JP2021071374A - Magnetic field measuring apparatus - Google Patents

Abstract

To provide a magnetic field measuring apparatus that can measure magnetic field signals of a plurality of measurement objects.SOLUTION: A magnetic field measuring apparatus has a digital FLL circuit including: an amplifier that amplifies a voltage output from a superconducting quantum interference device according to the intensity of a magnetic field; a plurality of low-pass filters that have filter characteristics in accordance with a plurality of measurement objects for which magnetic fields are measured, and perform anti-aliasing of signals output from the amplifier; a selection circuit that selects one of the plurality of low-pass filters according to the plurality of measurement objects; and an AD converter that converts an analog signal from the low-pass filter selected by the selection circuit into a digital signal by using a sampling frequency higher than a plurality of sampling frequencies corresponding to the plurality of measurement objects.SELECTED DRAWING: Figure 1

Description

The present invention relates to a magnetic field measuring device.

In biomagnetic field measurement using a superconducting QUantum Interference Device (SQUID), which is a superconducting ring having a Josephson junction, the measurement characteristics are non-linear. Therefore, linearization is performed using a FLL (Flux Locked Loop) circuit, and the magnetic field is measured. Hereinafter, the superconducting quantum interference device is also referred to as SQUID.

The FLL circuit includes an analog FLL system composed of only an analog circuit and a digital FLL system composed of a circuit that is once digitized and then digitized again. Since biomagnetic field measurement usually uses multiple channels, digital FLL is used in terms of suppressing variations between channels, reducing system costs, easiness of data processing, and advances in semiconductor technology. The method has been widely used.

A method for reducing costs by using a change amount counter that counts the number of periods of magnetic flux quantum and a reproduction counter that counts the frequency with respect to the magnetic flux to be measured in a magnetic field measuring device including SQUID and a digital FLL circuit is disclosed. (See Patent Document 1).

A magnetic field measuring device having a digital FLL circuit has been applied to a single application such as a magnetoencephalogram (MEG: Magnetoencephalograph), a magnetoencephalogram (MCG: Magnetocardiograph) or a spinometer (MSG: Magnetospinograph). This is because the sampling frequency and the like suitable for acquiring the biomagnetic field signal differ depending on the part to be measured. Therefore, one magnetic field measuring device cannot measure a plurality of magnetic field signals to be measured.

The disclosed technique has been made in view of the above problems, and an object of the present invention is to provide a magnetic field measuring device capable of measuring magnetic field signals of a plurality of measurement targets.

In order to solve the above technical problems, the magnetic field measuring device of one embodiment of the present invention includes an amplifier that amplifies the voltage output from the superconducting quantum interference element according to the strength of the magnetic field, and a plurality of measurements in which the magnetic field is measured. A plurality of low-pass filters having filter characteristics according to the target and performing antialiasing of the signal output from the amplifier, and a selection circuit for selecting one of the plurality of low-pass filters according to the plurality of measurement targets. Includes an AD converter that converts an analog signal from the low-pass filter selected by the selection circuit into a digital signal using a sampling frequency higher than the plurality of sampling frequencies corresponding to the plurality of measurement targets. It is characterized by having a digital FLL circuit.

It is possible to provide a magnetic field measuring device capable of measuring a plurality of magnetic field signals to be measured.

It is a block diagram which shows an example of the magnetic field measuring apparatus which concerns on 1st Embodiment of this invention. It is a block diagram which shows an example of the circuit structure around the low-pass filter of FIG. It is a block diagram which shows an example of the magnetic field measuring apparatus which concerns on 2nd Embodiment of this invention. It is a block diagram which shows an example of the magnetic field measuring apparatus which concerns on 3rd Embodiment of this invention.

Hereinafter, embodiments will be described with reference to the drawings. In each drawing, the same components may be designated by the same reference numerals, and duplicate description may be omitted.

