Noise Spectral Density Measurements Research Articles

Two-Wired Active Spring-Loaded Dry Electrodes for EEG Measurements.

Dry contact electrode-based EEG acquisition is one of the easiest ways to obtain neural information from the human brain, providing many advantages such as rapid installation, and enhanced wearability. However, high contact impedance due to insufficient electrical coupling at the electrode-scalp interface still remains a critical issue. In this paper, a two-wired active dry electrode system is proposed by combining finger-shaped spring-loaded probes and active buffer circuits. The shrinkable probes and bootstrap topology-based buffer circuitry provide reliable electrical coupling with an uneven and hairy scalp and effective input impedance conversion along with low input capacitance. Through analysis of the equivalent circuit model, the proposed electrode was carefully designed by employing off-the-shelf discrete components and a low-noise zero-drift amplifier. Several electrical evaluations such as noise spectral density measurements and input capacitance estimation were performed together with simple experiments for alpha rhythm detection. The experimental results showed that the proposed electrode is capable of clear detection for the alpha rhythm activation, with excellent electrical characteristics such as low-noise of 1.131 μVRMS and 32.3% reduction of input capacitance.

Magnetic Hysteresis and Barkausen Noise in Plastically Deformed Steel Sheets

The magnetic properties of steels are affected by plastic deformation, because the domain wall processes magnetoelastically interact with the dislocations and the residual stresses. The evolution of the magnetic hysteresis loop and its parameters with the type and degree of straining can thus provide a macroscopic signature of the underlying the mechanical and structural properties. Additional information can be achieved at a microscopic level through analysis of the Barkhausen noise, the signal generated by the stochastic flux variations associated with the discontinuous motion of the domain walls. Nondestructive methods for the structural evaluation of magnetic steels, devoted, in particular, to the investigation of work-hardening and state of internal stress following plastic straining, have therefore been developed in the literature, either through magnetic hysteresis or Barkhausen noise measurements. In this paper, we summarize significant results regarding the relationship between magnetic properties and plastic deformation in steel samples and the related experimental methods. Attention will be devoted, in particular, to the measurement and analysis of the Barkhausen noise spectral density and the way it relates to the macroscopic magnetic behavior and the structural properties.

InAsSb-based nBn photodetectors: lattice mismatched growth on GaAs and low-frequency noise performance

An InAsSb nBn detector structure was grown on both GaAs and native GaSb substrates. Temperature dependent dark current, spectral response, specific detectivity and noise spectral density measurements were then carried out. Shot-noise-limited figures of and were calculated (based upon the sum of dark current and background photocurrent) for the sample grown on GaAs and the sample grown on GaSb, respectively, at 200 K. Noise spectral density measurements revealed knee frequencies of between 124–337 Hz and ∼8 Hz, respectively. Significantly, these devices could support focal plane arrays capable of operating under thermoelectric cooling.

Room temperature direct detection of low frequency magnetic fields in the 100 pT/Hz0.5 range using large arrays of magnetic tunnel junctions

We developed a highly sensitive magnetic tunnel junction (MTJ) based magnetic field sensor device with a magnetic field sensitivity of 1.29 Ω/μT. For this, more than 1000 microstructured linear MgO based MTJ pillars were fabricated and connected in series. By current biasing the MTJ sensor device under small AC external magnetic fields (μ0HAC), a large AC output voltage proportional to μ0HAC is induced. Carrying out noise spectral density measurements at such conditions, we demonstrate that our MTJ sensor device is able to detect magnetic fields down to 115 pT/Hz0.5 at frequencies fAC < 200 Hz in an unshielded environment without amplification of the MTJ AC output voltage.

ENoise spectral density of single crystal YBaCuO films near to temperature of transition

Recently, the study of noise in high temperature critical superconductor is a adequate mean to search for disorder and dynamics of vortices in such materials [1], also to understand the origins of excess conducting noise [2-3], several causes are origins to explain the noise in HTSC’s: 1) the thermal fluctuation model [4], 2) the superconducting fluctuations model [5], 3) the magnetic flux noise [6]. In this investigation, we interest with measurements of the voltage noise spectral density S

Transversal Noise Current: An Excess Noise in CMOS Split-Drain Transistors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> An excess-noise current in CMOS Magnetic Sensitive Field Effect Transistor (MAGFET) split-drain transistors is investigated, and a new noise model is proposed. The model is based on the existence of the transversal noise current that stems from the inversion charge layer in the MOS transistor channel. This excess-noise current, along with the one predicted by the classical MOS transistor, produces the total split-drain noise current. Noise spectral density measurements were carried out to verify the proposed model for split-drain MAGFETs manufactured in 0.8- and 0.35-<formula formulatype="inline"><tex>$\mu\hbox{m}$</tex></formula> CMOS, with an equal geometric aspect ratio of 10 <formula formulatype="inline"><tex>$\mu\hbox{m}$</tex></formula>/10 <formula formulatype="inline"><tex>$\mu \hbox{m}$</tex></formula>. </para>

