Equivalent Magnetic Noise Level Research Articles

Modeling of Delta-E Effect Magnetic Field Sensors

In this article, we study magnetic field sensors based on the delta-E effect occurring in a magnetostrictive layer coupled to a piezoelectric resonator. The sensors consist in suspended beams that essentially vary in the bounded conditions at the edges. We examine a clamped-free and a double-clamped sensor. A finite-element model (FEM) is implemented and validated versus published experimental measurements. The latter are considered as reference results. A Butterworth–Van Dyke (BVD) model is also given in each case. The performances of these two types of resonators are pointed out and compared, especially in terms of sensitivity and equivalent magnetic noise level. The given approach gives the way of sensor optimization.

Impact of Adjustment of the Static Working Point on the 1/f Noise in a Negative Feedback GMI Magnetic Sensor

The resolution of giant magneto-impedance (GMI)-based magnetic sensor is limited by its equivalent magnetic noise level. It presents a white noise and a low-frequency 1/ f noise behavior. Previous works have showed that the white noise is mainly determined by the electronic conditioning noise, nevertheless, the 1/ f noise’s origin is not totally clear yet. To optimize the 1/ f noise, in this paper, first, we develop an optimized negative feedback GMI sensor. It contains of a low-distortion oscillator, a high-performance amplitude detector, and a GMI sensing element with adjustable static working point. And then, the sensitivity and the 1/ f noise level of the GMI sensor are studied when considering the electronic conditioning parameters and the static working point. The experiment results show that the adjustment of the static working point has a significant effect on the 1/ f noise at low frequencies. The optimal 1/ f noise is obtained by setting the static working point at the position where the GMI element possesses with the maximum intrinsic sensitivity. Finally, we achieve the GMI sensor noise performance of 20 pT/ $\surd $ Hz at white noise region and 100 pT/ $\surd $ Hz at 1 Hz.

Phase Modulation Noise of a Magneto(Elasto) Electric Sensor Operating as a Magnetometer in the Non-Linear Regime–Theoretical and Experimental Studies

In this paper, we present the equivalent magnetic noise of a multi-push-pull Magneto(Elasto)Electric sensor implemented as a magnetometer, with a phase modulation circuitry. Here, in this paper, either a magnetic field or an electric field carrier was modulated by a low magnetic field signal at the sensor resonant frequency, offering an optimal sensitivity, and thereby, the lowest equivalent magnetic noise level at this working point. These two modes of excitation are respectively called E/E (Electric Excitation/Electric detection) and M/E (Magnetic excitation/Electric detection). Based on the constitutive equations and the Nyquist theorem, we have determined the expected sensitivities and the equivalent magnetic noise for the phase modulation. The experimental results were compared to the modeling and to a former amplitude modulation method in order to analyze the coherence of our results and assess this method against the classical AM mode, respectively. At present, the main noise source contributions by using phase modulation are attributed to the electronic circuitry.

Evaluation of the Imaginary Part of the Magnetic Susceptibility, $\chi ^{\prime \prime }$ , and Application to the Estimation of the Low Frequency, 1/ $f$ , Excess Noise in GMI Sensors

The equivalent magnetic noise spectral density of giant magnetoimpedance (GMI)-based sensors exhibits a low-frequency excess noise inversely proportional to the frequency (1/f noise). In order to enhance the performance and reach the ultimate and intrinsic limit of the magnetometer, it is necessary to investigate the origin of this excess noise. Cross correlation measurements have permitted the demonstration that the excess low-frequency noise arises from the sensor, under defined excitation conditions, particularly the amplitude of the excitation current and the dc bias current. Given that the GMI effect is based on an impedance variation governed by the magnetization direction under an applied external magnetic field, the low-frequency magnetization fluctuations within the GMI wire have been considered to be a possible noise source. The fluctuation-dissipation theorem permits evaluation of the spectral power density of these fluctuations, which is proportional to the imaginary part of the susceptibility, $\chi ^{\prime \prime }$ . The latter may be evaluated by two different methods, based on the measurement of the real part of the wire impedance and on the evaluation of the circumferential hysteresis loop area. The results obtained by both methods are compared and discussed. The values of $\chi ^{\prime \prime }$ thus obtained are then used in order to predict the equivalent magnetic noise level at 1 Hz, and are compared with that measured at 1 Hz. The comparison is conducted for different amplitudes of the dc bias current, and the results obtained indicate that the proposed methods have the potential of predicting the excess noise in GMI sensors.