(First Embodiment)
FIG. 1 is a block diagram showing an example of a magnetic field measuring device according to the first embodiment of the present invention. For example, the magnetic field measuring device 100 (biomagnetic field measuring device) shown in FIG. 1 employs a digital FLL method, and is applicable to a magnetoencephalograph, a spinograph, and a magnetocardiograph. Further, the magnetic field measuring device 100 shown in FIG. 1 may be applied to the measurement of a nerve magnetic field or a muscle magnetic field.

表１は、用途（計測対象）毎の生体磁場信号の計測における磁気感度（Ｔ）、信号帯域（Ｈｚ）およびチャネル数の一例を示している。表１に示すように、生体磁場の計測に必要となる磁気感度、信号帯域およびチャネル数は、脊磁計（ＭＳＧ）、心磁計（ＭＣＧ）、脳磁計（ＭＥＧ）によりそれぞれ異なる。必要な信号帯域は、脊磁計、心磁計、脳磁計毎に異なるため、信号帯域をカバーするサンプリング周波数も異なる。後述するように、各用途でのサンプリング周波数は、例えば、信号帯域の数倍から１０倍程度である。

Table 1 shows an example of the magnetic sensitivity (T), signal band (Hz), and number of channels in the measurement of the biomagnetic field signal for each application (measurement target). As shown in Table 1, the magnetic sensitivity, signal band, and number of channels required for measuring the biomagnetic field differ depending on the spinometer (MSG), magnetocardiograph (MCG), and magnetoencephalograph (MEG). Since the required signal band differs for each spinometer, magnetocardiograph, and magnetoencephalogram, the sampling frequency covering the signal band also differs. As will be described later, the sampling frequency in each application is, for example, several times to ten times the signal band.

Since the spinometer inputs an electrical stimulus from the outside and measures the biomagnetic field induced by the electrical stimulus, the artifact (noise) caused by the electrical stimulus affects the measurement result. In general, artifacts are larger than the biomagnetic field and therefore require a wide dynamic range.

Further, in the measurement of the magnetic field, not only the sampling frequency used in the spinometer, the magnetocardiography, the magnetoencephalograph, etc., but also the sampling frequency used in the long-time mode, the environmental magnetic field measurement, etc. is required. The long-time mode is used, for example, in the diagnosis of epilepsy when the biomagnetic field is measured for a long time by a magnetoencephalograph while the subject is stimulated with sound, images, electricity, or the like. In addition, for example, in a state where vagus nerve stimulation (VNS; Vagus Nerve Stimulation) used for the treatment of epilepsy is used to electrically stimulate the vagus nerve of the subject, or when an external electrical stimulation is applied. When measuring the biomagnetic field with a cerebral magnetometer, a wide dynamic range is required to measure the biomagnetic field because large artifacts are generated by electrical stimulation.

In the environmental magnetic field measurement, by measuring the magnetic field of the ambient environment of the magnetic field measuring device 100 without measuring the biological magnetic field of the subject, it is confirmed whether the magnetic field measuring device 100 is operating normally, and the biological magnetic field is measured. It is determined whether measurement is possible.

Usually, in the measurement of a biomagnetic field by a magnetoencephalograph, data is measured for diagnosis in synchronization with an electroencephalogram (EEG). Further, in the measurement of the biomagnetic field by the magnetocardiography, the data is measured in order to make a diagnosis in synchronization with the electrocardiogram (ECG).

As described above, as the applications of the magnetic field measuring device 100 increase, the number of sampling frequencies used also increases, and circuits and controls for efficiently processing magnetic field signals in a wide signal band are required.

The magnetic field measuring device 100 shown in FIG. 1 has a SQUID 10 (superconducting quantum interference element) and a SQUID sensor 20. The SQUID 10 and the SQUID sensor 20 shown in FIG. 1 indicate one channel of the magnetic field measuring device 100. Although not particularly limited, for example, the magnetic field measuring device 100 has tens or hundreds of channels.

The SQUID 10 is a highly sensitive magnetic sensor that detects a magnetic field (magnetic flux) generated from a living body and penetrates a superconducting ring having a Josephson junction. For example, the SQUID 10 is configured by providing Josephson junctions at two locations on the superconducting ring.

