Transfer Function Measurements Research Articles

TE correlation coefficient of Planck legacy data

Testing deviations from the $\Lambda$CDM model using the Cosmic Microwave Background (CMB) power spectra requires a pristine understanding of instrumental systematics. In this work we discuss the properties of a new observable ${\cal R}^{TE}_{\ell}$, the correlation coefficient of temperature and E modes. We find that this observable is mostly unaffected by systematics introducing multiplicative biases such as errors in calibration, polarisation efficiency, beam and transfer function measurements. We discuss the dependency of this observable on the cosmological model and derive its statistical properties. We then compute the T-E correlation coefficients of Planck legacy data and compare them with expectations from the Planck best-fit $\Lambda$CDM and foreground model.

A Bayesian spherical harmonics source radiation model for sound field control.

In sound field reproduction and sound field control systems, the acoustic transfer functions between a set of sources and an extended reproduction area need to be accurately estimated in order to achieve good performance. This implies that large amounts of measurements should be performed if the area is large compared to the wavelengths of interest. In this paper, a method for reconstructing these transfer functions in highly damped conditions is proposed by using only a small number of measurements in the reproduction area. The source radiation is modeled with the spherical harmonics basis and its amplitude coefficients are fitted with Bayesian inference. This approach is validated in a sound field control experiment where a set of 12 control loudspeakers attenuate the sound pressure level generated by a set of six primary loudspeakers in a quiet zone while minimizing their radiation into a listening zone. The performance of the approach is studied by analyzing the sound field reconstruction and the sound field control performance. It is shown that it is possible to get-with few measurements and the source radiation model-results similar to those achieved using a dense grid of transfer function measurements.

Dynamic Modeling and Analysis of Buck Converter Based Solar PV Charge Controller for Improved MPPT Performance

Maximum power point tracking (MPPT) control is a key functionality in solar photovoltaic (PV)-based power conversion systems. A variety of perturbative MPPT control schemes are available in the literature, many of which are voltage-based techniques wherein the PV bus voltage is perturbed and set to the required level by an appropriate converter control that achieves the MPP tracking. However, a comprehensive plant model of the PV-fed converter system and a systematic control design of the PV bus voltage loop that facilitates the design of the MPPT control is not available in the literature. In this article, a detailed small-signal model is proposed for a single-stage PV-fed buck converter that acts as a battery charge-controller. The effects of parasitic storage elements present in the PV source as well as the interconnecting cable from rooftop are discussed in detail, along with the impact of series–parallel connection of the PV modules for power scaling. A full-order as well as a reduced-order model of the system is proposed and the various relevant transfer-functions are analytically derived. Based on this model and the control to converter input bus voltage transfer-function $\widetilde{v}_{\text{in}}/\widetilde{d}$ , a systematic design procedure for the PV bus voltage controller is proposed. It is shown that such a design facilitates the selection of perturbation period of a typical MPPT control algorithm for an improved tracking performance. The steps involved in the system model development and the control design are generalized and can be extended to other converter topologies as well. Active transfer-function measurements and experiments conducted on a 1-kW charge-controller hardware prototype validate the accuracy of the proposed model and the tracking performance of the MPPT control.

Technical Note: Accuracy of MTF measurements with an edge phantom at megavoltage x-ray energies.

Measurement of the modulation transfer function (MTF) is performed by evaluating the response of an imaging system to a predefined input. To obtain accurate results when using an edge phantom, the detector input signal must resemble an ideal step function. The MTF of megavoltage (MV) imagers used in radiotherapy has been measured with highly absorbing edge phantoms fabricated from thick metal blocks. This study investigates the influence of the edge phantom design on the accuracy of the resulting MTF. The MTF of an electronic portal imaging device (EPID) was measured at 6MV beam quality with four edge phantoms made of lead with 1.3, 3.3, 5.0, and 10.0cm thickness. Monte Carlo simulations were carried out for these and a selection of tungsten phantoms to determine the photon fluence at the imaging plane and quantify the systematic error in the MTF introduced by the edge phantom design. The measured MTF depends on the design of the edge phantom. The detector input signal of a thin phantom is affected by secondary radiation from the phantom itself, causing an overestimation of the MTF. The amount of secondary radiation can be reduced by increasing the phantom thickness or introducing an air gap between the phantom and the detector. Both methods introduce geometric unsharpness, which can result in an underestimation of the true MTF. Edge phantoms made from 4.0cm thick tungsten or 5.0cm thick lead induce comparatively small systematic errors of below 3% or 5%, respectively. When MTF measurements are conducted at MV energies, even a highly absorbing edge phantom will introduce a systematic error of several percent. Direct comparison of MTFs obtained with different edge phantoms should therefore be treated with caution.

