A method for evaluating the noise floor of magnetic shielding barrels based on cross-spectral analysis

Abstract

A method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis includes data acquisition, using two independent atomic magnetometers to simultaneously collect noise data at the center of the magnetic shielding barrel, windowing the discretized noise data of the two channels using a Hanning window to reduce near-sidelobe leakage, and Fourier transforming the windowed dual-channel data to obtain spectral data. Finally, a frequency-domain cross-correlation conjugate operation is performed to calculate the frequency domain characteristics of the common-mode components of the two atomic magnetometers, thereby obtaining the noise spectrum of the common-mode components of the two magnetometers, i.e., the noise floor of the magnetic shielding barrel. The present invention extracts the common-mode components between channels through cross-spectral analysis operations, thereby evaluating the noise floor of the magnetic shielding barrel, or the noise floor of the magnetic shielding environment. This method eliminates the need to develop and use higher-precision metrological magnetometers and relies solely on two less sensitive magnetometers to evaluate the noise floor of the magnetic shielding barrel. This reduces costs, eliminates the need to increase the equipment complexity of the extremely weak magnetic measurement device, and provides high reliability.

Description

Technical Field

[0001] The present invention relates to the fields of magnetic field measurement technology and weak signal detection, and in particular to a method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectrum analysis. Background Art

[0002] Measuring extremely weak biomagnetic fields is crucial for medical procedures and disease diagnosis. To measure brain magnetometry in the hundreds of femtoliters and heart magnetometry in the hundreds of picoliters, extremely weak magnetic field measurement devices require magnetic shielding to shield against the strong Earth's magnetic field and various electromagnetic devices. Therefore, the performance of these shielding devices directly impacts the sensitivity of the atomic magnetometer, the core sensor in the biomagnetic measurement device. Currently, the primary shielding device used for brain and heart magnetometry measurements is a shielding barrel. It typically consists of four layers of permalloy to shield low-frequency magnetic fields, and is constructed of pure aluminum to shield high-frequency fields. Key specifications for a magnetic shielding barrel include the shielding factor and the internal noise floor. Variability in shielding materials, structures, and processes directly impacts these performance factors. The noise floor of the shielding environment determines whether it is a major source of noise interference in the biomagnetic measurement device, making it crucial to measure the noise floor of the shielding barrel.

[0003] At present, the central noise floor of the fully enclosed magnetic shielding barrel designed and manufactured in China can reach 10fT / Hz. -1 / 2 Below, the internal combination of Permalloy and ferrite barrel can even reach as low as 1fT / Hz -1 / 2 The sensitivity of the most sensitive commercial magnetic measurement sensor at home and abroad is the atomic magnetometer, which is 30fT / Hz. -1 / 2 ,It is difficult to directly evaluate the shield barrel noise, so it is necessary to develop a more accurate metrological magnetometer. Summary of the Invention

[0004] In view of the above technical problems, the purpose of the present invention is to provide a method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis, which can be achieved by using 30fT / Hz -1 / 2 Sensitivity Commercial magnetometer, or magnetometer used in magnetic field detection device, with sensitivity comparable to commercial level, achieving 10fT / Hz -1 / 2 Even lower noise shielding buckets can be evaluated for their noise floor.

[0005] The technical solutions of the present invention are as follows:

[0006] A method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis, characterized by comprising the following steps:

[0007] Step 1: using a first atomic magnetometer to establish a first noise data acquisition channel through the center of the magnetic shielding barrel and form discrete voltage noise data of the first channel; at the same time, using a second atomic magnetometer to establish a second noise data acquisition channel through the center of the magnetic shielding barrel and form discrete voltage noise data of the second channel;

[0008] Step 2, performing Hanning windowing processing on the discrete voltage noise data of the first channel and the discrete voltage noise data of the second channel respectively;

[0009] Step 3: Perform discrete Fourier transform on the first channel windowed data and the second channel windowed data to obtain the first channel spectrum data V1 (f n ), and the second channel spectrum data V2(f n );

[0010] Step 4: V1(f n ) and V2(f n ) to perform cross-spectral calculation and obtain the power spectrum density X(f n ), using V1(f n ) to obtain the power spectrum density P1(f n ), using V2(f n ) to obtain the power spectrum density P2(f n ), using X(f n ) to obtain the noise floor of the magnetically shielded barrel.

