Hz Noise Research Articles

A CMOS Optoelectronic Transimpedance Amplifier Using Concurrent Automatic Gain Control for LiDAR Sensors

This paper presents a novel optoelectronic transimpedance amplifier (OTA) for short-range LiDAR sensors used in 180 nm CMOS technology, which consists of a main transimpedance amplifier (m-TIA) with an on-chip P+/N-well/Deep N-well avalanche photodiode (P+/NW/DNW APD) and a replica TIA with another on-chip APD, not only to acquire circuit symmetry but to also obtain concurrent automatic gain control (AGC) function within a narrow single pulse-width duration. In particular, for concurrent AGC operations, 3-bit PMOS switches with series resistors are added in parallel with the passive feedback resistor in the m-TIA. Then, the PMOS switches can be turned on or off in accordance with the DC output voltage amplitudes of the replica TIA. The post-layout simulations reveal that the OTA extends the dynamic range up to 74.8 dB (i.e., 1 µApp~5.5 mApp) and achieves a 67 dBΩ transimpedance gain, an 830 MHz bandwidth, a 16 pA/√Hz noise current spectral density, a −31 dBm optical sensitivity for a 10−12 bit error rate, and a 6 mW power dissipation from a single 1.8 V supply. The chip occupies a core area of 200 × 120 µm2.

AstroSat’s view of 4U 1735−44: spectral, temporal, and type I X-ray burst studies

ABSTRACT This study utilizes the simultaneous broad-band observations of 4U 1735−44 from AstroSat, offering enhanced spectral and temporal resolution, to investigate its spectral properties, temporal behaviour, and burst characteristics. Spectral, type I X-ray burst, and temporal analyses on 4U 1735−44 were performed using AstroSat/Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) observations. The hardness–intensity diagram from LAXPC-20 showed a positive correlation between hardness and intensity, with a pattern resembling the banana branch typical of atoll sources. Spectral analysis carried out in the 0.7–20.0 keV energy range, using the model combination – $\tt {constant}$$\times$$\tt {tbabs}$ ($\tt {nthcomp}$$+$$\tt {diskbb}$$+$$\tt {bbodyrad}$), suggested a cool accretion disc truncated at a large distance from the neutron star in the system. Time-resolved spectral studies of two type I X-ray bursts detected from the source revealed evidence of photospheric radius expansion, allowing for an estimation of the source distance. Temporal analysis showed the presence of low-frequency quasi-periodic oscillation at $\sim$69 Hz (3.3$\sigma$ significance with more than 99 per cent confidence) and prominent noise features below 30 Hz.

The Effect of Rhythmic Audio-Visual Stimulation on Inhibitory Control: An ERP Study.

Inhibitory control, as an essential cognitive ability, affects the development of higher cognitive functions. Rhythmic perceptual stimulation has been used to improve cognitive abilities. It is unclear, however, whether it can be used to improve inhibitory control. This study used the Go/NoGo task and the Stroop task to assess various levels of inhibitory control using rhythmic audio-visual stimuli as the stimulus mode. Sixty subjects were randomly divided into three groups to receive 6 Hz, 10 Hz, and white noise stimulation for 30 min. Two tasks were completed by each subject both before and after the stimulus. Before and after the task, closed-eye resting EEG data were collected. The results showed no differences in behavioral and EEG measures of the Go/NoGo task among the three groups. While both 6 Hz and 10 Hz audio-visual stimulation reduced the conflict effect in the Stroop task, only 6 Hz audio-visual stimulation improved the amplitude of the N2 component and decreased the conflict score. Although rhythmic audio-visual stimulation did not enhance response inhibition, it improved conflict inhibition.

