Transfer Function Measurements Research Articles

Transformer frequency response: a new technique to analyze and distinguish the low-frequency band in the frequency response analysis spectrum

Smart monitoring applications in all aspects of power systems are becoming increasingly popular because of the indirect effect on energy management and the smart grid. In this regard, high voltage equipment monitoring and real-time assessment are critical. Among these evaluations, transformer life and condition assessment has been considered for many years [1], [2]. Currently, advanced methods such as transfer function measurement are employed to determine the transformer condition [3]. Transfer function measurement is generally used for black box system identification [4]. This measurement might be implemented in the time domain as an impulse response or in the frequency domain using a swept signal; the latter approach is commonly called frequency response analysis (FRA) [5].

Modeling of topography measuring instrument transfer functions by time series models

The transfer function of topography measuring instruments contains important information for an understanding of the metrological characteristics. There are different methods for the estimation of the transfer function. For example, a measurement of the transfer function can be conducted with the aid of material measures that feature defined properties in the frequency domain. Another possibility is to determine the transfer function with the utilization of virtual measurements. However, these methods either require the development of a material measure specifically for this purpose or are only theoretical. We propose an approach that is common in signal processing and time series analysis for the application towards measuring instruments for geometrical product specification (GPS): filter design is used to estimate the instrument’s transfer function. With this approach, the transfer function can be determined with the aid of a measuring object with any well-known stochastic surface structure, as long as the manufacturing and measurement of the structures are possible. The general suitability of the approach for both stylus and optical measuring instruments is demonstrated, and the proposed time series model and the required signal pre-processing are optimized. Based on the results, a comparison of the model results with measured transfer functions is conducted and virtual measurements are performed in order to evaluate the accuracy of the determined models. In doing so, it was observed that the surface structure of the measuring object used has an influence on the quality of the results.

Regularized slanted-edge method for measuring the modulation transfer function of imaging systems

The slanted-edge method for modulation transfer function (MTF) measurement uses edge target images whose gray values are often affected by noise and other factors, decreasing its accuracy. We first analyze the ill-posedness in the edge spread function (ESF) regression caused by noise. Second, we propose a regularized slanted-edge method to solve this problem by incorporating a Tikhonov regularization term. Combined with varying precision weights, the ESF is solved using the variational principle, and the MTF is estimated using the regularized ESF. The regularized slanted-edge method is verified for Gaussian, gamma, and Rayleigh noise. The results show that our method improves the accuracy by 0.01-9.02% and 4.33% on average. The proposed method is more robust to noise and accurate than the slanted-edge method.

Measurement of Sound Transmission Loss in a Sound Barrier Filled with the Rice-Straw Particles by the Transfer Function and Laboratory Measurement Methods

The sound insulation capacity of a barrier is indicated by its transmission loss. In this study, the sound transmission loss of a sound barrier filled with rice-straw particles was measured by the transfer function method and a laboratory measurement method. The results of the two measurements were compared. The transmission losses measured by the two methods were similar above a frequency of 500 Hz. The loss increased greatly upon the introduction of a plywood to the sound barrier. The results of this study are expected to be used to design sound barriers for roads.

Experimental prediction of the vibration response of panels under a turbulent boundary layer excitation from sensitivity functions.

This study aims at validating an experimental method for characterizing the vibration behavior of panels excited by a turbulent boundary layer (TBL) excitation as a possible alternative to standard means like wind tunnels or in situ tests. The approach takes advantage of an explicit separation of the excitation contribution from the dynamic behavior of the panel. Based on the measurement of deterministic transfer functions on the panel, called "sensitivity functions," which are then combined with either measurements or a model of the wall-pressure fluctuations induced by the TBL excitation, the vibration response under such an excitation can be retrieved. For validation purposes, the wall-pressure fluctuations of the turbulent flow generated in an anechoic wind tunnel are measured with a flush-mounted microphone array. The decay rates and the convection velocity, which mainly characterize the excitation, are extracted from these measurements. The plate velocity response to this excitation is estimated following the proposed method using the measured sensitivity functions and the model of Mellen fed with experimentally estimated decay rates and convection velocity. A comparison between a directly measured vibration auto-spectrum under the actual flow and the one predicted following the suggested method shows satisfactory agreement.

