Measured Power Spectral Density Research Articles

Temporal contrast degradation from mid-spatial-frequency surface error on stretcher mirrors

Temporal contrast degradation due to mid-spatial-frequency error in chirped-pulse amplification stretcher optics is studied. Third-order cross-correlation measurements reveal a temporal peak that appears when using two different mirrors processed by magnetorheological finishing, despite an improvement in rms roughness compared to a third unprocessed mirror. Simulations based on measured power spectral density show how the actual impact on contrast is different from measurements using a typical bandwidth-limited third-order cross correlator. Strategies are proposed to avoid this type of contrast degradation while exploiting computer numerically controlled polishing techniques for enhancement of surface figure and roughness.

Low-frequency noise characterization of positive bias stress effect on the spatial distribution of trap in β-Ga2O3 FinFET

The reliability of a β-Ga2O3 thin-film field-effect transistor is investigated under positive-bias stress (PBS). The transistor has a tri-gate structure with a gate dielectric of Al2O3. By characterizing low-frequency noise (LFN), the spatial distribution of trap in the gate dielectric was quantitatively extracted. The measured power spectral density (PSD) followed a 1/f-shape due to trapping and de-trapping of the channel carriers to and from the gate dielectric. Notably, the vertical distribution of the traps perpendicular to the interface of β-Ga2O3 and Al2O3 was mapped

Assessment of power spectral density of microvascular hemodynamics in skeletal muscles at very low and low-frequency via near-infrared diffuse optical spectroscopies.

In this work, we used a hybrid time domain near-infrared spectroscopy (TD-NIRS) and diffuse correlation spectroscopy (DCS) device to retrieve hemoglobin and blood flow oscillations of skeletal muscle microvasculature. We focused on very low (VLF) and low-frequency (LF) oscillations (i.e., frequency lower than 0.145 Hz), that are related to myogenic, neurogenic and endothelial activities. We measured power spectral density (PSD) of blood flow and hemoglobin concentration in four muscles (thenar eminence, plantar fascia, sternocleidomastoid and forearm) of 14 healthy volunteers to highlight possible differences in microvascular hemodynamic oscillations. We observed larger PSDs for blood flow compared to hemoglobin concentration, in particular in case of distal muscles (i.e., thenar eminence and plantar fascia). Finally, we compared the PSDs measured on the thenar eminence of healthy subjects with the ones measured on a septic patient in the intensive care unit: lower power in the endothelial-dependent frequency band, and larger power in the myogenic ones were observed in the septic patient, in accordance with previous works based on laser doppler flowmetry.

Air/Water Interface Rheology Probed by Thermal Capillary Waves.

Atomic force microscopy (AFM) was used to study the interfacial rheology of air/water interfaces by investigating the thermal capillary fluctuations of surfactant-loaded interfaces. These interfaces are formed by depositing an air bubble on a solid substrate immersed in a surfactant (Triton X-100) solution. An AFM cantilever, in contact with the north pole of the bubble, probes its thermal fluctuations (amplitude of the vibration versus the frequency). The measured power spectral density of the nanoscale thermal fluctuations presents several resonance peaks corresponding to the different vibration modes of the bubble. The measured damping versus the surfactant concentration of each mode presents a maximum and then decreases to a saturation value. The measurements are in good agreement with the model developed by Levich for the damping of capillary waves in the presence of surfactants. Our results show that the AFM cantilever in contact with a bubble is a powerful tool to probe the rheological properties of air/water interfaces.

Identification of Cellular Signal Measurements Using Machine Learning

Spectrum awareness has a plethora of civilian and defense applications, such as spectrum resource management, adaptive transmissions, interference detection, and identification of threat signals. This article proposes an identification neural network (INN)-based model that identifies cellular signals from three different radio access technologies, namely global system for mobile (GSM) communications, universal mobile telecommunications service, and long-term evolution. The proposed INN identifies whether or not the measured power spectral density belongs to a certain cellular signal type. Two data collection approaches (DCAs) are considered: in-band and multiple-band. The over-the-air measurements for the two DCAs show that with low computational complexity, the proposed INN model provides an identification accuracy between 93% and 100%, with a false alarm (FA) rate between 0% and 10%.