Localization of real and tangent-law panned phantom sound sources in the frontal horizontal plane

Auditory source separations of as little as 1 degree are detectable. However, presenting auditory stimuli at small separations presents technical challenges, with loudspeaker separation limited by transducer diameter. An alternative procedure is to utilize phantom sources, with the perceived position of a single source determined by the relative output levels of two spatially separated loudspeakers. Therefore, it is important to determine whether real and phantom sources can be localized with the same precision. In the present experiment, listeners localized real (individual) sources and phantom sources computed using a tangent-law model giving the same nominal azimuthal angles as the real sources. Listeners used a laser pointer to indicate perceived source location. Infrared cameras detected pointer position with responses stored in terms of azimuth. Signals were broadband or narrowband (300-700 Hz and 3800–4200 Hz) noise, 100 or 500 ms in duration. Generally, phantom sources were localized with less precision than real sources, and high-frequency signals were localized with less precision than broadband or low-frequency signals, with no effect of duration. Results show that phantom sources are localized with sufficient accuracy and precision to be useful in assessing auditory spatial acuity, but they are not localized with the same precision as real sources.

Characterization and spatiotemporal variations of ambient seismic noise in eastern Bangladesh

This study analyses the ambient noise field recorded by the seismic network, TREMBLE, in Bangladesh, operational since late 2016. Horizontal-vertical spectral ratios confirm the placement of stations on sediment, many situated on thick sedimentary columns, consistent with local geology. Noise across the broadband spectrum is systematically examined. A high amplitude local microseism (0.4–0.8 Hz) is recorded, originating near the coast and modulated by local tides. The secondary microseism (0.15–0.35 Hz) correlates strongly with wave height in the Bay of Bengal and varies with seasons, with greater power and higher horizontal amplitude in the monsoon season when the wave height is highest. The microseism increases in amplitude and decreases in frequency as a tropical depression moves inland. The primary microseism (∼0.07–0.08 Hz) exhibits no seasonal changes in power but display strong horizontal energy which changes with seasons. Low frequency (0.02–0.04 Hz) noise on the horizontal components has a 24-h periodicity, due to instrument tilt caused by atmospheric pressure changes. A station located next to the major Karnaphuli River shows elevated energy at ∼5 Hz correlated to periods of high rainfall. Anthropogenic noise (∼4–14 Hz) is station-dependent, demonstrating changing patterns in human activity, such as during Ramadan, national holidays and the COVID pandemic. Our work holds implications for seismic deployments, earthquake, and imaging studies, while providing insights into the interaction between the atmosphere, ocean, and solid Earth.

Optical frequency comb generation using cascaded injection of semiconductor lasers

We study optical frequency comb (OFC) generation using cascaded injection of semiconductor lasers in this work. The OFC generation system is operated in two cascaded optical injection stages. When a master laser optically injects into the first stage with proper injection power and frequency, period-one (P1) dynamics are invoked in an optically injected semiconductor laser of the first stage. Another semiconductor laser in the second stage is then optically injected by the P1 dynamics. With proper injection power adjusted in the second stage, the P1 dynamics are regenerated, and the semiconductor laser relaxation oscillations (ROs) become undamped so that subharmonic oscillations appear. Because a subharmonic oscillation frequency is half of an oscillation frequency of the P1 dynamics, extra optical frequency components appear in the middle of the adjacent optical frequency components of the P1 dynamics, thus signaling OFC generation. The OFC signals exhibit at least 15 comb lines, resulting in a bandwidth greater than 140 GHz. Microwave comb signals are obtained after photodetection, although the microwave linewidth is on the order of a few megahertz because of the semiconductor laser noise. Thus, we propose a cascaded injection-locking scheme to stabilize the P1 dynamics and OFC signals. We have demonstrated pure microwave generations with a linewidth of less than 3 Hz and low phase noise.

