Low-frequency Distortion Research Articles

Lightning Current Measurement Method Using Rogowski Coil Based on Integral Circuit with Low-Frequency Attenuation Feedback.

A lightning current measurement method using a Rogowski coil based on an integral circuit with low-frequency attenuation feedback was proposed to address the issue of low-frequency distortion in the measurement of lightning currents on transmission lines using Rogowski coils. Firstly, the causes of low-frequency distortion in lightning current measurements using Rogowski coils were analyzed from the perspective of frequency domains. On this basis, an integration correction optimization circuit with a low-frequency attenuation feedback network was designed to correct the low-frequency distortion. The optimized integration circuit can also reduce the impact of low-frequency noise and the DC bias of the operational amplifier (op-amp) on the integration circuit due to the high low-frequency gain. Additionally, a high-pass filtering and voltage-divided sampling circuit has been added to ensure the normal operation of the integrator and improve the measurement range of the measurement system. Then, according to the relationship between the amplitude-frequency characteristics of the measurement system and the parameters of each component, the appropriate types of components and op-amp were selected to expand the measurement bandwidth. Finally, a simulation verification was conducted, and the simulation results show that this measurement method can effectively expand the lower measurement frequency limit to 20 Hz, correct the low-frequency distortion caused by Rogowski coils measuring lightning currents on transmission lines, and accurately restore the measured lightning current waveform.

Enhancing Contrast of Spatial Details in X-ray Phase-Contrast Imaging through Modified Fourier Filtering

In-line X-ray phase contrast imaging, which is simple to experiment with, provides significantly higher sensitivity, compared to conventional X-ray absorption imaging. The inversion of the relationship between recorded Fresnel diffraction intensity and the phase shift induced by the object is called phase retrieval. The transport of intensity equation (TIE), a simple method of phase retrieval, which is solved by the fast Fourier transform algorithm proposed by Paganin et al., has been widely adopted. However, the existing method suffers from excessive suppression of high-frequency information, resulting in loss of image details after phase retrieval, or insufficient detail contrast, leading to blurry images. Here, we present a straightforward extension of the two-distance FFT-TIE method by modifying the Fourier filter through the use of a five-point approximation to calculate the inverse Laplacian in a discrete manner. Additionally, we utilize a combination of continuous Fourier transform and a four-point approximation to compute the gradient operator. The method is evaluated by simulating samples with a shape similar to the resolution test map and by using a photograph of a dog for further evaluation. The algorithm that incorporates the modified gradient operator and the algorithm that solely utilizes the continuous Fourier transform for gradient computation were compared with the results obtained using the two-distance FFT-TIE method. The comparisons were conducted using the results obtained from two distances from the sample to the detector. The results show that this method improves the contrast of spatial details and reduces the suppression of high spatial frequencies compared to the two-distance FFT-TIE method. Furthermore, in the low-frequency domain, our algorithm does not lose much information compared to the original method, yielding consistent results. Furthermore, we conducted our experiments using carbon rods. The results show that both our method and the FFT-TIE method exhibit low-frequency distortion due to the requirement of close proximity between the absorption maps and the detector. However, upon closer inspection, our proposed method demonstrates superior accuracy in reproducing the finer details of the carbon rod fibers.

Hybrid-Modulation Hysteresis Scheme Based Decoupled Power Control of Grid-Connected Inverter

Most existing power control methods for grid-connected inverter are implemented in synchronous reference frame. In order to achieve soft-switching operation, those methods usually need additional design considerations, which could introduce complex computation. This article presents a control strategy for single-phase grid-connected inverter system with LCL filter that can be used for grid-connected battery/photovoltaic (PV) system, with the target to implement decoupled active and reactive power control capability in stationary frame and achieve soft-switching behavior for higher efficiency. In order to improve system efficiency, a new hybrid-modulation-based hysteresis current scheme is used to achieve soft-switching while improving the inherent zero-crossing distortion issue existing in traditional unipolar hysteresis current control. Multiproportional and resonant compensators are utilized to mitigate output low-frequency distortions further, together with a capacitor current feedback-based active damping design embedded in an inner loop. The proposed controller also demonstrates good robustness and immunity to distorted grid conditions. Simulation and experimental results are presented in this article with essential theoretical analysis to verify the validity of the proposed control strategy.