[0011] The first and second atomic magnetometers in step 1 are both 30fT / Hz -1 / 2 Sensitivity Commercial magnetometer, or magnetometer used to detect cardiac magnetism in cardiac magnetometry device, the photocurrent output by the magnetometer is converted into voltage through a transimpedance amplifier, and the discrete voltage data is collected by an analog-to-digital conversion collector.

[0012] Step 2 includes the following formula:

[0013]

[0014] Where W H (m) is the window function, M is the total number of sampling points of a single magnetometer, M is a positive integer, m is the sampling point sequence 0≤m≤M, m is an integer, and W H (m) is multiplied with the discrete voltage data to complete the windowing processing of the magnetometer discrete voltage data.

[0015] Step 3 includes the following formula:

[0016]

[0017] Where v1(m) is the discrete voltage noise data of the first channel, v2(m) is the discrete voltage noise data of the second channel, n is a positive integer, e is a natural constant, and i is an imaginary unit.

[0018] Step 4 includes the following formula:

[0019]

[0020] where f s is the sampling rate, It is the conjugate operation performed on V1(fn).

[0021] Step 4 includes: using the magnetic field voltage response coefficient of the magnetometer to perform conversion and calculate the noise floor of the magnetic shielding barrel.

[0022] Step 4 includes the following steps:

[0023] Step 41, retain X(f n ), which is the real part of X(f n )’s amplitude;

[0024] Step 42: For P1(f n ) and P2(f n ) and X(f n ), the average value within each 1Hz frequency range is taken as the power spectrum density value P1, P2 and X of each integer frequency point, P1 corresponds to P1(f n ), P2 corresponds to P2(f n ), X corresponds to X(f n );

[0025] Step 43: Take the square root of X to get the voltage noise amplitude. The unit of voltage noise amplitude is V / Hz. -1 / 2 , using the voltage-magnetic field conversion coefficient K of the magnetometer, the unit of K is V / nT, and K is divided by the voltage noise amplitude to obtain the noise floor value of the magnetic shielding barrel. The larger the value, the higher the noise floor.

[0026] The sensitive axes of the first atomic magnetometer and the second atomic magnetometer in step 1 are parallel, the intrinsic noises of the two magnetometers are non-correlated signals, and the difference in the responsivities of the two magnetometers does not exceed one order of magnitude.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The highest sensitivity of existing commercial atomic magnetometers is 30fT / Hz -1 / 2, it is difficult to directly evaluate the noise of the shielded barrel with lower noise, so it is necessary to develop higher-precision metrological magnetometers, and the existing higher-precision magnetometers are still in the laboratory stage. This method only requires two independent commercial atomic magnetometers, or any adjacent channels in the cardio-brain magnetic device, to evaluate the noise floor of the shielded barrel through an algorithm. It is low-cost and does not require complicating the original cardio-brain magnetic device. Furthermore, the operating environment of the cardio-brain magnetic device can also be tested and evaluated to assist in fault diagnosis and analyze the main interference sources of the cardio-brain magnetic device. In addition, this method essentially extracts the common-mode components between different sensors, and can also be applied to scientific research or commercial applications such as common-mode interference suppression.

[0029] The present invention is based on the Wiener-Hinchin theorem: the power spectral density of any wide-sense stationary random process with a constant mean is the Fourier transform of its autocorrelation function. The signals of the two-channel magnetometers are cross-correlated, and the result is Fourier transformed. By swapping the integration order of the cross-correlation and Fourier transform, the autocorrelation of the shielding barrel noise floor signals in the two channels can be derived. Therefore, the cross-spectral calculation yields the common-mode component of the two independent magnetometers, i.e., the noise floor of the magnetic shielding barrel. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The present invention is a flowchart of a method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis. Figure 1 The method comprises the following steps: 1, using a first atomic magnetometer to establish a first noise data acquisition channel through the center of a magnetic shielding barrel and form first channel discrete voltage noise data; and simultaneously using a second atomic magnetometer to establish a second noise data acquisition channel through the center of the magnetic shielding barrel and form second channel discrete voltage noise data; 2, performing Hanning windowing processing on the first channel discrete voltage noise data and the second channel discrete voltage noise data respectively; and 3, performing discrete Fourier transform on the first channel windowed data and the second channel windowed data respectively to obtain first channel spectrum data V1(f n ), and the second channel spectrum data V2(f n ); Step 4, V1(f n ) and V2(f n ) to perform cross-spectral calculation and obtain the power spectrum density X(f n ), using V1(f n ) to obtain the power spectrum density P1(f n ), using V2(f n ) to obtain the power spectrum density P2(f n ), using X(f n) to obtain the noise floor of the magnetically shielded barrel.