Experimental study on the effect of sound stimulation on hearing and behavior of juvenile black rockfish (Sebastes schlegelii)

Assessing the potential impacts of wind farm noise on fish is a crucial aspect of Environmental Impact Assessment (EIA) studies. There is increasing evidence of disturbances and effects on hearing and behavior in animals. The black rockfish (Sebastes schlegelii) is a commercially valuable rocky reef fish native to East Asia. However, empirical studies that measure the actual consequences are lacking. In this study, we used auditory evoked potentials (AEP) to assess the effects of dominant frequency noise emitted by offshore wind farms on the auditory sensitivity, hearing threshold, swimming, and feeding behavior of juvenile black rockfish. The experimental findings revealed that the most sensitive sound frequency was 200 Hz, with the lowest hearing threshold recorded at 86.4 ± 3.4 dB re 1 μPa. Following 3 and 7 days of exposure to 200 Hz noise at 110 dB, threshold shifts in black rockfish reached 19.0 dB and 13.3 dB, respectively. During the subsequent recovery phase, these shifts decreased to approximately 9.8 dB after 3 days, respectively. The noise-exposed group exhibited higher swimming duration, moving distance, and caudal fin swing frequency compared to the control group without noise exposure. Furthermore, noise prolonged the feeding rate of black rockfish. Our findings provide the first evidence of noise-induced temporary threshold shift and behavioral disturbances in juvenile black rockfish, implying potential fitness consequences associated with noise pollutant.

Low-Frequency Discontinuous Noise Diagnosis and Reduction of Air Conditioner Outdoor Unit

Abnormal discontinuous low-frequency noise occurs in the outdoor unit when the air conditioner is operating at 86~96 Hz. This noise has the characteristics of low frequency, strong penetration, long transmission distance, and poor sound quality, which can cause serious noise pollution. In order to reduce the low-frequency noise of the outdoor unit of the air conditioner, the discontinuous noise frequency is first located by testing the time-domain and frequency-domain maps of the noise. It is then compared with the frequency of fan rotation noise and structural radiation noise. It is found that the discontinuous noise is caused by the coupling of fan rotation noise and structural radiation noise. It is found that low-frequency noise improvement is significant when the frequency difference between two noise peaks is greater than 8 Hz or when a single noise peak is less than 25 dB(A) by studying the methods of frequency modulation and amplitude modulation of noise peaks. By adjusting the fan speed from 930 rpm to 940 rpm, the problem of 94 Hz noise is solved. Then, through testing the natural frequencies of the pipe and shell, it is found that the natural frequencies of the pipe are close to the excitation frequencies. The structural radiation noise is mainly caused by the pipe. Modal analysis is conducted on the pipe. When the normal boundary with the condenser and the normal boundary with the valve of the pipe is constrained by springs, the simulated values are closer to the measured values. Finally, structural optimization design is carried out on the pipe. When the U-tube of the exhaust pipe is shortened by 30 mm, a 30 g mass block is added to the cold inlet pipe, and a 97 g mass block is added to the lower part of the valve pipe of the suction pipe. The discontinuous noise of the optimized prototype is significantly improved.

Decreased Noise and Identification of Very Low Voltage Signals Using a Novel Electrophysiology Recording System.

The interpretation of intracardiac electrograms recorded from conventional electrophysiology recording systems is frequently impacted by powerline (50/60 Hz) noise and distortion due to notch filtering. This study compares unipolar electrograms recorded simultaneously from a conventional electrophysiology recording system and one of two 3D mapping systems (control system) with those from a novel system (ECGenius, CathVision ApS) designed to reduce noise without the need for conventional filtering. Unipolar electrograms were recorded simultaneously from nine consecutive patients undergoing catheter ablation for AF (five patients), atrioventricular nodal re-entrant tachycardia (three patients), or ventricular tachycardia (one patient) over the course of 1 week in 2020. The noise spectral power of the novel system (49-51 Hz) was 6.1 ± 6.2 times lower than that of the control system. Saturation artefact following pacing (duration 97 ± 85 ms) occurred in eight control recordings and no novel system recordings (p<0.001). High frequency, low amplitude signals and fractionated electrograms apparent on unfiltered novel system unipolar recordings were not present on control recordings. Control system notch filtering obscured His bundle electrograms observable without such filtering using the novel system and induced electrogram distortion that was not present on novel system recordings. Signal saturation occurred in five of seven control system recordings but none of the novel system recordings. In this study, novel system recordings exhibited less noise and fewer signal artefacts than the conventional control system and did not require notch filtering that distorted electrograms on control recordings. The novel recording system provided superior electrogram data not apparent with conventional systems.

