gain. The implemented circuit architecture could facilitate the wide frequency range from 1 to 3.3 Gbps with different length FR4-PCB traces, which brings as much as 25 dB loss. The replica control circuits are injected to provide a convenient way to regulate common-mode voltage for full differential operation. In addition, AC coupling is adopted to suppress the common input from the forward stage. A prototype chip was fabricated in 0.18-μm 1P6M mixed-signal CMOS technology. The actual area is 0.6 × 0.57 mm2 and the analog equalizer operates up to 3.3 Gbps over FR4-PCB trace with 25 dB loss. The overall power dissipation is approximately 23.4 mW.

The interaction of thiourea and formamidine disulfide in the dissolution of gold in sulfuric acid solutions

The mechanism of gold dissolution in acidic thiourea (Tu) solutions was investigated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV). CV over the potential range of −0.3 to 0.75 V vs. SCE shows peaks for the Tu/FDS (formamidine disulfide) and Au / Au ( Tu ) 2 + redox reactions. An oxidation peak associated with gold dissolution occurs at ϕ ⩾ 0.36 V vs. SCE. This peak is often not observed in the first scan and its presence and magnitude are sensitive to the positive sweep potential limit of the first scan. FDS addition to the solution or adsorbed on the electrode has a strong catalytic effect and makes gold dissolution peak shift to lower potential during the first scan. This catalytic effect is identical to that exerted by scanning to higher potential. Tu/FDS ratio has a significant effect on thiourea stability, and it should be about 10:1 for the minimum thiourea decomposition and maximum gold dissolution rate. EIS results at 0.5−0.7 V vs. SCE with only Tu initially present in solution reveal a high-frequency capacitive loop associated with double layer charging, a medium-frequency inductive loop associated with FDS adsorption, and a low-frequency capacitive loop related to Tu oxidation. It appears that FDS adsorption on the gold surface activates the dissolution of gold and inhibits the oxidation of Tu.

Interpretation of Low Temperature Solid Oxide Fuel Cell Electrochemical Impedance Spectra

Electrochemical impedance spectroscopy was performed on low temperature solid oxide fuel cells with yttria-stabilized zirconia electrolytes and different electrode materials and morphologies. Three loops are seen in a Nyquist plot; the high frequency loop is attributed to the electrolyte and series resistance. The intermediate and low frequency loops are influenced by the material and morphology of both electrodes. To clarify which elementary processes contribute to each loop, kinetic Monte Carlo simulations of a solid oxide fuel cell were performed to calculate the reaction rates for each elementary process. The rates fall into three groupings, allowing the identification of processes with corresponding features in the impedance spectra. Vacancy diffusion processes occur at the highest frequency, agreeing with the usual assignment of the high frequency loop with series resistance. Chemical reactions at the anode have an intermediate frequency, suggesting that the intermediate frequency loop is dominated by anode reactions. Low frequency reactions include electrochemical reactions, chemical reactions at the cathode, and water formation and desorption at the anode. This agrees with the experimental findings of the strong dependence of the low frequency loop on the bias voltage and the dominance of the cathode reactions in the low frequency regime.

Electrochemical characterization of a polybenzimidazole-based high temperature proton exchange membrane unit cell

This work constitutes detailed EIS (Electrochemical Impedance Spectroscopy) measurements on a PBI-based HT-PEM unit cell. By means of EIS the fuel cell is characterized in several modes of operation by varying the current density, temperature and the stoichiometry of the reactant gases. Using Equivalent Circuit (EC) modeling key parameters, such as the membrane resistance, charge transfer resistance and gas transfer resistance are identified, however the physical interpretation of the parameters derived from EC’s are doubtful as discussed in this paper. The EC model proposed, which is a modified Randles circuit, provides a reasonably good fit at all the conditions tested. The measurements reveal that the cell temperature is an important parameter, which influences the cell performance significantly, especially the charge transfer resistance proved to be very temperature dependent. The transport of oxygen to the Oxygen Reduction Reaction (ORR) likewise has a substantial effect on the impedance spectra, results showed that the gas transfer resistance has an exponential-like dependency on the air stoichiometry. Based on the present results and results found in recent publications it is still not clear what exactly causes the distinctive low frequency loop occurring at oxygen starvation. Contrary to the oxygen transport, the transport of hydrogen to the Hydrogen Oxidation Reaction (HOR), in the stoichiometry range investigated in this study, shows no measurable change in the impedance data. Generally, this work is expected to provide a basis for future development of impedance-based fuel cell diagnostic systems for HT-PEM fuel cell.

Interactions of Chloride and 3-Mercapto-1-Propanesulfonic Acid in Acidic Copper Sulfate Electrolyte

The individual roles of and the interactions between two additives, chloride ions and 3-mercapto-1-propanesulfonic acid (MPS), in copper electrochemical deposition from an acidic copper sulfate solution are studied with electrochemical methodologies such as linear sweeping voltammetry (LSV), chronoamperometry, and alternating-current electrochemical impedance spectroscopy (EIS). The LSV results reveal that the current density is not increased by MPS without the presence of chloride in the electrolyte. The EIS results reveal a low-frequency capacitive loop associated with chloride interaction with cupric ions and MPS and a high-frequency capacitive loop related to double-layer charging. A detailed reaction mechanism is proposed, and a mechanistic EIS model based on the reaction mechanism is developed. The EIS model predictions reflect, in a qualitative manner, the features observed in the EIS experimental data.