Characterizing the digital radiography system in terms of effective detective quantum efficiency and CDRAD measurement

The aim of this study was to assess the performance of a digital radiography system in terms of effective detective quantum efficiency (eDQE) for different tube voltages, polymethyl methacrylate (PMMA) phantom thicknesses and different grid types. The image performance of the digital radiography system was also evaluated by using CDRAD measurements at the same conditions and the correlation of CDRAD results with eDQE was compared. The eDQE was calculated via measurement of effective modulation transfer function (eMTF), effective normalized noise power spectra (eNNPS), scatter fraction (SF) and transmission factors (TF). SFs and TFs were also calculated for different beam qualities by using MCNP4C Monte Carlo simulation code. The integrated eDQE (IeDQE) over the frequency range was used to find the correlation with the inverse image quality figure (IQFinv) obtained from CDRAD measurements. The highest eDQE was obtained with 60 lp/cm grid frequency and 10:1 grid ratio. No remarkable effect was observed on eDQE with different grid frequency, but eDQE decreased with increasing grid ratio. A significant correlation was found between IeDQE and IQFinv.

Computational Temporal Ghost Imaging Using Intensity-Only Detection Over a Single Optical Fiber

We propose a method of computational temporal ghost imaging over a single optical fiber using simple optical intensity detection instead of coherent detection. The transfer function of a temporal ghost imaging system over a single optical fiber is derived, which is a function of the total fiber dispersion and the power density spectrum of the light source. In the simulations, we find that the bandwidth of the transfer function will decrease rapidly along with the increase of fiber length or light source bandwidth, resulting in a significantly distorted ghost image. To improve the image quality, we first calculate the transfer function by approximating the optical spectrum of the light source based on intensity detection as its power density spectrum. Then, we use an optimized Fourier deconvolution to compute a high-quality image by deconvoluting the measured transfer function from the spectrum of the distorted image. Our experiments achieve single-arm ghost images of a 5-Gb/s nonreturn-to-zero pulse object over a 200-km fiber, which have almost the same quality as the two-arm ones.

High-Resolution Extremity Cone-Beam CT with a CMOS Detector: Evaluation of a Clinical Prototype in Quantitative Assessment of Bone Microarchitecture.

A prototype high-resolution extremity cone-beam CT (CBCT) system based on a CMOS detector was developed to support quantitative in vivo assessment of bone microarchitecture. We compare the performance of CMOS CBCT to an amorphous silicon (a-Si:H) FPD extremity CBCT in imaging of trabecular bone. The prototype CMOS-based CBCT involves a DALSA Xineos3030 detector (99 μm pixels) with 400 μm-thick CsI scintillator and a compact 0.3 FS rotating anode x-ray source. We compare the performance of CMOS CBCT to an a-Si:H FPD scanner built on a similar gantry, but using a Varian PaxScan2530 detector with 0.137 mm pixels and a 0.5 FS stationary anode x-ray source. Experimental studies include measurements of Modulation Transfer Function (MTF) for the detectors and in 3D image reconstructions. Image quality in clinical scenarios is evaluated in scans of a cadaver ankle. Metrics of trabecular microarchitecture (BV/TV, Bone Volume/Total Volume, TbSp, Trabecular Spacing, and TbTh, trabecular thickness) are obtained in a human ulna using CMOS CBCT and a-Si:H FPD CBCT and compared to gold standard μCT. The CMOS detector achieves ~40% increase in the f20 value (frequency at which MTF reduces to 0.20) compared to the a-Si:H FPD. In the reconstruction domain, the FWHM of a 127 μm tungsten wire is also improved by ~40%. Reconstructions of a cadaveric ankle reveal enhanced modulation of trabecular structures with the CMOS detector and soft-tissue visibility that is similar to that of the a-Si:H FPD system. Correlations of the metrics of bone microarchitecture with gold-standard μCT are improved with CMOS CBCT: from 0.93 to 0.98 for BV/TV, from 0.49 to 0.74 for TbTh, and from 0.9 to 0.96 for TbSp. Adoption of a CMOS detector in extremity CBCT improved spatial resolution and enhanced performance in metrics of bone microarchitecture compared to a conventional a-Si:H FPD. The results support development of clinical applications of CMOS CBCT in quantitative imaging of bone health.

A hybrid real-time auralization system for binaural reproduction of virtual acoustic environments with simulated room acoustics adapted for hearing aid users

A recently developed binaural real-time reproduction system has been extended with an interface to research hearing aids allowing for the conduction of auditory research on subjects with hearing loss. Simulated hearing aid signals based on measurements of generic hearing aid-related transfer functions are additionally processed on a master hearing aid to emulate conventional hearing aid algorithms and played back through the hearing aid receivers. Designed for subjects with mild to moderate hearing loss, the system also facilitates the use of residual hearing capabilities by simulating an external sound field based on generic head-related transfer functions which is reproduced via loudspeakers and acoustic crosstalk cancellation filters. For increased ecological validity, plausible room acoustics are simulated using adjusted simulation models relying on geometrical acoustics. The proposed system was evaluated objectively on different levels by investigating the listening environment and various system components, running a benchmark analysis on the acoustical simulation and auralization, and measuring the combined system latency.

Near-field head-related transfer-function measurement and database of human subjects.