Direct Synthesis of Elastic and Stretchable Hierarchical Structured Fiber and Graphene-Based Sponges for Noise Reduction.

Noise pollution, as one of the three major pollutants in the world, has become a great burden on people's health and the global economy. Most present noise absorbers suffer large weight and inevitable compromise between good low-frequency (usually <1000 Hz) and high-frequency (typically >1000 Hz) noise reduction performance. This study presents a scalable strategy to directly synthesize ultrafine fiber sponges with ultrathin graphene-based vibrators by the synchronous occurrence of humidity-assisted electrospinning and electrospraying. The unique physical entanglements between reduced graphene oxide (rGO) nanosheets and ultrafine fibers endow hierarchical vibration structured fiber sponges (VSFSs) with excellent mechanical properties, which could withstand large shear strain (60%) and tensile stress (6000 times its weight) without damage and almost have no plastic deformation after 1000 compressions. Attribute to the vibration effect of ultrathin graphene-based vibrators and the viscous friction effect of porous fiber networks, the VSFSs achieve both good low-frequency (absorption coefficient of 0.98 in 680 Hz) and high-frequency sound absorption (absorption coefficients above 0.8 in 2000-6300 Hz) simultaneously. Furthermore, the noise reduction coefficient (NRC) of lightweight VSFSs (thickness of 30 mm) reaches 0.63, which could reduce high decibel noise by 24.4 dB, providing potential solutions for developing ideal noise-absorbing materials.

Evaluation of outcome of acoustic reflex tests in patients with type 2 diabetes mellitus: a cross-sectional study

PurposeType 2 diabetes mellitus (T2DM) may induce micro-vascular and macro-vascular changes that can lead to neuropathic changes which may affect the auditory pathway resulting in hearing loss. The study aims to evaluate the outcome of ipsilateral and contralateral acoustic reflex (AR) parameters and reflex decay tests (RDT) in patients with T2DM, and the relationship between average AR parameters, and duration and control of T2DM.MethodsAn analytical cross-sectional study was conducted in a tertiary care setup in 126 subjects which included 42 subjects with T2DM between 30 and 60 years of age, age-matched with 84 non-diabetic subjects. The subjects were evaluated for pure tone average (PTA), speech identification score (SIS), AR parameters [acoustic reflex threshold (ART), acoustic reflex amplitude (ARA), acoustic reflex latency (ARL)] and RDT.ResultsThe subjects with T2DM showed increased PTA in both ears when compared to the subjects with no disease. No significant difference was found in the SIS between both groups. There was no significant difference in the ART and ARL between the two groups. There was a significant difference in the ipsilateral and contralateral ARA at 500 Hz, 1000 Hz and broadband noise (BBN) when compared between the diabetic and non-diabetic groups. No significant difference was found between average AR parameters and duration and control of T2DM.ConclusionT2DM increases hearing thresholds and reduces ipsilateral and contralateral AR at lower frequencies and BBN. Duration and control of T2DM do not affect the AR parameters.

Heat-conducting elastic ultrafine fiber sponges with boron nitride networks for noise reduction

Industrial and traffic noise has become increasingly serious with the progress of industrialization. Most existing noise-absorbing materials suffer from poor heat dissipation and insufficient low-frequency (<1000 Hz) noise absorption, which not only reduces working efficiency but also leads to safety risks. Herein, heat-conducting elastic ultrafine fiber sponges with boron nitride (BN) networks were prepared by integrating direct electrospinning and impregnation method. The large acoustic contact area of ultrafine fibers and the vibration effect of BN nanosheets in a three-dimensional direction endow fiber sponges with good noise reduction, which can reduce white noise by 28.3 dB with a high noise reduction coefficient of 0.64. Moreover, thanks to good heat-conducting networks composed of BN nanosheets and porous structures, the obtained sponges exhibit superior heat dissipation with thermal conductivity of 0.159 W m−1 K−1. Besides, the introduction of elastic polyurethane and following crosslinking endow the sponges with good mechanical properties, which have almost no plastic deformation after 1000 compressions, and the tensile strength and strain are as high as 0.28 MPa and 75%. The successful synthesis of heat-conducting elastic ultrafine fiber sponges overcomes poor heat dissipation and low-frequency noise reduction of noise absorbers.