Theoretical and Experimental Investigation of the Effect of Pump Laser Frequency Fluctuations on Signal-to-Noise Ratio of Brillouin Dynamic Grating Measurement with Coherent FMCW Reflectometry.

Signal-dependent speckle-like noise has constituted a serious factor in Brillouin-grating based frequency-modulated continuous-wave (FMCW) reflectometry and it has been indispensable for improving the signal-to-noise ratio (S/N) of the Brillouin dynamic grating measurement to clarify the noise generation mechanism. In this paper we show theoretically and experimentally that the noise is generated by the frequency fluctuations of the pump light from a laser diode (LD). We could increase the S/N from 36 to 190 merely by driving the LD using a current source with reduced technical noise. On the basis of our experimental result, we derived the theoretical formula for S/N as a function of distance, which contained the second and fourth-order moments of the frequency fluctuations, by assuming that the pump light frequency was modulated by the technical noise. We calculated S/N along the 1.35 m long optical fiber numerically using the measured power spectral density of the frequency fluctuations, and the resulting distributions agreed with the measured values in the 10 to 190 range. Since higher performance levels are required if the pump light source is to maintain the S/N as the fiber length increases, we can use the formula to calculate the light source specifications including the spectral width and rms value of the frequency fluctuations to achieve a high S/N while testing a fiber of a given length.

Bayesian identification of bolted-joint parameters using measured power spectral density

A Bayesian method for the optimal estimation of parameters that characterize a bolted joint based on measured power spectral density is proposed in this article. Due to uncertainties such as measurement noise and modelling errors, it is difficult to identify joint parameters of a bolted structure accurately with incomplete measured response data. In this article, using the Bayesian probability framework to describe the uncertainty of the joint parameters and using the power spectrum of the structural response of the single-point/multi-point excitation as measurements, the conditional probability density function of the joint parameters is established. Then, the Bayesian maximum posterior estimation is performed by an optimization method. Two simplified bolted-joint models are built in the numerical examples. First, the feasibility of the proposed method in the undamped model is proved. Then, taking advantage of multi-point excitation, the identification accuracy of the proposed method in the damped model is improved. The numerical results show that the proposed method can accurately identify the stiffness and damping characteristics of joint parameters with good robustness to noise. Finally, the joint parameters of the finite element model for an aero-engine casing are identified by the proposed method with satisfactory accuracy.

Frequency domain approaches to locate forced oscillation source to control device

The location of the control devices that produce forced oscillation sources is crucial to the elimination of forced oscillations. This paper proposes two frequency domain approaches to reliably locate oscillation sources from mechanical parts and excitation systems of generators to their control devices. Firstly, the transfer function matrix from mechanical disturbances to mechanical powers is demonstrated to be inherently diagonally dominant. The property is used to monitor and preliminarily locate oscillation sources from mechanical parts. Moreover, this paper proposes a systematic method of power spectral density prediction to further locate oscillation source to control device by using the generator terminal voltage (or generator current) to predict the power spectral density of generator responses. It is shown that for non-source generators, the responses are determined by the generator terminal voltage. The generator with distinct mismatch between predicted and measured power spectral density is identified as the oscillation source. The mismatches between predicted and measured power spectral density of different responses are different, further enabling the method to identify the control device producing the oscillation source. The method of power spectral density prediction is used as a general method to provide further and detailed location results. Demonstrative examples of a 39-bus system and the 547-machine 8647-bus North China Power Grid show the effectiveness of the proposed methods.

Observation of microwave absorption and emission from incoherent electron tunneling through a normal-metal\u2013insulator\u2013superconductor junction

We experimentally study nanoscale normal-metal–insulator–superconductor junctions coupled to a superconducting microwave resonator. We observe that bias-voltage-controllable single-electron tunneling through the junctions gives rise to a direct conversion between the electrostatic energy and that of microwave photons. The measured power spectral density of the microwave radiation emitted by the resonator exceeds at high bias voltages that of an equivalent single-mode radiation source at 2.5 K although the phonon and electron reservoirs are at subkelvin temperatures. Measurements of the generated power quantitatively agree with a theoretical model in a wide range of bias voltages. Thus, we have developed a microwave source which is compatible with low-temperature electronics and offers convenient in-situ electrical control of the incoherent photon emission rate with a predetermined frequency, without relying on intrinsic voltage fluctuations of heated normal-metal components or suffering from unwanted losses in room temperature cables. Importantly, our observation of negative generated power at relatively low bias voltages provides a novel type of verification of the working principles of the recently discovered quantum-circuit refrigerator.