The SQUID 10 generates a voltage that changes periodically with respect to a change in magnetic flux penetrating the superconducting ring. Therefore, the magnetic flux penetrating the superconducting ring can be obtained by measuring the voltage across the superconducting ring with the bias current flowing through the superconducting ring. Hereinafter, the characteristic of the periodic voltage change in which the SQUID 10 is generated is also referred to as a Φ-V characteristic.

The SQUID sensor 20 has a digital FLL circuit 30 and a post-processing circuit 40 that process the magnetic field signal detected by the SQUID 10. The digital FLL circuit 30 includes an amplifier 31, a plurality of low-pass filters 32a, 32b, 32c, 32d, an AD (Analog-to-Digital) converter 33, a digital integrator 34, and a DA (Digital-to-Analog) converter 35. It has a voltage-current converter 36 and a feedback coil 37. Hereinafter, when the low-pass filters 32a, 32b, 32c, and 32d are described without distinction, they are also referred to as the low-pass filter 32. The feedback coil 37 arranged close to the SQUID 10 is mounted on a circuit board different from the circuit board of the digital FLL circuit 30 on which the amplifier 31, AD converter 33, and the like are mounted.

The amplifier 31 amplifies the output voltage generated by the SQUID 10 according to the strength of the magnetic field by the magnetic flux penetrating the SQUID 10, and outputs the amplified output voltage to the AD converter 33 via any of the low-pass filters 32.

The low-pass filters 32a, 32b, 32c, and 32d have different filter characteristics from each other, and one of the low-pass filters 32a, 32b, 32c, and 32d is selected when measuring the magnetic field. The selected low-pass filter 32 antialiases the signal output from the amplifier 31, and outputs the antialiased signal to the AD converter 33.

FIG. 2 is a block diagram showing an example of a circuit configuration around the low-pass filters 32a, 32b, 32c, and 32d of FIG. The digital FLL circuit 30 has a selection circuit 38 arranged between the output of the amplifier 31 and the input of each of the low-pass filters 32a, 32b, 32c, 32d.

The selection circuit 38 has an input terminal connected to the output of the amplifier 31 and four output terminals connected to the inputs of the low-pass filters 32a, 32b, 32c, and 32d, respectively. The selection circuit 38 transmits the signal amplified by the amplifier 31 to any of the low-pass filters 32a, 32b, 32c, and 32d according to the switching signal. For example, the switching signal is generated by a data processing device to which data is transferred from the thinning processing unit 41 (FIG. 1) according to the measurement target (use) of the magnetic field. For example, the switching signal is a 2-bit logic signal indicating any of four logic values, and the output of the amplifier 31 is output to any of the inputs of the low-pass filters 32a, 32b, 32c, and 32d according to the four logic values. Connect to. In the example shown in FIG. 2, the output of the amplifier 31 is connected to the input of the low-pass filter 32c.

The low-pass filter 32, which receives the signal via the selection circuit 38, outputs the antialiased signal to the AD converter 33. The other low-pass filter 32 that does not receive the signal stops operating. The outputs of the low-pass filters 32a, 32b, 32c, 32d are wired OR connected and connected to the input of the AD converter 33. An enable terminal may be provided in each low-pass filter 32, and only the low-pass filter 32 that has received the enable signal at the enable terminal may be operated.

The number of low-pass filters 32 mounted on the digital FLL circuit 30 may be two or more. For example, a low-pass filter having a cutoff frequency fc of 500 Hz may be further added to the four low-pass filters 32 shown in FIG. 2, and the selection circuit 38 may have five output terminals. In this case, the switching signal becomes a 3-bit logic signal.

For example, the cutoff frequency fc is set to 20 kHz for the low-pass filter 32a, 5 kHz for the low-pass filter 32b, 3 kHz for the low-pass filter 32c, and 1 kHz for the low-pass filter 32d. The filter characteristics (cutoff frequency fc, filter shape, order, attenuation factor, etc.) of each low-pass filter 32 are determined in consideration of antialiasing, noise suppression, and the like.