Oscillations in Gas Channels: II. Unraveling the Characteristics of the Low Frequency Loop in Air-Fed PEFC Impedance Spectra

The effect of oxygen depletion along the cathode flow channels on the local impedance response of a polymer electrolyte fuel cell (PEFC) operated on air and pure hydrogen was investigated using a novel experimental approach. The method combines the use of sectioned electrodes and local ac measurements in PEFCs. The results shown here give experimental proof that oxygen concentration oscillations in the cathode gas channels generated as a consequence of ac current flow evoke the formation of a low frequency capacitive loop in local /air-PEFC impedance spectra in downstream parts of the flow field. The loop shows positive or negative real parts at low frequencies depending on the air stoichiometry. The loop is not observed in the local spectrum when the ac current is applied locally and, consequently, the impact of upstream processes on the local impedance response is excluded. A mechanistic description of the processes occurring during ac measurements in /air-PEFCs was developed and validated by experiments. The results show that processes occurring along the flow channels of a PEFC cannot be excluded from the interpretation of PEFC impedance spectra even at rather high air stoichiometries and are in fact the most important phenomena at low modulation frequencies .

Indirect identification of H ads relaxation on different metals by electrochemical impedance spectroscopy

For the hydrogen evolution reaction, the coverage rate of the electrode surface by adsorbed hydrogen is generally difficult to evidence especially in presence of bubbles. In the present paper, a parallel competing reaction was incorporated to the interface by the addition of ferricyanide to a NaOH solution. This reaction was supposed to take place on the free electrode surface and allowed the relaxation of adsorbed hydrogen to be identified. Models taking into account hydrogen evolution, hydrogen absorption, and ferricyanide reduction were proposed. Three metals were tested: platinum, iron, and palladium, which absorb hydrogen in very different quantities. In these conditions, low frequency loops related to the H ads surface coverage appeared in the electrochemical impedance diagrams, whose properties depend on the tested metal. Very good agreement was found between the experimental data and model predictions.

A CMOS charge preamplifier for silicon drift detectors with on-chip JFET and feedback capacitor

In the framework of the Silicon Drift Detector for Hadronic Atom Research by Timing Application (Siddharta) project, we are developing the CMOS readout electronics for the analog processing of about 200 large-area (1 cm 2) silicon drift detectors (SDDs). The readout electronics have to provide an energy resolution better than 150 eV at 6 keV, when used with SDDs, and high long-term peak stability, in the order of few eVs, with respect to background variations during the experiment. The high-stability requirement imposes the choice of a charge preamplifier configuration as front-end stage. In our case, the peculiarity of the preamplifier is that the input JFET and the feedback capacitor are integrated in the detector chip, separated by the remaining part of the circuit, which is several centimeters away. In this work we describe a CMOS charge preamplifier designed to meet these specific constraints and characterized by the use of a drain feedback technique for the reset of the leakage current and signal charge. To this aim, a low-frequency loop is designed to actively control the drain of the SDD–JFET with full compatibility with the supplies limits of the CMOS technology. Because of the unpredictable value of the feedback capacitor, the circuit foresees the possibility to externally adjust the preamplifier decay time to achieve the correct pole-zero compensation with the fixed zero of the shaping amplifier. The main features of the circuit, as well as the results of the experimental characterization of a first prototype realized in the 0.35 μm CMOS AMS technology, are presented in the paper.

Steady-State and EIS Investigations of Hydrogen Electrodes and Membranes in Polymer Electrolyte Fuel Cells: I. Modeling

Electrochemical impedance spectroscopy (EIS) and steady-state models have been developed for the porous hydrogen electrode with water concentration dependence and water transport in a polymer electrolyte fuel cell membrane. Because the hydrogen electrode performance is influenced by its water content, the hydrogen electrode model was coupled to the membrane model. The EIS model for the hydrogen electrode gave three to four loops in the complex plane plots. The high-frequency semicircle was attributed to the Volmer reaction and the medium-frequency semicircle to the hydrogen adsorption. The additional low-frequency loops were connected to changes in the hydrogen electrode performance with water concentration, due to changes in kinetics or proton conductivity. Those loops appear in a frequency range depending on the water transport in the membrane, changing with , where is the water diffusivity and is the membrane thickness. Modeling of the membrane alone showed that the membrane gives rise to a loop in EIS. The difference between the high- and low-frequency intercepts of the loop is , where the high-frequency intercept is equal to the membrane resistance. The loop appears in the same frequency range as the hydrogen electrode low-frequency loops and thus overlaps.