Performance enhancement of AlGaN/InGaN MQW LED with GaN/InGaN superlattice structure

A gallium nitride (GaN)-based light-emitting diode (LED) structure with 10-period superlattice layers (SLs) of p -GaN/i -In0.2 Ga0.8 N, inserted between p -GaN and 12-period Al0.16 Ga0.84 N/In0.2 Ga0.8 N multiple quantum well (MQW) layers, is numerically designed and examined using physical device simulator, SILVACO. The SL structure acts as a confinement layer of holes that serves to mitigate the effects of efficiency droop and leakage current within LED. Consequently, inserting SL structure within epitaxial domain tends to improve the current-spreading capability of the device. The proposed structure is compared with the conventional LED without SL and enhanced LED performance has been demonstrated by the SL-based device. The measured power spectral density and luminous power of the under-investigated device are 8.27 W/cm eV at 433 nm and 3.1 W/cm at 12.32 kA/cm2, respectively. Correspondingly, the external QE of 25.4% has been determined that is higher than the earlier LED reported with 10-period SL structure of p -GaN/i -In0.2 Ga0.8 N and 12-period MQWs of GaN/indium GaN configuration. Moreover, another structure based on 30-period SL was also investigated and the output results exhibited no significant improvement in the efficiency due to thermionic emission and thickness-induced strain factors.

Features of noise in ultrathin gold nanowire structures

Bundles of ultrathin gold nanowires (Au NWs, 2 nm in diameter) were fabricated and subsequently assembled onto electrodes. Electrical measurements and noise spectroscopy techniques were applied for sample characterization. The peculiarities of noise behavior in the system of bundles of ultrathin gold nanowires were studied. The measured power spectral density of flicker noise was proportional to current squared, which reflects ohmic behavior in NW structures. Lorentzian-shaped components were revealed in the noise spectra. They are suggested to be the result of the participation of molecules adsorbed on the NW surface in transport phenomena. The presence of molecular interfaces was confirmed by high-resolution transmission electron micrographs. The adsorbed molecules play an important role in charge transport and therefore determine electrical and noise properties of the NW structures. The results should be taken into account for the development of NW devices for sensing and molecular electronics applications.

Structural damage identification based on power spectral density sensitivity analysis of dynamic responses

A new method is proposed to identify locations and severities of structural damages based on the power spectral density sensitivity analysis. Firstly, the structural responses and power spectral density under stationary and random excitations are calculated using pseudo excitation method. Then, the sensitivities of power spectral density with respect to the structural damage parameters are obtained. Finally, the finite element model updating method is adopted to identify the structural damages from the calculated and the simulated measured power spectral density. Two numerical examples demonstrate the satisfactory identification results obtained from the present method.

Design, fabrication and characterization of a low-noise Z-axis micromachined gyroscope

This paper presents the design, fabrication and characterization of a low-noise Z-axis micromachined gyroscope. A triple-mass scheme is adopted to obtain a decoupled mode and matched vibration structure. The gyroscope was fabricated based on a 30-μm silicon on insulator wafer. A DRIE, lag-effect-based, dicing-free process was used to easily and rapidly create the gyroscope devices without requiring normal dicing. The fabricated gyroscopes were vacuum packaged (pressure of approximately 2,000 Pa) and tested. The measured power spectral density of the noises is as low as 17.5 μV/?Hz, and the resolution of the fabricated gyroscope is 7.1 × 10---4°/s/?Hz (2.52°/h/?Hz).