表２は、用途（計測対象）毎の生体磁場信号の計測に必要とされるサンプリング周波数ｆｓとアンチエイリアスを実施するためのローパスフィルタのカットオフ周波数ｆｃとの一例を示している。表２に示すように、用途毎のサンプリング周波数ｆｓは、脊磁計（ＭＳＧ）で４０ｋｓｐｓ（sampling per second）、心磁計（ＭＣＧ）で５〜４０ｋｂｐｓ、脳磁計（ＭＥＧ）で５０ｓｐｓ〜１０ｋｓｐｓである。例えば、脳磁計において、１ｋｓｐｓ以下のサンプリング周波数ｆｓは、ノイズの検査等に使用される。

Table 2 shows an example of the sampling frequency fs required for measuring the biomagnetic field signal for each application (measurement target) and the cutoff frequency fc of the low-pass filter for performing antialiasing. As shown in Table 2, the sampling frequency fs for each application is 40 kps (sampling per second) for a spinometer (MSG), 5 to 40 kbps for a magnetocardiograph (MCG), and 50 sps to 10 kps for a magnetoencephalograph (MEG). For example, in a magnetoencephalograph, a sampling frequency fs of 1 kps or less is used for noise inspection and the like.

The sampling frequencies fs for inspection and frequency characteristic measurement, which are commonly used in a spinometer (MSG), a magnetocardimeter (MCG), and a magnetoencephalograph (MEG), are 1 kps and 40 kps, respectively. For example, the sampling frequency fs for inspection is used for measuring the environmental magnetic field, and the sampling frequency fs for measuring frequency characteristics determines whether or not the biomagnetic field signal can be measured by measuring the level of magnetic field noise. Used to do.

Since the cutoff frequency fc for each application is set to about 1/2 to 1/10 of the sampling frequency fs, it is equal to or less than the Nyquist frequency, which is 1/2 of the sampling frequency fs. Therefore, the occurrence of the folding phenomenon at the sampling frequency fs is prevented. For example, in this embodiment, when the cutoff frequency fc matches the cutoff frequency fc of the lowpass filter 32, the lowpass filter 32 that matches the cutoff frequency fc is selected. In applications where the cutoff frequency fc does not match the cutoff frequency fc of the lowpass filter 32, the lowpass filter 32 with a cutoff frequency fc lower than the Nyquist frequency is selected. In a magnetoencephalograph, for applications where the sampling frequency fs is 1 kps or less, it is preferable to select the low-pass filter 32d having the lowest cutoff frequency fc because the sampling frequency can be minimized and the amount of data is reduced.

In this embodiment, the antialiasing of the signal measured by the SQUID 10 for each application is performed by the low-pass filter 32, so that the low-pass filter for antialiasing does not have to be provided in front of the thinning processing unit 41 in the post-processing circuit 40.

Returning to FIG. 1, the AD converter 33 converts the analog signal antialiased by any of the low-pass filters 32 into a digital signal (voltage value) by sampling at a predetermined sampling frequency fs. That is, the AD converter 33 converts the voltage output from the SQUID 10 into a digital value according to the change in the magnetic field. The AD converter 33 outputs the digital value generated by the conversion to the digital integrator 34.

For example, the AD converter 33 samples data at a sampling frequency fs that is sufficiently larger than the maximum value of the sampling frequency fs for each application (measurement target). Although not particularly limited, the sampling frequency fs used in the AD converter 33 is, for example, 400 kHz. Further, the sampling frequency fs of the AD converter 33 is set to a common multiple (integer multiple) of the sampling frequency fs for each application.

When the sampling frequency fs of the AD converter 33 is not a common multiple of the sampling frequency fs for each application, it is necessary to provide a data complementing unit for complementing the data in front of the thinning processing unit 41. The data complement unit complements the data in order to make the sampling frequency fs of the data output from the digital integrator 34 an integral multiple (for example, the least common multiple) of the sampling frequency fs for each application, and the data including the complemented data. Is output to the thinning processing unit 41.