Near-field head-related transfer functions (HRTFs) of human subjects are essential to those researching spatial hearing. By using a carefully designed measurement system, near-field HRTFs of human subjects were measured and a database was constructed. The database includes 56 Chinese human subjects, seven source distances from 0.2 to 1.0 m, and 685 directions at each distance for each subject. In the present work, the technique of near-field HRTF measurement is outlined, the performance of the measurement system is assessed and validated, and the resultant database is reported. The database can provide fundamental data for future research.

Tilted-wire method for measuring resolution properties of CT images under extremely low-contrast and high-noise conditions.

Edge methods are predominantly used for modulation transfer function (MTF) measurements in computed tomography (CT) images reconstructed using iterative methods. However, edge methods employ a relatively large and distinct test object, which is intended to simulate relatively large and distinct clinical organs. If one wants to assess the image quality of a small low-contrast object that is visually indistinct against a noisy background, a small and indistinct test object is desired. Another concern is that information related to the signal amount is discarded during MTF measurements. Choosing a weak impulse as the ultimately small test object, we have developed a tilted-wire method, which is a type of point spread function (PSF) method compatible with extremely low contrast-to-noise ratio (CNR) conditions. The signal amount is measured as the PSF volume. We used two commercial CT systems to evaluate the measurement accuracy of the tilted-wire method. When ensemble-averaged images are used, one can measure the MTF even when the wire is indiscernible from noise. The measurement error under such conditions is a few percent for both the MTF and signal amount. We also applied the tilted-wire method to two hybrid iterative reconstruction methods, namely AIDR-3D and ASiR. The results show that the MTF of ASiR is completely CNR-dependent, but that of AIDR-3D is noise-dependent. The signal amount obtained with ASiR is unchanged from that obtained through filtered back-projection (FBP). The signal amount obtained with AIDR-3D is less than that obtained through FBP, depending on the noise level.

Contrast computation methods for interferometric measurement of sensor modulation transfer function

Accurate measurement of image-sensor frequency response over a wide range of spatial frequencies is very important for analyzing pixel array characteristics, such as modulation transfer function (MTF), crosstalk, and active pixel shape. Such analysis is especially significant in computational photography for the purposes of deconvolution, multi-image superresolution, and improved light-field capture. We use a lensless interferometric setup that produces high-quality fringes for measuring MTF over a wide range of frequencies (here, 37 to 434 line pairs per mm). We discuss the theoretical framework, involving Michelson and Fourier contrast measurement of the MTF, addressing phase alignment problems using a moiré pattern. We solidify the definition of Fourier contrast mathematically and compare it to Michelson contrast. Our interferometric measurement method shows high detail in the MTF, especially at high frequencies (above Nyquist frequency). We are able to estimate active pixel size and pixel pitch from measurements. We compare both simulation and experimental MTF results to a lens-free slanted-edge implementation using commercial software.

Perceptually motivated enhancement method for non‐uniformly illuminated images

Non‐uniformly illuminated images often suffer from low visibility in dark areas. Traditional methods usually enhance non‐uniformly illuminated images by bringing out the details in the dark areas, but easily result in over‐enhancement. Motivated by the Weber contrast model, the authors propose a perceptually inspired image enhancement method, which treats an image as a product of a luminance mapping (LM) transfer function and a contrast measure (CM) transfer function. The contribution of this proposed method is two‐fold. Firstly, they propose a progressive LM transfer function based on the sensitivity of the human visual system to emphasise changes at low brightness level and attenuates changes at high brightness levels. Secondly, they introduce a CM transfer function, which is based on a special implementation of a neural model of the human visual receptive field, to improve local intensity contrast. Experimental comparisons with some state‐of‐the‐art methods show that the proposed method can achieve both contrast enhancement and visual fidelity preservation.

Modified slanted-edge method for camera modulation transfer function measurement using nonuniform fast Fourier transform technique

ISO 12233 slanted-edge method experiences errors using fast Fourier transform (FFT) in the camera modulation transfer function (MTF) measurement due to tilt angle errors in the knife-edge resulting in nonuniform sampling of the edge spread function (ESF). In order to resolve this problem, a modified slanted-edge method using nonuniform fast Fourier transform (NUFFT) for camera MTF measurement is proposed. Theoretical simulations for images with noise at a different nonuniform sampling rate of ESF are performed using the proposed modified slanted-edge method. It is shown that the proposed method successfully eliminates the error due to the nonuniform sampling of the ESF. An experimental setup for camera MTF measurement is established to verify the accuracy of the proposed method. The experiment results show that under different nonuniform sampling rates of ESF, the proposed modified slanted-edge method has improved accuracy for the camera MTF measurement compared to the ISO 12233 slanted-edge method.

Frequency comb generation in a silicon ring resonator modulator.