Phase noise study of AlGaN/GaN HEMT X‐band oscillator

AlGaN/GaN high electron mobility transistor (HEMT) heterostructures were investigated by noise spectral density measurements. The heterostructure layers are grown without intentionally doping the barrier material and two-dimensional gas appears at the interface only due to polarization effects. Low-frequency noise spectra of the HEMT were studied in a wide range of applied voltages. They demonstrated a deviation from 1/f dependence at high applied drain-source voltages due to increasing dissipated power. The phase noise of a monolithic microwave integrated circuit oscillator based on the AlGaN/GaN HEMT amplifier was investigated. In spite of the low value of the resonator quality factor of 20, a low level of the phase noise of the oscillator was registered. The up-conversion factor was found to be as low as 15 MHz/V for frequency offset 100 kHz, demonstrating that AlGaN/GaN HEMTs offer an excellent potential for a wide range of microwave applications. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of the noise and correlations in harmonically mode-locked lasers

In a harmonically mode-locked laser multiple optical pulses propagate inside the laser cavity. The noise in different pulses inside the laser cavity is in general correlated. Information regarding the sign and magnitude of the noise correlations is contained in the distribution of the spectral weight among the supermode noise peaks that appear in the pulse energy and timing noise spectral densities. We show that the supermode noise spectrum obtained experimentally by measurement of the photodetector current noise spectral density can be used to determine the correlations in the energy and the timing noise of different pulses in the laser cavity. We also present simple models for the timing noise in harmonically mode-locked lasers that demonstrate the relationship between the noise correlations and the supermode noise peaks.

A new model for the low-frequency noise and the noise level variation in polysilicon emitter BJTs

Presents a new, physically-based model for the low-frequency noise in high-speed polysilicon emitter bipolar junction transistors (BJTs). Evidence of the low-frequency noise originating mainly from a superposition of generation-recombination (g-r) centers is presented. Measurements of the equivalent input noise spectral density (SIB) showed that for BJTs with large emitter areas (AE) S(I/sub B/) is proportional to 1/f, as expected. In contrast, the noise spectrum for BJTs with submicron A/sub E/ showed a strong variation from a 1/f-dependence, due to the presence of several g-r centers. However, the average spectrum has a frequency dependence proportional to 1/f for BJTs with large as well as small A/sub E/. The proposed model, based only on superposition of g-r centers, can predict the frequency-, current-, area-, and variation-dependency of with excellent agreement to the measured results.

Measurement and characterization of the frequency noise of a laser-diode-pumped single-frequency Tm-Ho:YAG laser

The frequency noise characteristics of a single-frequency 2.1-μm Tm-Ho:YAG laser have been thoroughly investigated. From the measurement of the frequency noise spectral density, performed by means of an interferometric technique, a short-term laser linewidth of 550 kHz over 1-ms time interval has been obtained. The frequency stability, defined as the two sample standard deviation of the frequency fluctuations with dead time T, has been also evaluated, obtaining values ranging from ∼250 kHz to ∼2.5 MHz for T values between 1 and 100 ms over an observation time of 100 ns. It has been shown through a numerical analysis that a significant improvement is achievable, attaining a frequency stability of ∼25 kHz for T up to 100 ms, by using the Pound–Drever locking technique in connection with a roto-vibrational line of the HBr molecule.

Low Frequency Noise and Random Telegraph Signal in a Multiple Quantum Well Infrared Photodetector

AbstractCurrent-voltage, voltage-time, and noise measurements were taken on a three well, twenty period, InGaAs/A1GaAs quantum well infrared photodetector at low temperature and low constant current bias. Data from the voltage-time measurements exhibited random telegraph signal (RTS) which is a manifestation of a single electron capture/emission event. It thus contributes to generation-recombination noise of the device. A model is proposed that allows extraction of number of mobile carriers, continuum state electron mobility, electron emission times and trap activation energy from the RTS data. These parameters may then be utilized to predict the excess noise spectrum. Data from the noise spectral density measurements show 1/f like noise at frequencies below I kHz. It can be shown to be a sum of Lorenztian curves with various electron emission times.The model proposed predicts that the local number of mobile charge carriers in any barrier section is less than 5 (total in the device around 300), there is nearly ballistic electron transport (mobility values around 3×105cm2/Vs), and the traps causing the RTS are in the barriers of the device and there are two of them in the device tested. The model applies to the linear mode of operation and assumes that the well regions are electron reservoirs, i.e. contacts, for the resistive barrier regions. The device is thus divided into a number of resistive portions in series. Each resistive portion is statistically independent from the others at low temperatures since the time the electrons spend in the wells is long compared to intra-well scattering times. This allows the electrons to fully thermalize and lose any statistical information from their previous continuum state trajectory.