PO-04-135 FEASIBILITY OF AN AUTOMATED MULTI-WAVEFORM CAPTURE DETECTION ALGORITHM FOR ANNOTATION OF CAPTURE DURING ATRIAL ARRHYTHMIAS

Standard defibrillation methods are perceived as intolerably painful and require a clinical procedure with anesthesia. There is a clinical need to find tolerable out-of-hospital alternatives to achieve the same result. Recent strategies have been based on delivering low energy pulse-fields and direct stimulation. Create a fast, accurate, automated, multi-waveform capture detection algorithm (MCDA) to annotate capture of cardiac tissue during atrial arrhythmias. Unipolar recordings were acquired while delivering stimulation during atrial arrhythmias from multiple electrodes in 10 patients consented for open-chest surgery. Stimulation was delivered through diagnostic catheters placed anteriorly and posteriorly on the left atrium. Signals were minimally filtered to preserve morphologic features. Signal noise was dominated by 50 Hz powerline interference, which was algorithmically extracted and subtracted. MCDA consistently identified captured “Q-S” morphology, while non-captured “R-S” patterns and far-field ventricular deflections were discarded. False positives were identified by insufficient morphologic correlation both through time and across neighboring electrodes. MCDA capture accuracy was evaluated by comparing to human-adjudicated annotation of capture (Control) in 26 records. MCDA robustness and accuracy was tested by comparing baseline (0 noise) against five levels (1 to 5x) of the extracted 50 Hz noise re-imposed upon the signal. Accuracy of the MCDA at baseline relative to Control was 85 ± 8.8% [(TP+TN)/(TP+TN+FP+FN)]. Sensitivity-specificity curve (Figure 1) confirms acceptable performance of the MCDA. Worst case error between the MCDA and Control with respect to all levels of re-imposed 50 Hz noise was < 6%. Relative error of baseline MCDA when compared to the 1x level of re-imposed 50 Hz noise was 12.3%. Signal-to-noise ratio decreased arithmetically, and the relative error increased linearly. MCDA speed is 10x compared to real-time. MCDA is fast, accurate, and performs well with and without the presence of 50 Hz noise in determining capture of cardiac tissue during atrial arrhythmias. MCDA can be applied as a fundamental step in providing an automated real-time solution for annotation of capture during atrial arrhythmias.

Seismic noise characterisation at a potential gravitational wave detector site in Australia

A critical consideration in the design of next-generation gravitational wave detectors is the understanding of the seismic environment that can introduce coherent and incoherent noise of seismic origin at different frequencies. We present detailed low-frequency ambient seismic noise characterisation (0.1–10 Hz) at the Gingin site in Western Australia. Unlike the microseism band (0.06–1 Hz) for which the power shows strong correlations with nearby buoy measurements in the Indian Ocean, the seismic spectrum above 1 Hz is a complex superposition of wind induced seismic noise and anthropogenic seismic noise which can be characterised using beamforming to distinguish between the effects of coherent and incoherent wind induced seismic noise combined with temporal variations in the spatio-spectral properties of seismic noise. This also helps characterise the anthropogenic seismic noise. We show that wind induced seismic noise can either increase or decrease the coherency of background seismic noise for wind speeds above 6 m s−1 due to the interaction of wind with various surface objects. In comparison to the seismic noise at the Virgo site, the secondary microseism (0.2 Hz) noise level is higher in Gingin, but the seismic noise level between 1 and 10 Hz is lower due to the sparse population and absence of nearby road traffic.