[0031] Figure 2 It is a schematic diagram of the results of magnetometer cross-spectrum analysis obtained by implementing the present invention to evaluate the noise floor of the magnetic shielding barrel. Figure 2 The horizontal axis is the frequency (Hz, scale value is 0, 10, ···, 90, 100), and the vertical axis is the magnetic noise spectral density (fT / Hz -1 / 2 ), the scale value is 10 0 ,10 1 ,···,10 4 ,10 5 ). Figure 2 The plus sign line in the middle corresponds to the measurement of the first atomic magnetometer, the dot line in the middle corresponds to the measurement of the second atomic magnetometer, and the circle line in the middle corresponds to the noise floor of the magnetic shielding barrel extracted by the cross-spectrum. Figure 2 Medium 8.6fT / Hz -1 / 2 It is the noise floor of the magnetic shielded barrel at 30Hz frequency.

[0032] Figure 3 This is a schematic diagram of the noise floor measurement results of a magnetic shielding barrel using a high-sensitivity dual-optical magnetometer. Figure 3 Medium 8.6fT / Hz -1 / 2 is the magnetic noise at 30Hz frequency, and Figure 2 This proves the effectiveness of the method of the present invention. DETAILED DESCRIPTION

[0033] Below is the attached figure ( Figure 1-Figure 3 ) and Examples illustrate the present invention.

[0034] Figure 1 The present invention is a flowchart of a method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis. Figure 2 It is a schematic diagram of the results of magnetometer cross-spectrum analysis obtained by implementing the present invention to evaluate the noise floor of the magnetic shielding barrel. Figure 3 This is a schematic diagram of the noise floor measurement results of a magnetic shielding barrel using a high-sensitivity dual-optical magnetometer. Figures 1 to 3 As shown, a method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis includes the following steps: step 1, using a first atomic magnetometer to establish a first noise data acquisition channel through the center position of the magnetic shielding barrel and form first channel discrete voltage noise data, and at the same time using a second atomic magnetometer to establish a second noise data acquisition channel through the center position of the magnetic shielding barrel and form second channel discrete voltage noise data; step 2, performing Hanning windowing processing on the first channel discrete voltage noise data and the second channel discrete voltage noise data respectively; step 3, performing discrete Fourier transform on the first channel windowed data and the second channel windowed data respectively, to obtain the first channel spectrum data V1(f n ), and the second channel spectrum data V2(fn ); Step 4, V1(f n ) and V2(f n ) to perform cross-spectral calculation and obtain the power spectrum density X(f n ), using V1(f n ) to obtain the power spectrum density p1(f n ), using V2(f n ) to obtain the power spectrum density P2(f n ), using X(f n ) to obtain the noise floor of the magnetically shielded barrel.

[0035] The first and second atomic magnetometers in step 1 are both 30fT / Hz -1 / 2 Sensitivity Commercial magnetometer, or magnetometer used to detect cardiac magnetism in cardiac magnetometry device, the photocurrent output by the magnetometer is converted into voltage through a transimpedance amplifier, and the discrete voltage data is collected by an analog-to-digital conversion collector.

[0036] Step 2 includes the following formula:

[0037]

[0038] Where W H (m) is the window function, M is the total number of sampling points of a single magnetometer, M is a positive integer, m is the sampling point sequence 0≤m≤M, m is an integer, and W H (m) is multiplied with the discrete voltage data to complete the windowing processing of the magnetometer discrete voltage data.

[0039] Step 3 includes the following formula:

[0040]

[0041]

[0042] Where v1(m) is the discrete voltage noise data of the first channel, v2(m) is the discrete voltage noise data of the second channel, n is a positive integer, e is a natural constant, and i is an imaginary unit.

[0043] Step 4 includes the following formula:

[0044]

[0045] where f s is the sampling rate, It is the conjugate operation performed on V1(fn).

[0046] Step 4 includes: using the magnetic field voltage response coefficient of the magnetometer to perform conversion and calculate the noise floor of the magnetic shielding barrel.

