Nyquist Frequency Research Articles

Enhancement of effective viewable information in integral imaging display systems with holographic diffuser: Quantitative characterization, analysis, and validation

A holographic diffuser can effectively improve the viewable quality of a large-scale integral imaging system. In this paper, a beam diffusion model of the holographic diffuser is designed and constructed to analyze the performance of holographic diffuser based integral imaging system quantitatively, and the mechanism improving the display effect is analyzed by characterization of visual-spatial frequency and effective visual 3D information. The theoretical analysis indicates that the holographic diffuser can effectively enlarge the visible range of reconstructed rays, overcome the Nyquist frequency limitation, and increase the effective visual-spatial frequency and the effective visual 3D information of the display system. The experimental results verify the theoretical prediction proving the holographic diffuser can effectively improve the display quality of the integral imaging display system.

Optical design and implementation of a compact and long focal length imaging system

In this paper, we demonstrate a compact and long focal length imaging system for building detection. This system is an all-spherical system. In order to avoid the use of aspheres, the system relies on two groups of correction elements to correct aberrations. This allows for short manufacturing cycles and simplified adjustments. The system works in the visible band from 460 nm to 750 nm. The focal length is 900 mm, and the optical length is only 280 mm. The designed modulation transfer function (MTF) is consistent with the diffraction limit. The MTF curve is 0.63 at the Nyquist frequency of 78 lp/mm. The optomechanical structure is developed based on the analysis consequence of tolerance and stray light, and a device is fabricated. We also propose a simplified method for measuring the space between the primary mirror and the secondary mirror in a compact coaxial optical system. According to the experimentation, the device is proven to meet the application requirement. Detailed descriptions of the design and implementation are provided.

Modified Passivity-Based Control for LCL-Filtered Grid-Tied Inverter with Output Admittance Reshaping

As a nonlinear control method, the Euler–Lagrange- (EL-) based passivity-based control (PBC) has been studied for grid-tied converters based on energy function to achieve better performance. However, the EL-based PBC method is dependent on an accurate mathematical model. In the traditional EL-based PBC research for LCL-filtered grid-tied inverter, the effect of the digital control delay is rarely considered and the stability under the grid impedance uncertainties is not discussed, especially in the capacitive grid or complex weak grid. To address these concerns, this study proposes a modified EL-based PBC method based on the output admittance reshaping for LCL-filtered grid-tied inverters. The system’ passive region is expanded by adding capacitor current feedback loop up to the Nyquist frequency. The potential resonance is thus eliminated irrespective of the grid impedance. Additionally, the stable region and control parameters design methods of the modified EL-based PBC method with inverter-side control are also carried out. To verify the correctness of the theoretical analysis, both simulation and experimental results are investigated from a 3 kW grid-tied inverter prototype.

A 9-Bit 500-ms/s 4-Stage Pipelined SAR ADC With Wide Input Common-Mode Range Using Replica-Biased Dynamic Residue Amplifiers

This paper presents a 9-bit pipelined successive-approximation-register (SAR) ADC consisting of 4-stage sub-SAR ADCs using replica-biased dynamic residue amplifiers. The replica-biased amplifier in the 1st stage keeps the output common mode constant over various input voltage conditions, which enables cascading multiple dynamic-amplifier-based pipeline stages. The replica-biased amplifiers from the 2nd to the last stage maintain the differential gain, eliminating the need for bit-weight calibrations. The 1st~3rd stages utilize a loop-unrolled SAR structure for high-speed conversion. A 9-bit 500MS/s pipelined SAR ADC is fabricated in 28nm CMOS process, and it achieves SNDR of 49.1dB and SFDR of 60.4dB at low frequency and SNDR of 45.3dB and SFDR 56.0dB at Nyquist frequency. The measured performance over wide input common-mode and temperature range validates the operation of the replica-biased dynamic amplifiers. The measured Walden figure of merit (FoMw) is 23.3fJ/conversion-step and Schreier FoM (FoMs) is 158.6dB, respectively.