Design and Implementation of an Optimized Double Closed-Loop Control System for MEMS Vibratory Gyroscope

This paper describes the development and experimental evaluation of a microelectromechanical system vibratory gyroscope using an optimized double closed-loop control strategy. An automatic gain control self-oscillation interface is used to resonate the gyroscope in the drive mode; the sense mode is controlled by a sixth-order continuous-time and force-feedback band-pass sigma-delta modulator. The parameters of both control loops are optimized by a genetic algorithm (GA). System level simulations show that the settling time of the drive mode self-oscillation is 125 ms, the root mean square displacement of the proof mass is in the sense mode, and the signal-to-noise ratio is 90 dB in a bandwidth of 64 Hz with a 200 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> /s angular rate input signal. The system is implemented using symmetrical and fully decoupled silicon on insulator gyroscope operating at atmospheric with the circuit implemented on printed circuit board. The measured power spectral density of the output bitstream shows an obvious band-pass noise shaping and a deep notch at the gyroscope resonant frequency. The measured noise floor is approximately -120 dBV/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> . In the drive mode, the relative drift of the resonant frequency and amplitude is 3.2 and 10.7 ppm for 1 h measurements, respectively. The settling time, scale factor, zero bias stability, and bandwidth of the gyroscope controlled by the optimized control system are 200 ms, 22.5 mV/ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> /s, 34 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> /h, and 110 Hz, respectively. This is compared with a non-optimized system for which the corresponding values are 300 ms, 17.3 mV/ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> /s, 58 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> /h, and 98 Hz; hence, by GA optimization a considerable performance improvement is achieved.

Experiments on Tree Reflectivity at C, X, K and Ka-bands

Dedicated field portable radars have been used to measure the reflectivity of identified deciduous and coniferous trees at C to Ka frequency bands with perpendicular illumination. Actual test radars, their electrical parameters including antennas and calibration devices are described. First results show summer time reflectivities from −30 to −10 dB (m2/m2) for deciduous trees and from −30 to 0 dB (2/m2) for coniferous samples, depending on frequency band. Late autumn and spring measurements show particularly well in Ka-band the absence of leaves. Variations in histogram detail, from one frequency to another, exceed 10 dB despite the cumulative probabilities for all bands are quite similar. The measured power spectral density of deciduous trees in the spring was typically 20 dB/Hz at X-band and 30 dB/Hz in Ka-band. Midsummer tests gave values close to 15 and 25 dB/Hz.

Resonant diffusion of energetic electrons by narrowband Z mode waves in Saturn's inner magnetosphere

AbstractBanded emissions near 5 and 20 kHz are observed by the Cassini Radio and Plasma Wave Science (RPWS) instrument and are identified as Z mode waves in the regions where the plasma frequency is below the electron gyrofrequency. Using wave parameters derived from Gaussian fits to the measured power spectral density for narrow‐band Z mode waves and the ambient density and magnetic field information at L = ~4, we compute the bounce‐averaged diffusion coefficients for the Saturnian radiation belt electrons. Our results indicate that 5 kHz Z mode emissions can cause resonant scattering of electrons from ~2 MeV to tens of megaelectron volts, while 20 kHz waves can interact with electrons from hundreds of kiloelectron volts to ~2 MeV. In principle, the rate of momentum diffusion can be an order of magnitude larger than that of pitch angle diffusion and cross diffusion for electrons at equatorial pitch angles &gt; ~10°, suggesting that Z mode waves on Saturn are responsible for local acceleration of radiation belt energetic electrons at intermediate pitch angles.

Fundamental quantum noise mapping with tunnelling microscopes tested at surface structures of subatomic lateral size

We present a measurement scheme that enables quantitative detection of the shot noise in a scanning tunnelling microscope while scanning the sample. As test objects we study defect structures produced on an iridium single crystal at low temperatures. The defect structures appear in the constant current images as protrusions with curvature radii well below the atomic diameter. The measured power spectral density of the noise is very near to the quantum limit with Fano factor F = 1. While the constant current images show detailed structures expected for tunnelling involving d-atomic orbitals of Ir, we find the current noise to be without pronounced spatial variation as expected for shot noise arising from statistically independent events.