[0047] Step 4 includes the following steps: Step 41, retain X(f n ), which is the real part of X(f n ) amplitude; Step 42: For p1(f n ) and P2(f n ) and X(f n ), the average value within each 1Hz frequency range is taken as the power spectrum density value P1, P2 and X of each integer frequency point, P1 corresponds to p1(f n ), P2 corresponds to P2(f n ), X corresponds to X(f n ); Step 43: Take the square root of X to obtain the voltage noise amplitude, the unit of voltage noise amplitude is V / Hz -1 / 2 , using the voltage-magnetic field conversion coefficient K of the magnetometer, the unit of K is V / nT, and K is divided by the voltage noise amplitude to obtain the noise floor value of the magnetic shielding barrel. The larger the value, the higher the noise floor.

[0048] The sensitive axes of the first atomic magnetometer and the second atomic magnetometer in step 1 are parallel, the intrinsic noises of the two magnetometers are non-correlated signals, and the difference in the responsivities of the two magnetometers does not exceed one order of magnitude.

[0049] The present invention discloses a method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis, which relates to the fields of magnetic field measurement technology and weak signal detection. The method comprises the following steps: data acquisition, using two independent atomic magnetometers to simultaneously collect noise data at the center of the magnetic shielding barrel; windowing the discretized noise data of the two channels using a Hanning window to reduce near-sidelobe leakage; Fourier transforming the windowed dual-channel data to obtain spectral data; and finally performing a frequency-domain cross-correlation conjugate operation to calculate the frequency-domain characteristics of the common-mode components of the two atomic magnetometers. The noise spectrum of the common-mode components of the two magnetometers is analyzed to obtain the noise floor of the magnetic shielding barrel. The present invention uses cross-spectral analysis to extract the common-mode components between the two magnetometers from the data collected by the two-channel magnetometers, thereby evaluating the noise floor of the magnetic shielding barrel or the noise floor of the magnetic shielding environment. This method eliminates the need to develop and use higher-precision metrological magnetometers and relies solely on two less sensitive magnetometers to evaluate the noise floor of the magnetic shielding barrel. This reduces costs, eliminates the need to increase the complexity of the extremely weak magnetic field measurement device, and provides high reliability.

[0050] A method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis collects data from two independent atomic magnetometer channels and performs windowing, Fourier transform, and cross-spectral calculations on each channel. Finally, the noise floor of the magnetic shielding barrel is measured using a cross-spectral density plot. The method includes the following steps:

[0051] S1: Two independent atomic magnetometers are used to simultaneously collect noise data at the center of a magnetic shielding barrel. The photocurrent output by the magnetometer is converted into a voltage through a transimpedance amplifier, and the discrete voltage data is collected through an analog-to-digital converter.

[0052] S2: The discrete voltage noise data collected from the two channels is windowed using a Hanning window to reduce near-sidelobe leakage.

[0053] S3: Perform discrete Fourier transform on the windowed dual-channel data to obtain spectrum data V1 (f n ) and V2(f n );

[0054] S4: Convert the spectrum data of magnetometers 1 and 2 into power spectrum and divide it by the frequency resolution to obtain the power spectrum density P1(f n ) and P2(f n ), which is used to analyze the noise of magnetometers 1 and 2. If the power spectral density obtained in this step is greater than the power spectral density obtained by the cross-spectrum, it indicates that the shielding barrel noise floor is low, and the cross-spectrum result is the magnetic shielding barrel noise floor. If the power spectral density obtained in this step is similar to the cross-spectrum result, it indicates that the current magnetic shielding barrel noise is high, and the magnetic shielding barrel noise floor can be directly obtained from the magnetometer power spectral density data;

[0055] The spectrum V1(f n ) and V2(f n ) to perform cross-spectral calculation and calculate the power spectrum density data X(f n ), the magnetic field voltage response coefficient of the magnetometer is used for conversion to calculate the magnetic noise floor of the shielding barrel.

[0056] The two independent atomic magnetometers refer to atomic magnetometers that use different lasers and different analog-to-digital conversion acquisition channels, so that the intrinsic noise of the two magnetometers is non-correlated signal, and the response of the two magnetometers needs to differ by no more than one order of magnitude.

[0057] The analog-to-digital converter collects discrete voltage data, and the sampling rate is downsampled to 4 kSPS (4,000 samples per second). In order to make the power spectrum density obtained later smooth enough, the collection time is required to be more than 30 minutes.

[0058] The Hanning window function expression in step S2 is:

[0059]

[0060] Where M is the total number of sampling points of a single magnetometer, and m is the sampling point sequence 0≤m≤M. H(m) is the window function, which is multiplied by the discrete voltage data to complete the windowing processing of the magnetometer discrete voltage data.