Bayesian inference for modal identification in ducts with a shear flow.

An in-duct modal decomposition technique is described. The basis for the technique is to consider the decomposition as an inference problem. Using transfer function measurements at the duct walls, a Bayesian inference is conducted to evaluate the acoustic modal coefficients in the presence of uncertainties. These uncertainties encompass model errors, microphone measurements error, and uncertainty on the flow profile. The formalism of the direct problem of modal decomposition in a ducted shear flow is first developed. The case of a circular cross-section duct is then treated without and with a flow, using synthetic noisy signals for the inference problem.

Methods of DC/DC converter transfer function measurements, based on data acquired in the time domain

Methods of DC/DC converter transfer function measurements, based on data acquired in the time domain

Analysis of the Dynamics of Premixed Methane and Biogas Flames Based on Cross-correlation Maps

ABSTRACT In this study, the cross-correlation method (CCM) has been investigated in detail and applied to analyze the dynamic response of premixed methane and biogas flames. CCM mapping was proposed in a recent study as an effective, and notably simple, method to evaluate the actual contribution of different flame zones to its global dynamic response. The physical interpretation of cross-correlation maps has been further investigated and validated by means of spatially resolved measurements of the local flame transfer function along the flame. The results obtained are consistent with vortex shedding being the phenomenon governing the dynamics of the flames studied. Methane and biogas flames exhibited different response to fluctuations. The analysis of cross-correlation maps revealed changes in the location of regions with strongest response to velocity fluctuations. Furthermore, this method was applied to determine the variations in the convective length associated with the phase delay between the oscillations in velocity and in heat release rate. In spite of the broad differences among the flames studied, all of them were consistent with the perturbations traveling along the flame at approximately one half of the flow velocity, in coincidence with the findings of some recent works.

On the Influence of Transfer Function Noise on Sound Zone Control in a Room

Sound zones are valuable in scenarios where multiple people are present in the same room but want to listen to individual audio content without wearing headphones. The purpose of sound zone methods is to minimize the acoustic leakage between the zones by controlling multiple loudspeakers. This requires knowledge of how the loudspeakers interact with the room and radiate sound to the zones. That interaction is characterized by the transfer functions between the loudspeakers and microphones sampling the sound field in the zones. In this paper, the effect on the acoustic separation due to inherent noise in in situ transfer function measurements is investigated. The attainable separation is analyzed in the frequency range 20–300 Hz by means of the eigenfunctions of a rectangular room. The concept of observable degrees of freedom is introduced to indicate the number of active eigenfunctions, which are different within the zones at a given frequency. Likewise, controllable degrees of freedom indicate whether each source can excite the active eigenfunctions independently. It is argued that high separation can be achieved when the observable degrees of freedom are fewer than the controllable, and the target sound field can be described by the observable degrees of freedom. However, to attain this high separation it is a requirement that the details in the transfer functions associated with these degrees of freedom can be resolved in the presence of the measurement noise. For both simulated and experimental conditions the transfer functions are estimated using Bayesian inference and the uncertainty in the estimates is used to automatically regularize the sound field control. This regularization is seen to improve the performance when the measurement noise is correlated between the microphones and have little effect when the noise is uncorrelated.