Improved Parallel Magnetic Resonance Imaging Reconstruction with Sampling based on Normal Distribution

Generalized autocalibrating partially parallel acquisitions (GRAPPA) is a k-space-based magnetic resonance imaging (MRI) reconstruction algorithm that obtains coefficients by acquiring k-space center data with the Nyquist frequency requirement, fitting these data as auto-calibration signal (ACS). The under-sampled k-space data are then filled using the linear correlation of the neighboring points of the k-space. At high reduction factors, images reconstructed by using GRAPPA have high levels of noise even when the number of ACS lines is significantly increased. NL-GRAPPA (Non-linear GRAPPA) can improve GRAPPA for a comparatively impressive image quality. However, it is extremely difficult to reduce the sampling time significantly because of the high number of ACS lines that must be sampled. In this paper, we proposed an undersampling parallel MRI (pMRI) reconstruction technique based on the ideology of normal distribution to improve the traditional GRAPPA and NL-GRAPPA methods. The proposed method mainly takes advantage of the fact that the data in the center of the k-space had a greater impact on the imaging quality than the data at the edges, and then samples the data based on the ideology of normal distribution, i.e., discarding part of the data at the edges of the k-space. The experimental results show that our proposed method can significantly reduce the number of sampled lines and have higher imaging quality with a lower number of ACS lines.

Beyond Nyquist in Frequency Response Function Identification: Applied to Slow-Sampled Systems

Fast-sampled models are essential for control design, e.g., to address intersample behavior. The aim of this paper is to develop a non-parametric identification technique for fast-sampled models of systems that have relevant dynamics and actuation above the Nyquist frequency of the sensor, such as vision-in-the-loop systems. The developed method assumes smoothness of the frequency response function, which allows to disentangle aliased components through local models over multiple frequency bands. The method identifies fast-sampled models of slowly-sampled systems accurately in a single identification experiment. Finally, an experimental example demonstrates the effectiveness of the technique.

Catalogue of solar-like oscillators observed by TESS in 120-s and 20-s cadence

Context. The Transiting Exoplanet Survey Satellite (TESS) mission has provided photometric light curves for stars across nearly the entire sky. This allows for the application of asteroseismology to a pool of potential solar-like oscillators that is unprecedented in size. Aims. We aim to produce a catalogue of solar-like oscillators observed by TESS in the 120-s and 20-s cadence modes. The catalogue is intended to highlight stars oscillating at frequencies above the TESS 30-min cadence Nyquist frequency with the purpose of encompassing the main-sequence and subgiant evolutionary phases. We aim to provide estimates for the global asteroseismic parameters vmax and ∆v. Methods. We applied a new probabilistic detection algorithm to the 120-s and 20-s light curves of over 250 000 stars. This algorithm flags targets that show characteristic signatures of solar-like oscillations. We manually vetted the resulting list of targets to confirm the presence of solar-like oscillations. Using the probability densities computed by the algorithm, we measured the global asteroseismic parameters vmax and ∆v. Results. We produce a catalogue of 4177 solar-like oscillators, reporting ∆v and vmax for 98% of the total star count. The asteroseismic data reveal a vast coverage of the Hertzsprung-Russell diagram, populating the red giant branch, the subgiant regime, and extending towards the main sequence. Conclusions. A crossmatch with external catalogues shows that 25 of the detected solar-like oscillators are a component of a spectroscopic binary, and 28 are confirmed planet host stars. These results provide the potential for precise, independent asteroseismic constraints on these and any additional TESS targets of interest.

The Ewald sphere/focus gradient does not limit the resolution of cryoEM reconstructions

In our quest to solve biomolecular structures to higher resolutions in cryoEM, care must be taken to deal with all aspects of image formation in the electron microscope. One of these is the Ewald sphere/focus gradient that derives from the scattering geometry in the microscope and its implications for recovering high resolution and handedness information. While several methods to deal with it has been proposed and implemented, there are still questions as to the correct approach. At the high acceleration voltages used for cryoEM, the traditional projection approximation that ignores the Ewald sphere breaks down around 2–3 Å and with large particles. This is likely not crucial for most biologically interesting molecules, but is required to understand detail about catalytic events, molecular orbitals, orientation of bound water molecules, etc. Through simulation I show that integration along the Ewald spheres in frequency space during reconstruction, the “simple insertion method” is adequate to reach resolutions to the Nyquist frequency. Both theory and simulations indicate that the handedness information encoded in such phases is irretrievably lost in the formation of real space images. The conclusion is that correct reconstruction along the Ewald spheres avoids the limitations of the projection approximation.

Design of Airborne Large Aperture Infrared Optical System Based on Monocentric Lens.