Si-JFET devices and related noise behavior under irradiation

Monolithic N-channel junction field effect transistors (NJFETs) dc characteristics, small signal parameters and noise have been studied from 300 K down to cryogenic temperatures before and after irradiation with 60 Co γ-rays and fast neutrons (1 MeV). Radiation induced effects on dc parameters and noise are reviewed. Noise spectral density measurements performed at various temperatures have shown that the radiation induces a noise increase which is temperature and frequency dependent.

An epitaxially-grown charge modulation device

A charge modulation device (CMD) has been fabricated in a p-type epitaxial layer grown from the buried-channel silicon region of a charge-coupled device (CCD). Construction of the CMD directly above the CCD buried-channel and over the oxidized CCD transfer gates lowers the effective sense capacitance while providing isolation of the CMD source/drain regions. Responsivity values of 28 and 66 /spl mu/V/e for feedback and no feedback conditions, respectively, were measured dynamically on test devices. Input-referred noise values of approximately four electrons r.m.s. were calculated from noise spectral density measurements assuming a low-pass filter 3 dB cutoff frequency of 5 MHz and correlated double sampling.

Instrumentation for high accuracy noise characterisation of front-end devices in detector applications

The paper describes the instrumentation that was developed for the noise characterisation of field-effect and bipolar transistors. Very accurate measurements of the series noise spectral density in the frequency range 100 mHz — 100 MHz and of the equivalent noise charge at processing times ranging from 10 ns to 10 μs are made possible. A review of results on various device types shows how these data are used in view of applications to a broad range of experimental conditions in elementary particle and nuclear physics.

Noise spectral density measurements of a radiation hardened CMOS process in the weak and moderate inversion

The authors have measured the noise of MOS transistors of the United Technology Microelectronics Center (UTMC) 1.2- mu m radiation-hardened CMOS P-well process from the weak to moderate inversion region. The noise power spectral densities of both NMOS and PMOS devices were measured from 1 kHz to 50 MHz. The bandwidth was chosen such that the important components of the spectral densities such as the white thermal noise and the 1/f noise could be easily resolved and analyzed in detail. The effects of different device terminal DC biases and channel geometries on the noise are described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurement of semiconductor laser amplifier noise figure in coherent optical transmission system

We report the first detailed investigation of the effect of semiconductor laser amplifier noise figure on the sensitivity of a heterodyne receiver. Independent measurement of the amplifier spontaneous noise spectral density confirms that local oscillator-spontaneous emission beat noise accounts for all of the observed penalty.

Noise spectral density as a device reliability estimator

The measurement of noise spectral density is proposed as an estimator of solar cell device reliability. The use of associated temperature and bias stresses is investigated as a non-destructive rapid test method. A novel technique which employs the cooling rather than the heating of the device was utilized. In certain cases the predominant failure mechanisms may be derived. Data are presented on indium-tin oxide/silicon as well as p-n junction devices.

Temperature effects in silicon avalanche diodes

Gain-voltage curves for reach-through avalanche photodiodes are computed using the ionization rate data of van Overstraeten and de Man and Lee et al. and compared with measured curves. Noise spectral density measurements are also made and compared with McIntyre's avalanche noise theory. Agreement between computed and measured k-values as obtained from the noise measurements and between the computed and measured gain-voltage relationship can only be obtained with the ionization rate data of Lee et al. Gain-voltage curves were measured over the temperature range −17 to +55°C and compared with computed curves in which temperature dependence of the ionization rate data of Lee et al. is computed using the modified Baraff theory. Excellent agreement between theory and experiment is obtained.

Noise Characteristics in Silicon Photodiodes

Measurements of noise spectral density in the avalanche multiplication region have been made from 1 MHz to 4 GHz for four silicon photodiodes. The noise spectral density has been compared with the existing theories. At low frequencies the microplasma on-off noise, the theory of which has been successfully applied to one diode containing a single bistable microplasma, is pronounced. At high frequencies (≥100 MHz) one of the photodiodes exhibited a uniform-avalanche shot noise, which has been well described by McIntyre's theory with k=0.1, where k is the ratio of the hole ionization coefficient to the electron ionization coefficient. The rf (1 MHz∼1 GHz) bias has been found to give rise to shifts of the I–V curves to higher voltages and changes in the noise spectral density. In some cases the noise spectral density is reduced. However, whether the noise is reduced or not depends on the property of microplasmas contained in the diode.