Wideband UHF DQPSK Backscatter Communication in Reverberant Cavity Animal Cage Environments

Many neuroscience experiments with animal subjects require free motion of the animal within a metal cage environment. In such cage environments, wireless communication with implanted devices, e.g., neural recording and stimulation is particularly challenging because the metal cage walls form a reverberant cavity with dense multipath. In the case of backscatter communication with the implanted device, the multipath challenge is particularly acute because of the round-trip nature of the backscatter channel. This paper demonstrates the reverberant cavity effect via measurement of the channel transfer function inside a metal cage used for nonhuman primate research in the 902–928 MHz ultrahigh-frequency industrial, scientific, and medical band. A reduced-size ceramic patch antenna developed for the Neurochip neural recording and stimulation device was affixed to a saline tissue proxy, while a commercial air-dielectric patch antenna was affixed to the ceiling of the cage. A measured 3 dB channel bandwidth greater than 6.5 MHz with a port-to-port insertion loss between 14 and 37 dB was achieved at 126 surveyed locations within the cage volume. A 6.25 Mb/s backscatter data uplink using a differential quadrature phase shift keying constellation was successfully validated inside the cage, with effectively 0% packet error rate for all but two of the surveyed locations. The simulation and experimental results show good agreement and reveal that wideband backscatter communication systems can perform well despite the significant multipath inside the reverberant cage environment.

Investigation of speckle imagery spectral estimation challenges for modulation transfer function measurements

Efforts to extend speckle-based focal plane array modulation transfer function measurements beyond the detector Nyquist frequency have unearthed challenging spectral estimation issues. In an attempt to better understand the task of speckle imagery spectral estimation, we explore the nuances of various estimation techniques, making comparisons using both real speckle imagery and simulated data. Parameters and features of the techniques investigated include number of image realizations, the size of image realizations, and applications of windows to speckle imagery spectral estimation. Real-world testing considerations such as laser stability and the challenge of collecting significant numbers of independent image realizations are addressed in the analysis. Results from this research show the advantage increasing the number of realizations has on estimation variance, the robustness of smaller realization segments when battling speckle field imagery spatial nonuniformities, the benefits of windowing image segments with regard to power spectral density estimation accuracy, and the impact that the increasing aperture area has on system signal-to-noise ratio.

Rapid acquisition of the 3D MRI gradient impulse response function using a simple phantom measurement.

To provide a simple tool for rapid measurement of the 3D gradient modulation transfer function (GMTF) of clinical MRI systems using a phantom. Knowledge of the transfer function is useful for gradient chain characterization, system calibration, and improvement of image reconstruction results. Starting from the well-established thin slice method used for phantom-based measurement of the 1D GMTF, we add phase encoding to partition the thin slices into voxels that act as localized field probes. From the signal phase evolution measured at the 3D voxel positions, the GMTF can be derived for cross and higher order spatial terms represented by spherical harmonics up to 3rd order. Using spherical phantoms, 16 GMTFs representing all terms up to 3rd order harmonics can be determined in a scan time of <2 min. A large voxel volume of >1 mL yields high SNR, enabling signal acquisition using the system's body coil. The method is applied for improving system calibration and for characterizing the effect of additional hardware in the bore. The presented method seems well-suited for rapid measurement of the GMTF of a clinical system, as it delivers high-quality results in a short scan time.

Detection and quantitative diagnosis of axial displacement of transformer winding by frequency response analysis

Transfer functions of a transformer whose winding is axially displaced are measured in several connection schemes of frequency response analysis (FRA), and the sensitivities of the measured transfer functions in these connection schemes to the axial displacement are evaluated. For the objective evaluation, some numerical indices are employed, and it is found that the inductive inter-winding measurement is the most sensitive to the axial displacement. Furthermore, a simple circuit model is proposed to reproduce the measured transfer functions. All the parameters of the proposed circuit model can be determined from the measured transfer functions in FRA. The proposed model can also reproduce the measured transfer functions with the axial displacement by varying one parameter. The variation of the parameter in reproducing the transfer function with the axial displacement show good linear relation to the amount of the axial displacement. This indicates the possibility of a quantitative diagnosis of the axial displacement by FRA.