Multidimensional Pulsewidth Modulation for Cascaded Split-Source Inverter

Cascaded split-source inverter (CSSI) is a single-stage modular multilevel structure. This article proposes the multidimensional pulsewidth modulation (MD-PWM) for this topology. All multilevel modulations are a particular version of this approach. The principles of this geometrical method are based on selecting the optimal points and determining the switching sequences and time intervals. By considering the operational principles of CSSI, this method is modified to accurately control the voltage of dc links, charge inductors with constant duty cycles, control dc and ac sides of cells independently, and reduce the reverse recovery effects of input diodes. MD-PWM, with a low computational burden, can be implemented for CSSI with any number of voltage levels. The output power can be distributed equally or unequally among cells, and both buck and boost operation modes are realized for each cell. Also, this method is extended for the operation of the converter with unequal input sources. For an asymmetric topology with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> cells, harmonics are located at multiples of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> times switching frequency. Thus, this strategy eliminates low-frequency distortions from the output voltage of asymmetric CSSI. The simulation and experimental results verify the performance and accuracy of the proposed technique and mathematical relations.

DC-Balanced Even-Dimensional CAP Modulation for Visible Light Communication

Carrierless amplitude and phase (CAP) is a high spectral efficiency (SE) modulation scheme for visible light communication (VLC) systems. For N-dimensional (N-D) CAP, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> pulse shaping filters (PSFs) should satisfy zero inter-symbol interference (ISI) and zero inter-channel interference (ICI) conditions. In a practical VLC system, direct-current-balanced (DC-balanced) CAP PSFs are required to avoid severe low-frequency distortion. In this paper, we discuss the possibility of designing perfect DC-balanced N-D PSFs based on zero ISI and zero ICI conditions. Perfect PSFs should be spectrally efficient and flattened. We first prove that it's impossible to design perfect DC-balanced PSFs if the dimension is odd, but it's possible to design perfect DC-balanced PSFs if the dimension is even. Based on our proposed theories, we use a modified minimax optimization method to design a group of perfect DC-balanced 4-D PSFs. A simulation based on the bandpass VLC channel is set up to compare the bit error rate (BER) performance of odd-dimensional and even-dimensional PSFs. The simulation shows that even-dimensional PSFs outperform odd-dimensional PSFs. Moreover, a VLC system based on a laser diode is demonstrated. The experimental results show that the BER performance of even-dimensional CAP (2-D or 4-D CAP) is much better than that of odd-dimensional CAP (3-D CAP) in the bandpass VLC system. We show that the novel 4-D CAP has the nearly identical BER performance as classical 2-D CAP and it can be a good candidate multiuser modulation scheme for bandpass VLC systems.

Управление нелинейными динамическими процессами трехфазных рекуперирующих преобразователей с пространственно-векторной модуляцией

A control system for nonlinear dynamic processes of threephase regenerative converters is considered. The system is built on the basis of the target-oriented control and allows to provide the desired dynamic mode without additional parametric synthesis. It eliminates the contradiction between the requirements for the switching frequency or the parameters of the transient process in the system and the requirements for the desired dynamic mode when the system parameters change over a wide range. A control system for nonlinear dynamic processes based on the method of directing to the purpose is proposed, that allows solving the specified problem. When implementing the system under consideration, it is assumed to evaluate the coordinates of fixed points of the desired mode based on measurements on a real control object with subsequent digital processing of the received signals in order to isolate the main harmonics. The simulation of the considered system and the efficiency of the proposed control system for nonlinear dynamic processes are performed. The proposed approach can be applied in other power conversion systems with low- frequency periodic impacts in conditions of lowfrequency distortion of the shape of currents or voltage as a result of bifurcations.

Chirp characterization of SOAs under sub-nanosecond electro-optical switching using heterodyne signal post-processing

A technique to characterize amplitude distortions and optical carrier frequency variations (chirp) using optical heterodyne reception and data post-processing is presented. Predominantly, dynamic performances of electro-optical switching action based on semiconductor optical amplifiers (SOA) are analyzed, through the measuring of the SOA chirp imposed on the switched optical carrier. The employment of nearly linear SOAs is preferable in comparison with its nonlinear counterparts since the slower operational state transitions are compensated by narrower guard times and less drastic transient behaviors. At its best operational condition, the most linear SOA achieved chirp oscillations below 0.4GHz. Its short transient behavior indicates the possibility to achieve optical package switching with low latency and low amplitude and frequency distortions.

Artificial-Intelligence-Enhanced Mobile System for Cardiovascular Health Management.