Conventional reconnaissance camera systems have been flown on manned aircraft, where the weight, size, and power requirements are not stringent. However, today, these parameters are important for unmanned aerial vehicles (UAVs). This article provides a solution to the design of airborne large aperture infrared optical systems, based on a monocentric lens that can meet the strict criteria of aerial reconnaissance UAVs for a wide field of view (FOV) and lightness of airborne electro-optical pod cameras. A monocentric lens has a curved image plane, consisting of an array of microsensors, which can provide an image with 368 megapixels over a 100° FOV. We obtained the initial structure of a five-glass (5GS) asymmetric monocentric lens with an air gap, using ray-tracing and global optimization algorithms. According to the design results, the ground sampling distance (GSD) of the system is 0.33 m at 3000 m altitude. The full-field modulation transfer function (MTF) value of the system is more than 0.4 at a Nyquist frequency of 70 lp/mm. We present a primary thermal control method, and the image quality was steady throughout the operating temperature range. This compactness and simple structure fulfill the needs of uncrewed airborne lenses. This work may facilitate the practical application of monocentric lens in UAVs.

Optical Design and Analysis of Theia: A Scientific-Grade Multispectral Imager for Nanosatellites

This article describes the optical design and analysis of Theia: a miniature, scientific-grade, multispectral Earth observation imager for nanosatellites that can be used for quantitative remote sensing. The instrument is designed to take images with a 5% radiometric accuracy throughout its three-year lifetime. The sustained accuracy is achieved with a post-launch calibration module. The instrument can capture frames at a 33-m ground sampling distance at an orbit height of 650 km. The design can take diffraction-limited images with a modulation transfer function of at least 0.13 at the sensor’s Nyquist frequency, which is comparable with Sentinel-2’s MultiSpectral Instrument. The instrument works in two user-defined bands, fits inside one CubeSat unit, and weighs 600 g. The successful analysis of the system is presented, including optical performance, radiometric, stray light, and tolerance analyses.

Baroclinic Tidal Cusps from Satellite Altimetry

Abstract The spatially averaged frequency spectrum of sea level has been computed at 4 cycle-per-year resolution and a Nyquist frequency of 0.5 cycles per hour using dual-satellite crossover data from the Jason and CryoSat-2 satellite altimeter missions. The novelty of the analysis is that it reveals unambiguous peaks due to high-frequency tidal signals, even after removing the predicted barotropic tide, without the usual aliasing caused by altimeter sampling. The tidal continuum, that is, a tidal cusp, is present in the spectrum in the diurnal and semidiurnal tidal bands, and a Lorentzian model spectrum has been fit within each band to identify the properties of the non-phase-locked tidal variability. An interesting feature of the semidiurnal tidal continuum is the unambiguous presence of an inner and an outer band, characterized by different Lorentzian bandwidths of roughly (180 day)−1 and (30 day)−1. Considering different latitude ranges, it is clear that the tidal continuum is most prominent in the range from −30° to 30° latitude. Within this range, it is found that 1.05-cm2 variance is associated with the semidiurnal continuum, and slightly less than half of this variance, 0.41 cm2, is associated with the slower, (180 day)−1 bandwidth, variability. The ratio of non-phase-locked to total baroclinic variability is about 62% in this latitude band, a value that is consistent with previous model-based estimates for this quantity. Quantification of the properties of the tidal continuum poleward of 30° latitude is not possible with the present data, due to the small size of the tidal signal compared to the mesoscale variability and other sources of noise.

Passivation of Grid-Following VSCs: A Comparison Between Active Damping and Multi-Sampled PWM

This article compares different strategies used to enhance the stability properties of grid-following voltage-source converters (VSCs). Because of digital delays, VSC admittance exhibits a nonpassive zone, which introduces negative damping and may destabilize the grid-connected operation. It is shown that typically used active damping (AD) strategies only bring positive impact up to a certain frequency, while deteriorating admittance properties around and above the Nyquist frequency. Multi-sampled pulsewidth modulation (MS-PWM) greatly extends the passive admittance region, using only a single-loop current controller. Experimental admittance measurements are performed on a single-phase VSC, up to twice the switching frequency. Subsequently, different grid-connected scenarios are tested to show that MS-PWM retains stable operation, where AD methods cause instability. This article also offers analytic modeling and experimental measurements of noise propagation for compared strategies. It is shown that derivative-based AD is not highly sensitive; however, MS-PWM offers additional noise suppression.