[0061] The discrete Fourier expression in step S3 is:

[0062]

[0063] Where v1(m) is the discrete voltage data of magnetometer 1, v2(m) is the discrete voltage data of magnetometer 2, V1(f n ) is the fast Fourier transform spectrum of magnetometer 1, V2(f n ) is the fast Fourier transform spectrum of magnetometer 2, f n is a frequency sequence.

[0064] The power spectrum density expression in step S4 is:

[0065]

[0066] where f s is the sampling rate, P1(f n ) is the power spectral density of the first magnetometer, P2(f n ) is the power spectral density of the second magnetometer.

[0067] The magnetic noise in the magnetic shielding barrel is a wide-sense stationary random process with a constant mean, and the cross-spectral expression is:

[0068]

[0069] Among them, X(f n ) is the power spectrum density of the common mode components of the two channel magnetometers obtained by cross-spectral analysis, It is the conjugate operation of V1(f).

[0070] Step S4 further comprises:

[0071] S41: Keep X(f n ), which is the amplitude of the power spectral density;

[0072] S42: The power spectrum density P1 (f n ) and the power spectral density P2(f n ), and X(f n ), and take the average value within each 1 Hz frequency range as the power spectrum density value of the corresponding integer frequency point;

[0073] S43: Calculate the voltage noise amplitude (unit: V / Hz) by revising the result of S42. -1 / 2), the magnetic noise floor inside the shielding barrel is calculated by dividing the voltage-to-magnetic-field conversion coefficient of the magnetometer, also known as the responsivity K (unit: V / nT), by the voltage noise amplitude. The larger the value, the higher the noise floor.

[0074] To facilitate implementation by those skilled in the art, the present invention will be described using a magnetically enclosed shielding barrel with an outer diameter of 45 cm and a length of 90 cm, consisting of four inner layers of permalloy and an outermost layer of aluminum. The demagnetized barrel is a common laboratory magnetic shielding barrel configuration. The present invention provides a method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis, comprising the following steps:

[0075] S1: Using two independent atomic magnetometers, the atomic source uses 87 Rb is heated to 120 degrees Celsius and the static magnetic field surrounding the magnetometer is compensated. The magnetometer uses a 795nm laser for atomic pumping to polarize rubidium alkali metal atoms. A photoelectric converter converts the intensity of the pump light after it passes through the atomic gas chamber into a change in photocurrent. This is then converted into a voltage signal by a transimpedance amplifier in the electrical acquisition module for acquisition. A 1kHz modulated magnetic field is applied in a direction perpendicular to the pump light direction. The direction of the modulated magnetic field is the sensitive direction of the atomic magnetometer. Here, the sensitive direction is set to the axial direction of the magnetic shielding barrel. The present invention requires that the sensitive axes of the two magnetometers be parallel. Independence refers to the fact that the two magnetometers use different lasers and different analog-to-digital conversion acquisition channels, so that the intrinsic noise of the two magnetometers is uncorrelated, and the responsivity of the two magnetometers must not differ by more than an order of magnitude. Furthermore, the analog-to-digital acquisition device acquires discrete voltage data, and the sampling rate is downsampled to 4kSPS from the sampling rate of the analog-to-digital acquisition device. To ensure a smooth power spectrum, the acquisition time required was over 30 minutes. The voltage signal was demodulated at 1kHz using the phase-locked amplifier module within the electrical acquisition module to obtain the discrete voltage data v1(m) and v2(m) from the two atomic magnetometers. Furthermore, synchronized data acquisition between the two magnetometers was crucial for the subsequent cross-spectral analysis to effectively extract the common-mode component.

[0076] S2: The discrete voltage noise data collected from the two channels is windowed using a Hanning window to reduce near-sidelobe leakage.

[0077] In order to better reduce the interference signal adjacent to the main lobe in the spectrum, that is, the near side lobe leakage, the Hanning window is used. The window function coefficients applied use the root mean square value, not the maximum value, so choose The window function expression is:

[0078]

[0079] Where M is the total number of sampling points of a single magnetometer, and m is the sampling point sequence 0≤m≤M. H (m) is the window function, which is multiplied by the discrete voltage data to complete the windowing processing of the magnetometer discrete voltage data.