The number of patients with cardiovascular diseases is rapidly increasing in the world. The workload of existing clinicians is consequently increasing. However, the number of cardiovascular clinicians is declining. In this paper, we aim to design a mobile and automatic system to improve the abilities of patients’ cardiovascular health management while also reducing clinicians’ workload. Our system includes both hardware and cloud software devices based on recent advances in Internet of Things (IoT) and Artificial Intelligence (AI) technologies. A small hardware device was designed to collect high-quality Electrocardiogram (ECG) data from the human body. A novel deep-learning-based cloud service was developed and deployed to achieve automatic and accurate cardiovascular disease detection. Twenty types of diagnostic items including sinus rhythm, tachyarrhythmia, and bradyarrhythmia are supported. Experimental results show the effectiveness of our system. Our hardware device can guarantee high-quality ECG data by removing high-/low-frequency distortion and reverse lead detection with 0.9011 Area Under the Receiver Operating Characteristic Curve (ROC–AUC) score. Our deep-learning-based cloud service supports 20 types of diagnostic items, 17 of them have more than 0.98 ROC–AUC score. For a real world application, the system has been used by around 20,000 users in twenty provinces throughout China. As a consequence, using this service, we could achieve both active and passive health management through a lightweight mobile application on the WeChat Mini Program platform. We believe that it can have a broader impact on cardiovascular health management in the world.

Analysis and Improvement of the Effect of Distributed Parasitic Capacitance on High-Frequency High-Density Three-Phase Buck Rectifier

For high-density three-phase buck rectifier (3ph-BR), such as a standard full-brick size 3ph-BR, there exist obvious parasitic capacitances distributed between its dc-link output and the system ground, which lead to low-frequency distortion of input currents, and thus, serious deterioration of the input current total harmonics distortion (THD). The effect of the distributed parasitic capacitances becomes more obvious for the 3ph-BR under light load conditions. To improve the input current quality of the high-density 3ph-BR, a modified 3ph-BR is introduced in this article. The modified 3ph-BR has one low-frequency current path which realizes power transfer from ac input to dc output, and two high-frequency current paths which provide low-impedance path for high-frequency currents. The high-frequency current caused by the parasitic capacitance distributed between the dc-link output and the system ground circulates internally, rather than flow to the ac input side. Thus, input current THD of the modified 3ph-BR can be reduced. A 1-kW experimental prototype of the modified 3ph-BR with 200 kHz switching frequency and standard full-brick size is built to verify the analysis results.

One-Bit ΣΔ-Encoded Stimulus Generation for On-Chip ADC Test

This paper presents an application of the [Formula: see text] modulation technique to the on-chip dynamic test for analog-to-digital converters (ADCs). The required stimulus such as a single- or two-tone signal is encoded into one-bit [Formula: see text] sequence, which is applied to an ADC under test through a driving buffer and a simple low-pass reconstruction filter. By a systematic approach, we select the order and type of a [Formula: see text] modulator and develop a frequency plan suitable for the spectral measurement. In this way, we achieve a high dynamic range suitable for spectral harmonic and intermodulation distortion tests for ADCs. For high frequency measurements (up to the Nyquist frequency), we propose a novel low-pass/band-pass modulation scheme that allows to avoid harmful effects of the low-frequency quantization noise. Also we address the distortion components which originate from the buffer imperfections for a nonreturn-to-zero waveform representing the encoded stimulus. We show that the low-frequency distortion components can be cancelled by using a simple iterative predistortion technique supported by measurements with a DC-calibrated ADC. By correlation between low- and high-frequency components also the high frequency distortions can be largely reduced. The presented techniques are illustrated by simulation results of an ADC under test.

Understanding of scanning-system distortions of atomic-scale scanning transmission electron microscopy images for accurate lattice parameter measurements

Atomic-scale scanning transmission electron microscopy (STEM) imaging has opened up the possibility of studying the local lattice parameters of crystalline materials. To ensure more accurate measurements, low-frequency distortions of the images should be properly calibrated, which requires a better understanding of their causes. Although the major possible causes are sample drift and the scanning systems of microscopes, their effects are intricate because the rates of sample drifts differ in respective measurements. In the present study, low-frequency distortions of STEM images and their dependence on scan rotations were evaluated by measuring the lattice parameters of a reference specimen, strontium titanate. The distortions due to sample drifts and the scanning system of a microscope were separately calculated and corrected using affine transformations. In the as-observed images, the length scales in the x and y directions were underestimated by 0.4–1.2% and 2.7–3.6%, respectively, with shear distortions of 0.6°–1.2°, and the magnitudes of the underestimation and shear distortions were dependent on the scan rotations. On the basis of these findings, a methodology was proposed for the correction of distortions for accurate measurement of the lattice parameters of materials.