A charge sensitive preamplifier for high peak stability in spectroscopic measurements at high counting rates

In this paper, a charge preamplifier designed to achieve high stability in spectroscopic measurements at different counting rates is presented. The preamplifier operates with the input JFET and the capacitor directly integrated on the silicon detector, in the specific case a Silicon Drift Detector (SDD). The motivation of the work is to overcome the peak shift, typically in the order of few percents and then comparable with the energy resolution achievable at 6 keV with a SDD, which can be measured when the on-chip JFET is operated in the conventionally used source follower configuration. In the proposed circuit, the SDD anode voltage is stabilized by means of a low-frequency loop which operates according to the drain feedback technique. Moreover, the implemented design, differently from previous designs also based on a continuous reset of the leakage + signal currents, allows to obtain a sufficiently fixed decay time constant of the preamplifier with respect to event rate variations. This feature allows to maintain an optimal pole-zero compensation by the shaping amplifier when the event rate changes relevantly during the measurement. In an experimental characterization of the preamplifier with a SDD, a Mn-Kalpha peak-shift within /spl plusmn/0.03% (i.e., /spl plusmn/2 eV at 6 keV) has been measured changing the rate from few kcounts/s up to 400 kcounts/s.

Simultaneous EIS and in situ microscope observation on a partially blocked electrode application to scale electrodeposition

A novel technique allowed EIS and microscopical observation of the interface between electrode and solution to be coupled in situ. The electrochemical reduction of oxygen was investigated by using this device when the electrode surface was progressively covered by an insulating layer of scale deposit. It was shown that two time domains can be distinguished from the initial time of the experiment corresponding to a bare electrode up to a totally blocked surface. In the first part of the experiment, EIS showed one time constant which characterizes the charge transfer process of water reduction. When the percentage of coverage reached more than 80%, this latter electrochemical contribution to EIS was negligible and the reduction process of oxygen was highlighted by two time constants observed from impedance diagrams. The high frequency response characterized the charge transfer process and the low frequency loop identified the diffusion process of oxygen. An electrochemical model was proposed to explain the shape of the impedance spectra plotted with respect to the time of coverage. The good correlation between the experimental and the fitted model curves led to the change versus time of the different parameters of the equivalent circuit. It notably showed that the double layer capacity was directly proportional to the active surface measured by the microscope observation until the surface was totally covered, according to the expectations in the experimental conditions used here.

Extrapolating Near-Field Emissions of Low-Frequency Loop Transmitters

This paper investigates the emission of fields and field attenuation rates from electrically small transmitting loop antennas in the presence of a general half-space. Considerations are made for the practical effects of antenna loop dimension and source-to-sensor impedance coupling. The resulting field attenuation rates correlate well with open area test site (OATS) measurements. The tables provided may be readily used to extrapolate measured low-frequency emissions to common regulatory limit locations.

A TWO LOOP OPTIMAL CONTROL OF FLEXIBLE DRIVE TRAIN VARIABLE SPEED WIND POWER SYSTEMS

This paper presents a two loop optimal control structure for variable speed fixed pitch horizontal axis wind turbines (HAWT) having flexible drive trains. That structure, formed by a low frequency loop and a high frequency loop, is the result of the frequency separation principle adopted in the wind modelling. The optimality of the whole system is expressed by the trade-off between the energy conversion maximization and the minimization of the total load excitation that induces the drive train's mechanical stress. This optimal problem is treated within a complete linear quadratic stochastic approach, whose effectiveness was proved by numerical simulation.

Steady-state and impedance investigations of anodic dissolution of p-Ge in alkali

The electrodissolution of p-Ge in aqueous NaOH solutions (pH 12–13) has been investigated with steady-state and impedance techniques. In the potential region where the process is under kinetic control, the impedance patterns are characterised by a high frequency capacitive loop, associated with the space charge, largely merged with another due to charge transfer resistance and double layer capacitance, and two low frequency loops changing from inductive to capacitive upon increasing the potential. At larger potential, under mixed control, the same high frequency features are observed, while a convection diffusion impedance dominates the low frequency response. The data relevant to pure kinetic control are interpreted with a simplified kinetic model, based on the classical scheme of Beck and Gerischer [Z. Electrochem., Ber. Bunsenges. Physik. Chem. 63 (1959) 500], neglecting transport effects and assuming only three electrochemical steps: (i) oxidation, involving water molecules, of Ge(0) to dihydroxylated Ge(II) by two holes; (ii) oxidation by one hole of Ge(II)–Ge(III) in a step requiring OH −; (iii) oxidative dissolution of Ge(III) proceeding either by hole capture or electron injection, leaving behind a surface Ge(0) species. Diagrams calculated for the limiting cases of pure depletion layer control reproduce the essential features of the experimental ones.

Electrochemical Impedance Spectroscopy Investigation of Bath Aging in Damascene Process Chemistries

Electrochemical impedance measurements were performed on copper bath solutions during copper deposition and subsequent aging of the bath. The results show a correlation between the low-frequency capacitive loop of the electrochemical impedance and copper deposit roughness. The deposit became smoother as the bath aged and reached an optimal value, after which the deposit became rougher. The optimal surface smoothness correlated with a minimum in the low-frequency capacitive loop diameter of the impedance. The results suggest that electrochemical impedance measurements may be used to monitor changes in the copper bath that affect copper deposit quality and superfilling capability.