We report on the generation of an optical comb of highly uniform in power frequency lines (variation less than 0.7 dB) using a silicon ring resonator modulator. A characterization involving the measurement of the complex transfer function of the ring is presented and five frequency tones with a 10-GHz spacing are produced using a dual-frequency electrical input at 10 and 20 GHz. A comb shape comparison is conducted for different modulator bias voltages, indicating optimum operation at a small forward-bias voltage. A time-domain measurement confirmed that the comb signal was highly coherent, forming 20.3-ps-long pulses.

Accuracy and Precision of Edge-Based Modulation Transfer Function Measurement for Sampled Imaging Systems

The real-time, edge-based, multidirectional modulation transfer function (MTF) measurement system has elucidated the MTF characteristics of sampled imaging systems. However, the accuracy and precision of edge-based MTF measurements are not well understood quantitatively because of the complicated shift-variant nature of the sampling process by the image sensor and the binning process in the edge-based method for producing a supersampled edge spread function. The edge-based MTF measurement is valid only when the edge positions imaged are assumed to be distributed uniformly relative to the sampling sites. This study demonstrates that precision, which is dependent on the edge angle, is counterintuitively high at a higher oversampling ratio of the binning regardless of the smaller pixel counts per bin even with camera noise, and that significantly high precision can be achieved by taking an ensemble average of MTFs computed over a single cycle of the binning phase, even at low oversampling ratios, with high accuracy through proper corrections to compensate for the attenuation of the MTF values due to the discrete processing of the edge-based method.

Adapting Hearing Devices to the Individual Ear Acoustics: Database and Target Response Correction Functions for Various Device Styles.

To achieve a natural sound quality when listening through hearing devices, the sound pressure at the eardrum should replicate that of the open ear, modified only by an insertion gain if desired. A target approximating this reference condition can be computed by applying an appropriate correction function to the pressure observed at the device microphone. Such Target Response Correction Functions (TRCF) can be defined based on the directionally dependent relative transfer function between the location of the hearing device microphone and the eardrum of the open ear. However, it is unclear how exactly the TRCF should be derived, and how large the benefit of individual, versus generic, correction is. We present measurements of Head-Related Transfer Functions (HRTF) at the eardrum and at 9 microphone locations of a comprehensive set of 5 hearing device styles, including 91 incidence directions, and recorded in 16 subjects and 2 dummy heads. Based on these HRTFs, individualized and generic TRCF were computed for frontal (referred to as free-field) and diffuse-field sound incidence. Spectral deviations between the computed target and listening with the open ear were evaluated using an auditory model and virtual acoustic scenes. Results indicate that a correction for diffuse-field incidence should be preferred over the free field, and individual correction functions result in notably reduced spectral deviations to open-ear listening, as compared with generic correction functions. These outcomes depend substantially on the specific device style. The HRTF database and derived TRCFs are publicly available.

基于月球目标的FY-2G在轨调制传递函数评估

基于月球目标的FY-2G在轨调制传递函数评估

Traceable calibration for a digital real-time oscilloscope with time interleaving architecture

Impairments of analog-to-digital converters (ADCs) used in digital real-time oscilloscopes (DRTO) have caused inevitable signal distortions in measurements. To calibrate these errors with traceability, we propose a novel method that consists of two steps. First, each transfer function of the ADCs is measured using pulse trains from a photodiode calibrated up to 110 GHz. Each data set of the ADCs is superimposed to convert the repetitive pulse to a single pulse to solve the under-sampling problem of the separated data depending on each ADC. Then, the signals of the device under test (DUT) are also separated and superimposed depending on the ADCs, and they are calibrated in the frequency domain based on the measured transfer functions. After a calibration process, the data set is reconverted to the time domain to achieve traceable calibration. To verify our method, we have measured the output of another 70 GHz photodiode with a calibrated DRTO. In terms of results, time-interleaved errors are suppressed by more than 24 dB up to the bandwidth of the DRTO.

Estimating and Correcting the Device-Under-Test Transfer Function in Loaded Reverberation Chambers for Over-the-Air Tests

We assess the potential error in measurements of the power transfer function corresponding to a reverberation chamber set-up when different antenna types are used for the reference and device-under-test measurements. We derive a mathematical description of the transfer function that accounts for differences in the amount of unstirred energy, represented by the spatially averaged K factor, arising from various antenna types. Our results show that loaded chamber configurations, combined with reference/device antenna pairs having significantly different radiation patterns, can result in statistically significant errors in the prediction of the transfer function. If it is possible to obtain an estimate of the K factor associated with the device antenna, this correction can improve the estimate of the transfer function that would be experienced by the device under test. Finally, we develop a method that could be used in standardized test methods to bound the uncertainty associated with the unknown K factor for common antenna types.