Absolute Deflection Measurements in a Micro- and Nano-Electromechanical Fabry-Perot Interferometry System.

Fabry-Perot laser interferometry is a common laboratory technique used to interrogate resonant micro- and nano-electromechanical systems (MEMS/NEMS). This method uses the substrate beneath a vibrating MEMS/NEMS device as a static reference mirror, encoding relative device motion in the reflected laser power. In this work, we present a general approach for calibrating these optical systems based on measurements of large-amplitude motion that exceeds one half of the laser wavelength. Utilizing the intrinsic nonlinearity of the optical transduction, our method enables the direct measurement of the system's transfer function (motion-to-detected-voltage). We experimentally demonstrate the use of this technique to measure vibration amplitudes and changes in the equilibrium position of a MEMS/NEMS device using monolithic silicon nitride and silicon cantilevers as sample systems. By scanning the laser along a cantilever surface, we spatially map static and dynamic deflection profiles simultaneously, and then compare the static profile against results from a commercial optical profilometer. We further demonstrate extension of our calibration technique to measurements taken at small amplitudes, where the optical transduction is linear, and to those taken in the frequency domain by a lock-in amplifier. Our aim is to present a robust calibration scheme that is independent of MEMS/NEMS materials and geometry, to completely negate the effects of nonlinear optical transduction, and to enable the assessment of excitation forces and MEMS/NEMS material properties through the accurate measurement of the MEMS/NEMS vibrational response.

Measurement and analysis of modulation transfer function of digital image sensors

With the ceaseless development of technology, optical imaging systems have been extensively used in numerous different fields. As a result, verifying the performance of such systems has taken on significant importance. To test these systems, a commonly used metric is the modulation transfer function (MTF) because it reflects the spatial-frequency response of optical systems. To determine how best to measure the MTF of an image sensor, we use the slanted-knife-edge method to measure optical patterns and a black-and-white face sensor to receive patterns, following which we numerically calculate the MTF. We use the MTF of the Nyquist frequency (50 lp/mm) of an image sensor as the measured value, and the slanting angle for the slanted-knife-edge method ranges from 2° to 10°, which is known to provide accurate results. We then compare and analyze the experimental data and explore how inaccurate slanting angles affect the measured MTF. Finally, we propose a “program correction method” in which we input a corrected slanting angle into the region-of-interest program to correct for errors in the measurement of the MTF. The corrected parameters provide a more accurate measurement of the MTF of the given pattern.

Transfer function measurement for the SSRF SRF system

A digital transfer function measurement system has been embedded in the low-level radio frequency (LLRF) system of the storage ring of the Shanghai Synchrotron Radiation Facility. The measurement results indicate that the decreased control accuracy at high current is primarily owing to ripples from the high-voltage power supply, the transient beam loading effect, and the digital aliasing effect. The current LLRF algorithm is not able to suppress these disturbances.

Modeling and Analysis of the Noise Performance of the Capacitive Sensing Circuit with a Differential Transformer.

Capacitive sensing is a key technique to measure the test mass movement with a high resolution for space-borne gravitational wave detectors, such as Laser Interferometer Space Antenna (LISA) and TianQin. The capacitance resolution requirement of TianQin is higher than that of LISA, as the arm length of TianQin is about 15 times shorter. In this paper, the transfer function and capacitance measurement noise of the circuit are modeled and analyzed. Figure-of-merits, including the product of the inductance L and the quality factor Q of the transformer, are proposed to optimize the transformer and the capacitance measurement resolution of the circuit. The LQ product improvement and the resonant frequency augmentation are the key factors to enhance the capacitance measurement resolution. We fabricated a transformer with a high LQ product over a wide frequency band. The evaluation showed that the transformer can generate a capacitance resolution of 0.11 aF/Hz1/2 at a resonant frequency of 200 kHz, and the amplitude of the injection wave would be 0.6 V. This result supports the potential application of the proposed transformer in space-borne gravitational wave detection and demonstrates that it could relieve the stringent requirements for other parameters in the TianQin mission.