Overview of distributed acoustic sensing technology and recently acquired data sets

Fiber optic distributed acoustic sensing (DAS) is a recent innovation utilized primarily in the seismic community for measuring seismic acoustics signals at low frequencies (single digit Hz and below). The technique utilizes strain rates in a fiber optic cable, observed via the backscatter of light pulses, to measure the acoustic field. Recently, the capabilities of this technology to measure higher frequency acoustic fields (10s to 100s of Hz) have been explored. Low frequency marine mammals calls at ∼20 Hz and ship noises have been successfully recorded, and a recent experiment demonstrated the capability to record up to ∼500 Hz. This talk provides an overview of DAS technology and introduces two recent experiments for studying water column acoustics with DAS. A 4-day experiment conducted in November 2020 as part of the Ocean Observatories Initiative (OOI) provides data along two fiber optic cables extending west from the coast of Oregon by 65 km and 95 km, reaching depths of 590 m and 1575 m, respectively. DASCAL22, a recent experiment from October 2022, simultaneously recorded data using DAS at 2 kHz sampling rate on a cable extending 3.54km at ∼100 m depth and multiple moored hydrophones placed close to the DAS cable, allowing direct comparison between a new and existing technology.

Aerospace grade acoustic metamaterial design, modeling, and experimental validation

Vertical Take-Off and Landing Urban Aerial Mobility vehicle propulsion systems use rotors and blades to achieve lift, which results in heightened noise at the blade passing frequency. These frequencies must be attenuated to reduce cabin discomfort and comply with noise exposure limits. The design must absorb 350 Hz noise, be fire retardant to comply with aviation standards, and be maximally one inch thick. This paper outlines the design, computational simulation, and experimental validation of a novel aerospace-grade acoustic metamaterial. It contributes to the field of computational analysis of new metamaterial structures and experimental validation using prototype Helmholtz unit cells. Achieving maximum sound absorption with the aerospace requirements of minimal mass and fire safety led to the consideration of Nomex honeycomb Helmholtz resonator acoustic panels. Their sub-wavelength attenuation, tunability, lightweight and fire resistance make them an optimal fit for this application. A novel phenomenon was observed in experimental impedance tube testing where multiple Nomex honeycomb cells acted as one resonator cavity. This discovery allowed for samples of only one inch in thickness (cell depth) to achieve 83% sound absorption at 348 Hz. Experimental values match well with the calculated analytical solution using the Helmholtz equation and simulated sound absorption in COMSOL MultiPhysics. The acoustic metamaterial achieves tunable noise attenuation with minimal thickness, weight, and fire suppression using a novel design.

A 43.3-μW Biopotential Amplifier With Tolerance to Common-Mode Interference of 18 Vpp and T-CMRR of 105 dB in 180-nm CMOS

In this article, we present an electrocardiogram (ECG) amplifier that has a large total CMRR (TCMRR) regardless of contact impedance mismatch and a large tolerance to common-mode interference (CMI). To achieve these features, an adaptive TCMRR enhancing loop is implemented in parallel with a charge-pump-based common-mode suppressing loop (CMSL). It adjusts the CMI current through each contact impedance so that common-mode (CM) to differential-mode (DM) conversion is minimized. We also propose a fast settling technique for the adaptive loop so that contact impedance variation can be tracked fast enough to allow robust ECG acquisition. A prototype chip fabricated in 180-nm CMOS achieves TCMRR larger than 105 dB even when there is contact impedance mismatch of up to 30%. It also achieves tolerance to CMI of 18 VPP at 60 Hz and input-referred noise of 1.90 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mu }\rm V_{rms}$ </tex-math></inline-formula> while consuming 43.3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{W}$ </tex-math></inline-formula> .