Parameter optimization for a high-order band-pass continuous-time sigma-delta modulator MEMS gyroscope using a genetic algorithm approach

This paper describes a novel multiobjective parameter optimization method based on a genetic algorithm (GA) for the design of a sixth-order continuous-time, force feedback band-pass sigma-delta modulator (BP-ΣΔM) interface for the sense mode of a MEMS gyroscope. The design procedure starts by deriving a parameterized Simulink model of the BP-ΣΔM gyroscope interface. The system parameters are then optimized by the GA. Consequently, the optimized design is tested for robustness by a Monte Carlo analysis to find a solution that is both optimal and robust. System level simulations result in a signal-to-noise ratio (SNR) larger than 90 dB in a bandwidth of 64 Hz with a 200° s−1 angular rate input signal; the noise floor is about −100 dBV Hz−1/2. The simulations are compared to measured data from a hardware implementation. For zero input rotation with the gyroscope operating at atmospheric pressure, the spectrum of the output bitstream shows an obvious band-pass noise shaping and a deep notch at the gyroscope resonant frequency. The noise floor of measured power spectral density (PSD) of the output bitstream agrees well with simulation of the optimized system level model. The bias stability, rate sensitivity and nonlinearity of the gyroscope controlled by an optimized BP-ΣΔM closed-loop interface are 34.15° h−1, 22.3 mV °−1 s−1, 98 ppm, respectively. This compares to a simple open-loop interface for which the corresponding values are 89° h−1, 14.3 mV °−1 s−1, 7600 ppm, and a nonoptimized BP-ΣΔM closed-loop interface with corresponding values of 60° h−1, 17 mV °−1 s−1, 200 ppm.

The effects of laser-sheet thickness on dissipation measurements in turbulent non-reacting jets and jet flames

The effects of laser-sheet thickness on planar laser measurements of scalar gradients in turbulent flows are studied. Experiments are performed in the near field of a turbulent, non-premixed, axisymmetric jet flame and in the near field of a non-reacting, isothermal turbulent jet. Laser Rayleigh scattering provides two-dimensional measurements of the instantaneous temperature and mixture fraction fields in the flame and non-reacting jet, respectively. The effect of spatial resolution on measurements of the mean dissipation and the power spectral density of axial temperature and mixture-fraction gradients is examined. The effect of varying the laser-sheet thickness is compared to that of spatial filtering within the image plane. Measurements of the mean dissipation and power spectral density are significantly less sensitive to resolution degradation in the non-differentiated dimensions than in the differentiated dimension. For example, on the jet flame centreline, the dissipation-cut-off microscale, which is determined from the measured power spectral density, is overestimated by 9% when the beam-waist thickness is increased from a 1/e-squared width of 160 µm to 624 µm. In contrast, spatial filtering along the direction of differentiation with a smoothing kernel of 624 µm width produces a bias of 76% in the cut-off microscale. These results experimentally confirm the theoretical analysis of previous studies. A simple spatial model illustrates the origin of this difference and approximately predicts its magnitude for both planar and line measurements. A criterion for matching in-plane and out-of-plane resolution is established. For many planar gradient measurements, considerably less out-of-plane resolution is needed than in-plane resolution. The combined effects of noise and spatial averaging on the dissipation measurements are also briefly examined.

Ultra-Low Vibration Pulse-Tube Cryocooler Stabilized Cryogenic Sapphire Oscillator With ${\hbox{10}}^{-16}$ Fractional Frequency Stability

A low maintenance long-term operational cryogenic sapphire oscillator has been implemented at 11.2 GHz using an ultra-low-vibration cryostat and pulse-tube cryocooler. It is currently the world's most stable microwave oscillator employing a cryocooler. Its performance is explained in terms of temperature and frequency stability. The phase noise and the Allan deviation of frequency fluctuations have been evaluated by comparing it to an ultra-stable liquid-helium cooled cryogenic sapphire oscillator in the same laboratory. Assuming both contribute equally, the Allan deviation evaluated for the cryocooled oscillator is sigma_y = 1 x 10^-15 tau^-1/2 for integration times 1 < tau < 10 s with a minimum sigma_y = 3.9 x 10^-16 at tau = 20 s. The long term frequency drift is less than 5 x 10^-14/day. From the measured power spectral density of phase fluctuations the single side band phase noise can be represented by L_phi(f) = 10^-14.0/f^4+10^-11.6/f^3+10^-10.0/f^2+10^-10.2/f+ 10^-11.0 for Fourier frequencies 10^-3<f<10^3 Hz in the single oscillator. As a result L_phi approx -97.5 dBc/Hz at 1 Hz offset from the carrier.