[0080] S3: Perform discrete Fourier transform on the windowed dual-channel data to obtain spectrum data V1 (f n ) and V2(f n );

[0081] The discrete Fourier expression in this step is:

[0082]

[0083] S4: Convert the spectrum data of magnetometers 1 and 2 into power spectrum and divide it by the frequency resolution M*f s , obtain the power spectrum density diagram of the two-channel magnetometer, evaluate the noise spectrum of each channel, and use it to compare the results of S5 cross-spectral analysis to verify the correctness of common mode extraction;

[0084] The power spectrum density expression in this step is:

[0085]

[0086] where f s is the sampling rate, P1(f n ) is the power spectral density of the first magnetometer, P2(f n ) is the power spectral density of the second magnetometer.

[0087] The frequency spectra of the two magnetometers are cross-spectral operated to calculate the frequency domain characteristics of the common mode components of the two atomic magnetometers. The common mode component is the magnetic noise floor, that is, the magnetic noise of the magnetic shielding barrel. The shielding barrel noise floor is calculated through the response coefficient of the magnetometer.

[0088] The cross-spectral expression is:

[0089]

[0090] Among them, X(f n) is the power spectrum of the common-mode component of the two-channel magnetometers obtained by cross-spectral analysis. This formula assumes that the response voltage v1(t) acquired by the analog-to-digital converter of magnetometer 1 and the response voltage v2(t) acquired by the analog-to-digital converter of magnetometer 2 have the following characteristics: v1(t) = s(t) + n1(t) and v2(t) = s(t) + n2(t), respectively. Here, s(t) is the common-mode component of the two magnetometers, i.e., the magnetic noise of the magnetometer's operating environment; n1(t) is the uncorrelated noise floor of magnetometer 1 and the uncorrelated electrical noise introduced during signal processing; and n2(t) is the uncorrelated noise floor of magnetometer 2 and the uncorrelated electrical noise introduced during signal processing. Therefore, when performing a cross-correlation operation on the two channels, only the term s(t) + s(t-τ), i.e., the autocorrelation term of the shielding barrel magnetic noise, remains; the remaining terms are zero, where s(t-τ) is the delay τ of s(t). Therefore, the result is Fourier transformed, and the order of integration is exchanged. By utilizing the time-shift property of the Fourier transform, the expression of the shielding barrel noise power spectrum can be obtained. Furthermore, the power spectrum needs to be converted to units to evaluate the noise floor of the magnetic shielding barrel. Therefore, step S4 further includes:

[0091] S41: Keep X(f n ), which is the amplitude of the power spectral density;

[0092] S42: The power spectrum density P1 (f n ) and the power spectral density P2(f n ), and X(f n ), take the average value within each 1 Hz frequency range as the power spectrum density value at the integer frequency point;

[0093] S43: Calculate the voltage noise amplitude (unit: V / Hz) by revising the result of S42. -1 / 2 ), the magnetic noise floor inside the shielding barrel is calculated by dividing the voltage-to-magnetic-field conversion coefficient of the magnetometer, also known as the responsivity K (unit: V / nT), by the voltage noise amplitude. The larger the value, the higher the noise floor.

[0094] Finally, the cross-spectrum analysis results of the noise floor of the shielded magnetic shielding barrel are as follows Figure 2 As shown in the figure, magnetometers 1 and 2 synchronously collect 60 minutes of 4kSPS magnetic shielding barrel noise data, which does not contain any signal. Since the magnetic shielding barrel for biomagnetism is based on the characteristics of biological signals, it usually focuses on the 2-100Hz frequency range. Therefore, this example evaluates the noise floor of the magnetic shielding barrel at a frequency point near 30Hz. As shown by the dotted line in the figure, the magnetometer evaluated by the method of the present invention is 8.6fT / Hz -1 / 2Furthermore, in order to verify the correctness of the evaluation results, a high-sensitivity dual-light magnetometer with a 10mm air chamber developed in the laboratory was used, and a calibrated coil was used in a magnetic shielding environment to apply a 30Hz, 100pT average calibration magnetic field to obtain the magnetometer response coefficient K. Figure 3 Peaks appear at 30Hz and 60Hz. Figure 3 The magnetic noise measured at the same position is also 8.6fT / Hz. -1 / 2 , which proves the effectiveness of the method of the present invention.