Learning to synthesize: robust phase retrieval at low photon counts

The quality of inverse problem solutions obtained through deep learning is limited by the nature of the priors learned from examples presented during the training phase. Particularly in the case of quantitative phase retrieval, spatial frequencies that are underrepresented in the training database, most often at the high band, tend to be suppressed in the reconstruction. Ad hoc solutions have been proposed, such as pre-amplifying the high spatial frequencies in the examples; however, while that strategy improves the resolution, it also leads to high-frequency artefacts, as well as low-frequency distortions in the reconstructions. Here, we present a new approach that learns separately how to handle the two frequency bands, low and high, and learns how to synthesize these two bands into full-band reconstructions. We show that this “learning to synthesize” (LS) method yields phase reconstructions of high spatial resolution and without artefacts and that it is resilient to high-noise conditions, e.g., in the case of very low photon flux. In addition to the problem of quantitative phase retrieval, the LS method is applicable, in principle, to any inverse problem where the forward operator treats different frequency bands unevenly, i.e., is ill-posed.

Dynamic Inertia Response Support by Energy Storage System with Renewable Energy Integration Substation

In recent years, the expansion of renewable energy in electric power systems has been increasing at such a rapid pace that it has started affecting frequency stability. Renewable generators connected to the grid produce variable amounts of power, and in most cases have no inherent inertia response (IR) to the system frequency. Therefore, the high penetration of renewable generators in the system results in low inertia and frequency distortion. If renewable generators account for a high proportion of the supply in a power system, the use of energy storage systems (ESSs) with frequency-support algorithms (in the place of synchronous generators) can stabilize the network. The participation of ESSs in frequency support must be organized precisely, so that they are fully devoted to their own purpose. In this paper, the frequency-support parameters of ESSs are calculated for achieving stable frequency response from a network. An estimation and calibration process is conducted during the active power-order change of the ESSs in the substation, and is verified through electromagnetic-transients-including-DC (EMTDC)-based simulations.

Randomized spectral sampling for efficient simulation of laser propagation through optical turbulence

We present a new method for the generation of atmospheric turbulence phase screens based on the frequency shift property of the Fourier transform. This method produces low spatial frequency distortions without additional computation time penalties associated with methods using subharmonic subgrids. It is demonstrated that for simulations of atmospheric turbulence with finite outer scales, the performance of our method with respect to the statistical phase structure function of the screen meets or exceeds other methods with respect to agreement with theory. We outline small-scale accuracy issues associated with modelling non-Kolmogorov spectral power laws using existing techniques, and propose a solution. For simulations of long-range propagation through atmospheric optical turbulence, our method provides various advantages over standard methods.

Research and Compensation of Frequency Response of LiNbO3 Phase Modulator for High-Performance Fiber-Optic Gyroscope

A lithium niobate (LN) phase modulator is widely spread because ofhigh electro-optic coefficients. Particularly, it is a highly required component for a closed-loop fiber-optic gyroscope (FOG) of navigation and tactical grade. However, low-frequency distortions of LN phase modulator frequency response can degrade the FOG performance. This paper deals with the research and compensation of frequency response of an LN phase modulator. It is a complex task to measure the response of the LN phase modulator in a FOG because of its Sagnac interferometer. Therefore, the special and novel modulation/demodulation algorithm is proposed. To the best of our knowledge and belief, the proposed method is the first published method that provides the measurement of the pulse response of a phase modulator as a part of a closed-loop FOG. The pulse response obtained in the proposed way allows getting appropriate high order transfer function of a phase modulator. For the first time to the best of our knowledge, the humidity dependence of the LN phase modulator frequency response is found. Influence of the phase modulator frequency response on FOG angular random walk, bias drift, and scale factor nonlinearity and instability is theoretically analyzed and mathematically simulated. The algorithmic method of the phase modulator response compensation is proposed and experimentally confirmed. The experimental results show that the proposed measurement technique of the phase modulator transfer function allows creating the compensating inverse filter that provides phase modulator response being close to flat. The application of the proposed method in the FOG operating mode allows reducing unwanted spikes to the noise level.