A 33.33 Gb/s/wire pin-efficient 1.06 pJ/bit wireline transceiver based on CNRZ-5 for Chiplet in 28 nm CMOS

This paper presents a low-power, high-bandwidth wireline transceiver for chiplet. The transceiver uses a new technology of correlated non-return to zero (CNRZ) to improve the pin efficiency and reduce the noise of the common mode and crosstalk (CMN and XTALK). The transmitter adopts a 4:1 multiplexer (MUX) and a special voltage-mode driver [source series terminated (SST)] to improve the power efficiency. The receiver utilizes a low jitter clock-and-data alignment (CDA) based on high resolution phase interpolator (PI) and a MINI-PLL with differential filter based to improve the jitter performance of the receiver’s clock. In addition, a special continuous-time linear equalizer (CTLE) is introduced into the multiple-input compare (MIC) based decode circuit to reduce the receiver’s bit error rate (BER). This transceiver is designed with a 28 nm CMOS process technology and supplied with 0.9/1.2 V. Simulation results show that this transceiver can operate at 200 Gb/s with the 33.3 Gb/s single-wire data rate and a power consumption of 1.06 pJ/b. The transceiver’s BER is less than 1E-15 and eye-wide-opening is 12.02 ps (48.08% UI), which is under 10 dB channel loss at 20 GHz Nyquist frequency.

LED communications with linear complexity compensation of dynamic nonlinear distortion.

An in-depth study of the second order Volterra kernel of light emitting diodes (LEDs) is provided, with a special focus on compensation of dynamic nonlinear distortion generated by the LED when used as a communications signal transmitter. It is shown that from the factorization of the kernel in the frequency domain follows a simplified time-domain model for LED nonlinearity, consisting of three operations: two linear filtrations and squaring. Next, using the pth inverse theory, the corresponding block structures of nonlinear pre- and postdistorters are derived. As it turns out, they exhibit linear computational complexity. It is shown that the model coefficients defining the kernel may be estimated up to the Nyquist frequency. Numerical results indicate that pre- and postdistortion achieve the same performance with respect to receiver signal to noise power ratio (SNR) and have a considerable advantage over linear equalizers. However, in a practical scenario, the predistortion increases the peak to average power ratio of the transmitted signal (unless it is not high already), possibly leading to performance inferior to postdistortion. Finally, it is shown that the simplified equalizer based on the LED model has roughly identical performance to the regular, quadratic Volterra equalizer.

Modeling and suppressing the wavefront degeneration in a CGH interferometric null test.

The computer-generated hologram (CGH) enables the ultra-high accuracy of surface measurement but causes the wavefront degeneration in the optical system. In this article, we give a high-accuracy analytical simulation of the wavefront degeneration in null test by the elliptical Gaussian model. We propose an analytical expression of instrumental transfer function (ITF) for the CGH null test without knowing the phase distribution of CGH, which gives an efficient instruction to suppress the wavefront degeneration. The ITF of the interferometric null test for a ∅3m aspheric mirror can be optimized from 0 to 0.65 at 0.4 Nyquist frequency.

A 1.5-GS/s 6-bit Single-Channel Loop-Unrolled SAR ADC With Speculative CDAC Switching Control Technique in 28-nm CMOS

This paper presents a 1.5-GS/s 6-bit single-channel loop-unrolled successive approximation register (SAR) analog-to-digital converter (ADC) using speculative capacitive DAC (CDAC) switching control technique. The proposed SAR ADC achieves a high sampling rate by eliminating additional delays in typical loop-unrolled SAR ADCs related to settling time constraints in their CDACs. Specifically, the CDACs are duplicated and controlled in speculative ways so that the CDAC outputs passage to their next values before completing the regeneration operation of comparators, thereby improving timing constraints for successive approximations. The switching power overhead from the CDAC speculation is mitigated by introducing an energy-efficient CDAC control technique that produces desired voltage transients with minimal power overheads. The prototype of the proposed SAR ADC is fabricated in a 28-nm CMOS technology and occupies an active area of 0.0038-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The design consumes 5.8 mW from a 1.2-V supply. The ADC achieves 1.5-GS/s sampling frequency with a 31-dB SNDR at a low input frequency and a 28.6 dB at the Nyquist frequency without applying any offset calibration techniques, achieving the highest sampling frequency among the 6-bit single-channel loop-unrolled SAR ADCs reported.