Seismic Monitoring of Machinery through Noise Interferometry of Distributed Acoustic Sensing

AbstractApplication of distributed acoustic sensing (DAS) in seismic studies has benefited from its high-density acquisition, environmental adaptation, and low-cost deployment. Nevertheless, the great potential of such observations in seismic research across scales is far from explicit. To test the feasibility of DAS for small-scale seismic monitoring in the urban city, we conducted a one-week field experiment with three ∼72 m long fiber-optic cables, and eight seismometers at the campus of southern marine science and engineering Guangdong laboratory (Guangzhou). Stable high-frequency (2–8 Hz) noise correlation functions (NCFs) were successfully retrieved between DAS channels from continuous in situ noise recording. The observed NCFs are highly asymmetrical, indicating the nonuniform distribution of the noise sources. Beamforming analysis of the seismic data demonstrates that the noise sources are stable daily with consistent direction and slowness. Temporal variation of the NCFs shows that the observed stable signals emerge simultaneously with the machinery operating time of the campus. NCF modeling with spatially varying source spectra reveals that a localized source in the nearby office building fitted the observations well. Accordingly, ground vibration of operating machinery is suggested to account for the temporal and spatial features retrieved from the observed NCFs. Our study demonstrates that DAS has great potential in high-resolution source localization and characterization, as well as temporal monitoring (∼hours) using urban anthropogenic seismic sources.

An Improved Altimeter in-Orbit Range Noise-Level Estimation Approach Based on Along-Track Differential Method

Satellite radar altimeters are advanced remote sensing devices that play an important role in observing the global marine environment. Accurately estimating the noise level of altimeter in-orbit ranging data is crucial for evaluating the payload performance, analyzing sea conditions, and monitoring data quality. In this study, we propose an approach based on the differential processing of along-track odd–even data sequences for altimeter in-orbit range noise-level estimation. Using the long-term along-track data sequence can notably improve the issue in the existing method in that the noise level is underestimated owing to the utilization of a relatively short data segment. On the basis of an analysis of the influence of low-frequency components on noise-level estimation, the mathematical formulas of the above differential method were deduced, and the efficacy of the approach in assessing the noise level of altimeter in-orbit data was demonstrated by simulation experiments. This method was used to estimate the noise levels of the 20 Hz datasets of Jason-3 and Sentinel-6, and the idea of the time-domain difference was extended to the frequency domain. The statistical results showed that the 20 Hz noise levels at the significant wave height (SWH) = 2 m were 7.41 cm (Jason-3 low-resolution (LR) mode), 6.66 cm (Sentinel-6 LR mode), and 3.13 cm (Sentinel-6 high-resolution (HR) mode). The power spectrum density analysis further verified its accuracy. By reprocessing the 20 Hz data of Sentinel-6 into 10, 5, and 1 Hz, the effectiveness of the along-track odd–even differential method to directly evaluate the noise level of 1 Hz data was explored, and the impact of ocean signals such as swells on noise-level estimation in synthetic aperture mode was discussed.

An Automatic Loop Gain Enhancement Technique in Magnetoimpedance-Based Magnetometer

A low-power, low-noise, and high-bandwidth magnetometer that utilizes the magnetoimpedance (MI) element as a sensor head is presented. The MI element has a high sensitivity, and it can be implemented in the mm-scale through the MEMS process. The analog front-end (AFE) circuit of the magnetometer includes a digital calibration scheme that automatically enhances the loop gain of the system, resulting in high bandwidth and low-noise characteristics. The AFE circuit is designed based on a switched-capacitor (SC) approach, and its dedicated switching scheme can suppress the folded noise of an amplifier. A single-coil magnetic negative feedback architecture with correlated double sampling (CDS) enables to achieve a high dynamic range (DR) and stable passband gain in addition to simplifying the structure of the MI element. The AFE chip of the magnetometer is implemented in a 0.18- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> CMOS process, and it achieves an 8-pT/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\surd $ </tex-math></inline-formula> Hz noise floor within a 31-kHz bandwidth and the DR of 96 dB, where the power consumption is 1.97 mW.