Personalized signal-independent beamforming for binaural hearing aids.

The effect of personalized microphone array calibration on the performance of hearing aid beamformers under noisy reverberant conditions is studied. The study makes use of a new, publicly available, database containing acoustic transfer function measurements from 29 loudspeakers arranged on a sphere to a pair of behind-the-ear hearing aids in a listening room when worn by 27 males, 14 females, and 4 mannequins. Bilateral and binaural beamformers are designed using each participant's hearing aid head-related impulse responses (HAHRIRs). The performance of these personalized beamformers is compared to that of mismatched beamformers, where the HAHRIR used for the design does not belong to the individual for whom performance is measured. The case where the mismatched HAHRIR is that of a mannequin is of particular interest since it represents current practice in commercially available hearing aids. The benefit of personalized beamforming is assessed using an intrusive binaural speech intelligibility metric and in a matrix speech intelligibility test. For binaural beamforming, both measures demonstrate a statistically signficant (p < 0.05) benefit of personalization. The benefit varies substantially between individuals with some predicted to benefit by as much as 1.5 dB.

Parameter study of friction damping ring for railway wheels based on modal analysis

A straight web wheel with friction damping rings in different configurations was tested in a laboratory. A total sound pressure level (SPL) test of the wheel under different preload conditions and damping ring installation positions was conducted, and the existence of an “optimum preload” was indicated. Modal tests and analyses of the wheel under different configurations of the damping rings were conducted and verified using a vibration transfer function measurement. The results indicate that a greater number of damping rings clearly improves their vibration mitigation effects. Furthermore, the installation of the damping ring on the inside groove of the wheel has a superior effect on the first four modes, whereas an outside installation has a better effect on the last two modes. Using a two-degree-of-freedom system, the “optimum contact stiffness” existing between the damping ring and the wheel at each mode is confirmed, where the variation tendency of the contact stiffness is to reach and exceed the optimum value from low frequency to high frequency as the preload value increases.

Aspects of the measurement of the acoustic transfer function in a cavitation tunnel

Model scale experiments represent one of the most effective, and probably the most commonly used, approaches for the study of cavitating propeller noise. Despite this, the acquirement of consistent data of propeller noise from tests in model scale facilities is still a challenging task. Data collected with such an approach are usually affected by several scale effects and facility related issues which make such experiments still very complex. Among others, the effect of the facility itself as acoustical surrounding must be carefully addressed in order to properly estimate propeller source levels in ideal free field conditions. This is currently carried out, when possible, by measuring facility transfer functions and applying such functions as a correction to measured noise spectra. The development of robust procedures for the measurement of these transfer functions is thus very important. In the present work, this problem is addressed by discussing in details techniques, merits and possible problems related to this topic.

On the Influence of Continuous Subject Rotation During High-Resolution Head-Related Transfer Function Measurements

In recent years, the required time for high resolution individual head-related transfer function (HRTF) measurements has been reduced by the use of microphone or loudspeaker arrays and with parallelization of measurement signals. While a short measurement time is beneficial during HRTF measurement of any resolution, it is essential for the acquisition of high resolution individual HRTF measurements since subjects cannot remain motionless over extended periods of time. This decrease in measurement time can be achieved by using continuous rotation during the measurement, instead of step-wise subject positioning. This paper examines the influence of such continuous measurement rotation with sine sweep measurements and the necessary subsequent post-processing. The goal is to identify a rotation speed, from a predefined set of speeds, that does not have an audible influence on the measured HRTF itself. To this end, a subjective and objective comparison between the two measurement modes is presented in this paper. Five HRTF measurements of an artificial head are acquired using different rotation speeds. They are compared objectively to a reference measurement of the same head performed with step-wise measurement rotation in the same measurement setup. A subjective listening experiment examines whether the found differences between the measurements do result in an audible difference. The objective evaluation showed expected trends regarding the dependency of the error on the measurement speed and is used to validate the used signal post-processing. The subjective evaluation showed that with a rotation of 3.8 $^\circ$ /s no audible differences to the reference are present.