A 10-Gbps CTLE design using split-length input pair MOS Transistors

ABSTRACT The equaliser is an indispensable block inside the receiver in Serial Link systems. It is used to moderate the high-frequency loss of the signal in the channel. A new technique is described to improve the performance of the Continuous-Time Linear Equaliser (CTLE). This technique utilises split-length device (SLD) in order to boost the output impedance of the SLD. The proposed CTLE is designed and simulated in 65 nm CMOS technology. Post-layout simulation results demonstrate that the proposed technique has more peaking than conventional CTLE by an order of 1.5 dB at Nyquist frequency. The Proposed design has a vertical eye-opening of 220 mV and a horizontal eye-opening of 0.35 UI. It consumes 1.56 mW from a 1.2 V supply. A FoM of 13.1 fJ/bit/dB is achieved.

Accurate High-Frequency Modeling of the Input Admittance of PWM Grid-Connected VSCs

This article addresses high-frequency admittance modeling of current-controlled voltage source converters (VSCs). Recent studies have shown that harmonic instability may also occur at frequencies above the Nyquist frequency. To form an accurate multiple-frequency model in this frequency range, sidebands that originate from modulation and sampling must be examined. In this article, an accurate small-signal model is developed taking into account an adequate digital pulsewidth modulator (DPWM) representation, which allows to predict dependence of the frequency response on the steady-state dc (SS-dc) operating point. It is shown that when center-pulse sampling is implemented, pulsewidth modulation sidebands do not create additional loops leaving only sampling sidebands to be considered. Using the same approach as for SS-dc operation, a model that accurately represents admittance measurements during sinusoidal ac (S-ac) operation is developed. Its basis is a novel DPWM model suitable for S-ac regime, which allows to predict dependence of the VSC’s input admittance on the grid voltage magnitude. Experimental and simulated admittance measurements, performed on a single-phase two-level VSC during various SS-dc and S-ac regimes, match with the proposed models up to twice the sampling frequency.

Aerodynamic loads correction method based on wavelet packet transform in high-frequency force balance tests

High-frequency force balance (HFFB) technique is a commonly used method for acquiring wind-induced response of tall buildings in wind tunnel tests. However, the resonance buffeting of the balance-model system (BMS) results in the distortion of measured aerodynamic loads. When the signal distortion is complicated due to additional vibration modes and noise interference, the existing methods fail to obtain correction results. Thus, a new correction method based on wavelet packet transform (WPT) is proposed in this paper to address the issue. The distorted signals are decomposed by WPT, and the relationship between wavelet packet coefficients and power spectral density (PSD) estimation is established. Then, the reference line that conforms to the aerodynamic attenuation characteristics is extracted by a single-degree-of-freedom (SDOF) correction method combined with a parameter identification method. Subsequently, the wavelet packet coefficients of SDOF correction results are adjusted according to the reference line. Finally, the time history of the aerodynamic load after correction is reconstructed by inverse WPT (IWPT). The proposed method is validated by a numerical example and applied to a HFFB test. Compared with the existing methods, the proposed method can address more complex distortion scenes and extend the correction frequency range to the Nyquist frequency.

Diagnosis of Water Failures in Proton Exchange Membrane Fuel Cells via Physical Parameter Resistances of the Fractional Order Model and Fast Fourier Transform Electrochemical Impedance Spectroscopy

AbstractThe identification of water status is the foundation for fuel cell water management, which is helpful to fuel cell reliability and longevity. In this article, a novel and reliable method for diagnosing the hydration condition of proton exchange membrane fuel cells (PEMFCs) was presented using a fractional-order model (FOM) to represent the PEMFCs impedance. The results show that the mean root-mean-squared error (RMSE) and mean absolute percentage error (MAPE) between the proposed model and experimental data (in normal, drying, or flooding cases) are about 0.034 and 0.473, respectively. The fast Fourier transform–electrochemical impedance spectroscopy technique (FFT-EIS) was used as an alternative technique that is simple and efficient to electrochemical impedance spectroscopy (EIS). The PEMFCs hydration state is monitored by observing the changing effect of the physical resistor values (membrane resistance, polarization, and diffusion resistances) of the proposed model. These resistors, characterized by their high sensitivity to the drying and flooding of PEMFCs, affect the Nyquist impedance spectra and frequency spectrum amplitudes at low and high frequencies. Based on the obtained results, it is concluded that the proposed strategy can be used to develop new domains in which the PEMFCs’ hydration states can be properly predicted.