Feedforward Modulation for the Neutral-Point-Clamped Converter With Confined Capacitor Voltage Ripples and Reduced Switching Power Losses

This article presents a new modulation technique with feedforward compensation for the three-phase three-level neutral-point-clamped converter. With the proposed technique, the capacitor voltage ripples are allowed to vary within certain limits. This enables an optimized design of the converter since the maximum capacitor voltages are predefined. Furthermore, the proposed modulation technique is able to reduce switching power losses compared to modulation techniques that eliminate capacitor voltage ripples completely. The proposed technique is therefore a tradeoff solution between aforementioned techniques and traditional modulation techniques where the capacitor voltage ripples are not limited. In the proposed technique, if the capacitor voltages are within the tolerable specified range, all the phases switch in two consecutive voltage levels in steady state. When the capacitor voltages go beyond the specified limits, one of the three phases is chosen to switch in three levels to inject an appropriate neutral point current for capacitor voltage balance. As the capacitor voltages are allowed to oscillate, the modulation technique is provided with feedforward compensation to avoid producing low-frequency distortion on the output voltages of the converter. Experimental results are presented operating under different loading conditions.

A Digital Filtering Method for Eliminating DC Offset in Ambulatory ECG Signals

The amplifier is easy to saturation because of polarization voltage in ambulatory ECG acquisition. The traditional way is using analog high-pass filter to eliminate, but the output tends to have a residue. If upgrading high-pass filter cutoff frequency, it will lead to low frequency distortion of ECG signals. In this paper, a Savitzky-Golay (SG) smoothing filter has been designed by combining the single edge point M and the polynomial order p, which can fit the polarization voltage components of ECG signals, filter useful components and get drift-free ECG signals by using the subtraction algorithm. The results of ECG filtering experiment verify the feasibility of the SG smoothing filter, and show the filtered ECG signal without any losses of useful components.

Adiabatic regularization of power spectrum and stress tensor of relic gravitational wave without low-frequency distortion

Adiabatic regularization is a method to remove UV divergences in quantum fields in curved spacetime. For relic gravitational wave generated during inflation, regularization on all k-modes of the power spectrum to 2nd adiabatic order, and of the energy density and pressure to 4th order, respectively, causes low-frequency distortions. To avoid these, we regularize only the short modes inside the horizon during inflation (corresponding to the present frequencies f ≳ 109 Hz), and keep the long modes intact. Doing this does not violate the energy conservation since the k-modes of RGW are independent of each other during inflation. The resulting spectra are UV convergent and simultaneously free of low-frequency distortion, and these properties remain in the present spectra after evolution, in contrast to regularization at the present time which has some distortion or irregularities. The spectra generally exhibit quick oscillations in frequency domain, even if the initial spectra during inflation have no oscillations. This pattern is due to the interference between the positive and negative frequency modes developed during cosmic expansion, and may be probed by future RGW detections.

Enhanced magnetoelectric voltage in ferrite/PZT/ferrite composite for AC current sensor application

In this paper, we report the fabrication of a rare-earth free current sensor based on a PZT/NiCoZn-ferrite magnetoelectric (ME) trilayer composite disk. To improve the sensitivity of the sensor, the structure uses an in-plane series connection, which increases the ME voltage by two for a fixed volume. Then, we propose a full characterization of the sensor: electrical modeling, low and high frequency limits, sensitivity, linearity, distortion and resolution. The device is also evaluated under sinus, square, and triangle waveform currents, which are excitation signals commonly used in the field of electrical engineering. The current sensor shows high current sensitivity (90 mV/A), good linearity from 0.001 to 30 A and low added impedance (0.01 Ω) in a frequency range of 10 Hz–30 kHz for a very compact structure (4 cm3). It also exhibits a high resolution of 1 mA (for a 1 A peak signal) and good stability.

Remarks on correcting ionospheric distortions in L-band radar interferometry

The L-band synthetic aperture radar (SAR) interferometry (InSAR) technique has a lower accuracy due to ionospheric phase distortions. Recently, a multiple-aperture interferometry (MAI)-based ionospheric correction method has been proposed. Using four types of ionosphere-distorted interferograms, the performance of the correction method was evaluated and then analyzed the feasibility of the correction method. The test interferograms contained severe azimuth streaking, low-frequency ionospheric phase distortion and drastic phase change due to the ionosphere. The results showed that (i) the existence and magnitude of ionospheric phase distortions can be recognized from MAI interferograms and (ii) the MAI-based ionospheric correction method efficiently reduced severe azimuth streaking and low-frequency distortion but did not mitigate the drastic phase change perfectly. It is allowed (i) to determine whether a given SAR interferogram has an ionospheric distortion and (ii) to predict whether the ionospheric distortion can be corrected by using the MAI-based ionospheric correction method.