Low Frequency Excess Noise Source Investigation of Off-Diagonal GMI-Based Magnetometers

The equivalent magnetic noise spectral densities of off-diagonal giant magnetoimpedance (GMI)-based magnetometers exhibit significant low-frequency excess noise, proportional to 1/f noise. As it represents a serious limitation to the ultimate sensing performances of high sensitivity magnetometers, possible sources of this 1/f noise are under investigation. Low-frequency magnetization fluctuations have been proposed as the noise source in the case of classical GMI-based sensors. Here, we apply this model to off-diagonal GMI-based magnetometers. This requires the inclusion of magnetization fluctuation noise sources, in addition to white noise sources from electronic conditioning in the GMI effect equations. A pessimistic scenario is presented, predicting the upper limit of low-frequency excess noise from material characteristics. The equivalent magnetic noise level is then computed from the sensitivity of each term of the sensing element impedance matrix to the magnetization angle at the static working point (for both axial and circumferential static magnetic field) and to conditioning circuitry. Based on this, it appears that magnetization fluctuations similarly affect all modes of operation of the two-port network sensing element, inducing identical impedance fluctuations. It also appears that this noise depends only upon the static equilibrium condition. This condition is governed by the effective anisotropy of the magnetic wire and by both axial and circumferential static components of the working point.

Investigation of a Bending-Mode Magneto(Elasto)electric Sensor Using a Phase Modulation With a PLL

Bending-mode magnetostrictive-piezoelectric sensors clamped at its two extremities show promising results for an enhancement of its magnetic field sensitivity by using a phase modulation (PM) technique. In this operating mode, the sensor is excited by an external carrier at its bending resonant frequency. The results of such a magnetic field sensing by a magneto(elasto)electric bilayer when using a PM technique with a phase locked loop circuitry are presented. The aim of this method is to follow the phase shift at the bending resonant frequency of the sensor, which is proportional to the applied magnetic field for the small-signal regime. We obtained a magnetic sensitivity of ~1.5 kV/T and an equivalent magnetic noise level around 80 nT/$\surd $ Hz at 2 Hz. These performances are presently limited by the noise from our electronics, suggesting that there is room for further improvements.

Mechanical Noise Limit of a Strain-Coupled Magneto(Elasto)electric Sensor Operating Under a Magnetic or an Electric Field Modulation

The mechanical noise limit of a strain-coupled magneto(elasto)electric composite has been investigated when a magnetic or an electric field modulation is applied to sense a low-frequency magnetic field and access dc field measurement capabilities. The sensitivity and noise of such a composite sensor were derived from constitutive equations based on the piezoelectric and ferromagnetic material properties. The analysis was used to evaluate the equivalent noise floor of the composite sensor and to explain the origin of noise by constituting a mechanically coupled electromagnetic model. Experimental measurements revealed a good fit with the proposed model. For example, an equivalent magnetic noise level of ~60 pT/Hz at 1 Hz with dc capability was achieved using an appropriate field modulation.

Noise Behavior of High Sensitive GMI-Based Magnetometer Relative to Conditioning Parameters

The lowest measurable magnetic field of a high sensitive magnetometer based on giant magnetoimpedance (GMI) effect will be determined by its noise level. Numerous previous works have showed that the equivalent magnetic noise level of GMI magnetometer presents a 1/f behavior at low frequencies and a white noise. Currently, performances of the magnetometer are still limited by the electronic conditioning as well as the intrinsic equivalent magnetic noise of the GMI sensor. To improve sensor performances, particularly at low frequency, we have investigated two focuses on research: 1) reduce the electronic noise of the conditioning electronic by increasing the voltage sensitivity, in units of V/T and 2) reduce the intrinsic equivalent low frequency sensor noise. As previously reported, the equivalent magnetic noise at 1 Hz can be reduced by increasing the excitation current in addition to a dc bias current. By working on these bias conditions, on the given samples, we almost 1 pT/√Hz in white noise region and 15 pT/√Hz at 1 Hz.

Investigations on the Equivalent Magnetic Noise of Magneto(Elasto)Electric Sensors by Using Modulation Techniques

The equivalent magnetic noise of the magnetoelectric (ME) layered composite sensors has been investigated for various modulation techniques. The ME thin film response to an electric modulation or a magnetic modulation can be sensed by using either a charge amplifier or a coil wound around the sample and then demodulated by a synchronous detector. The equivalent magnetic noise for these excitations methods has been compared. As expected, the low-frequency fluctuations can be lowered when the magnetoelectric sensor is operated in a modulation mode. Results show that these methods can give the same level of equivalent magnetic noise for a certain strain-excitation. In theory, mechanical noise appears as the only dominant noise source after the demodulation process in the case of a certain strong amplitude excitation carrier signal. By using these modulation techniques, an equivalent magnetic noise level of 10-100 pT/Hz at 1 Hz was achieved with DC capability.