[0095] The above process belongs to offline data processing. After data acquisition, it is necessary to complete the data processing on a computer. Each step is completed through the response function of Matlab to evaluate the noise floor of the magnetic shielding barrel. It can also be described in C language and burned into the array magnetometer control system through compiled software. It is used to evaluate the operating environment noise of the array atomic magnetometer device and perform fault analysis.

[0096] This method is based on the Wiener-Hinchin theorem: the power spectral density of any generalized stationary random process with a constant mean is the Fourier transform of its autocorrelation function. By processing cross-spectral data, two magnetometers are used to assess the noise floor of a magnetically shielded barrel, which is significantly lower than the magnetometer noise floor. This approach is cost-effective and simple to use. The method is also applicable to various magnetically shielded spaces and can be used in other fields to extract common-mode components for analysis, demonstrating its high practical value in engineering applications.

[0097] Any content not described in detail in this specification is prior art known to those skilled in the art. It should be noted that the above description is intended to help those skilled in the art understand the present invention, but does not limit the scope of protection of the present invention. Any equivalent substitution, modification, improvement, and / or simplification of the above description that does not depart from the essence of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis, characterized in that: The following steps are involved: Step 1: using a first atomic magnetometer to establish a first noise data acquisition channel through the center of the magnetic shielding barrel and form discrete voltage noise data of the first channel; at the same time, using a second atomic magnetometer to establish a second noise data acquisition channel through the center of the magnetic shielding barrel and form discrete voltage noise data of the second channel; Step 2, performing Hanning windowing processing on the discrete voltage noise data of the first channel and the discrete voltage noise data of the second channel respectively; Step 3: Perform discrete Fourier transform on the first channel windowed data and the second channel windowed data to obtain the first channel spectrum data V1 (f n ), and the second channel spectrum data V2(f n ); Step 4: V1(f n ) and V2(f n ) to perform cross-spectral calculation and obtain the power spectrum density X(f n ), using V1(f n ) to obtain the power spectrum density P1(f n ), using V2(f n ) to obtain the power spectrum density P2(f n ), using X(f n ) to obtain the noise floor of the magnetic shielding barrel; Step 4 includes: using the magnetic field voltage response coefficient of the magnetometer to convert and calculate the noise floor of the magnetic shielding barrel; Step 4 includes the following steps: Step 41, retain X(f n ), which is the real part of X(f n )’s amplitude; Step 42: For P1(f n ) and P2(f n ) and X(f n ), the average value within each 1Hz frequency range is taken as the power spectrum density value P1, P2 and X of each integer frequency point, P1 corresponds to P1(f n ), P2 corresponds to P2(f n ), X corresponds to X(f n ); Step 43: Take the square root of X to get the voltage noise amplitude. The unit of voltage noise amplitude is V / Hz. -1 / 2 , using the voltage-magnetic field conversion coefficient K of the magnetometer, the unit of K is V / nT, and K is divided by the voltage noise amplitude to obtain the noise floor value of the magnetic shielding barrel. The larger the value, the higher the noise floor.

2. The method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis according to claim 1, characterized in that: The first and second atomic magnetometers in step 1 are both 30fT / Hz -1 / 2 Sensitivity Commercial magnetometer, or magnetometer used to detect cardiac magnetism in cardiac magnetometry device, the photocurrent output by the magnetometer is converted into voltage through a transimpedance amplifier, and the discrete voltage data is collected by an analog-to-digital conversion collector.

3. The method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis according to claim 1, characterized in that: Step 2 includes the following formula: Where W H (m) is the window function, M is the total number of sampling points of a single magnetometer, M is a positive integer, m is the sampling point sequence 0≤m≤M, m is an integer, and W H (m) is multiplied with the discrete voltage data to complete the windowing processing of the magnetometer discrete voltage data.

4. The method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis according to claim 1, characterized in that: Step 3 includes the following formula: Where v1(m) is the discrete voltage noise data of the first channel, v2(m) is the discrete voltage noise data of the second channel, n is a positive integer, e is a natural constant, and i is an imaginary unit.

5. The method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis according to claim 1, characterized in that: Step 4 includes the following formula: where f s is the sampling rate, It is the conjugate operation performed on V1(fn).

6. The method for evaluating the noise floor of a magnetic shielding barrel based on cross-spectral analysis according to claim 1, characterized in that: The sensitive axes of the first atomic magnetometer and the second atomic magnetometer in step 1 are parallel, the intrinsic noises of the two magnetometers are non-correlated signals, and the difference in the responsivities of the two magnetometers does not exceed one order of magnitude.

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