The digital integrator 34 integrates the change in the voltage of SQUID 10 (to be exact, the amplified voltage output from the amplifier 31) from the lock point, which is the starting point of each period of the Φ-V characteristic, and integrates the voltage value. The integrated value is output to the DA converter 35. Further, the digital integrator 34 outputs the integrated voltage value to the thinning processing unit 41 of the post-processing circuit 40.

The DA converter 35 converts the voltage value (digital signal) integrated by the digital integrator 34 into a voltage, and outputs the converted voltage to the voltage-current converter 36. The voltage-current converter 36 converts the voltage received from the DA converter 35 into a current, and outputs the converted current to the feedback coil 37.

The feedback coil 37 generates a magnetic flux by the current received from the voltage-current converter 36, and feeds back the generated magnetic flux to the SQUID 10 as a feedback magnetic flux. That is, the feedback coil 37 generates a magnetic field received by the SQUID 10 based on the current from the voltage-current converter 36. As a result, the voltage generated by the SQUID 10 can be maintained near the lock point (linear region) of the Φ-V characteristic, and the biomagnetic field signal can be obtained accurately.

The thinning processing unit 41 is a digital signal received from the digital integrator 34, and the data sampled by the AD converter 33 is thinned out, for example, a number of data corresponding to the highest sampling frequency fs in the measurement target. To get. The thinned-out data is transferred to a data processing device (not shown) such as a personal computer or a server. For example, the thinning processing unit 41 thins out the data sampled by the AD converter 33 at 400 kHz to 1/10, and outputs the data corresponding to the sampling frequency fs of 40 kps. For example, the data processing device is included in the magnetic field measuring device 100.

The data processing device stores the transferred data in the storage device. When the sampling frequency fs of the measurement target is equal to the sampling frequency fs obtained by the thinning by the thinning processing unit 41 (the highest sampling frequency fs of the measurement targets), the data processing device keeps the data stored in the storage device as it is. It can be used. That is, when the sampling frequency fs of the measurement target is 40 ksps, antialiasing is performed by the low-pass filter 32a (fc = 20 kHz), the data sampled by the AD converter 33 is thinned out to 1/10, and then the data processing is performed as it is. Can be done. In this case, the data processing device can generate image data showing the waveform of the magnetic field signal or the like using the data read from the storage device, and display the image indicated by the generated image data on a display device (not shown). ..

When the sampling frequency fs of the measurement target is 10 kps, antialiasing is performed by the low-pass filter 32c having a cutoff frequency fc of 3 kHz, and the data sampled at 400 kHz is thinned to 1/10 by the thinning processing unit 41. The data processing device further thins the data received from the thinning processing unit 41 to 1/4 (10 kHz), and then performs data processing.

When the sampling frequency fs of the measurement target is 5 kps, antialiasing is performed by the low-pass filter 32d having a cutoff frequency fc of 1 kHz, and the data sampled at 400 kHz is thinned to 1/10 by the thinning processing unit 41. The data processing device further thins the data received from the thinning processing unit 41 to 1/8 (5 kHz), and then performs data processing.

As described above, when the sampling frequency fs to be measured is a frequency at which aliasing noise can be removed by antialiasing by the low-pass filters 32a to 32d, the magnetic field measuring device 100 can perform data processing only by thinning processing. In other words, it is possible to eliminate the need for antialiasing of the data before thinning by the thinning processing unit 41.

On the other hand, when the sampling frequency fs of the measurement target is 2 kps or less, antialiasing is performed by the low-pass filter 32d having a cutoff frequency fc of 1 kHz, and the data sampled at 400 kHz is thinned to 1/10 by the thinning processing unit 41. .. The cutoff frequency fc for removing the aliasing noise needs to be about 1/3 to 1/5 of the sampling frequency fs. Therefore, when the sampling frequency fs of the measurement target is 2 kps or less, the data output from the thinning processing unit 41 may include aliasing noise.

Therefore, the data processing device performs antialiasing with a cutoff frequency fc of about 1/3 of the sampling frequency fs for the data transferred from the thinning processing unit 41. Then, the data processing device thins the antialiased data to the number of samples corresponding to the sampling frequency fs of the measurement target, and performs data processing using the thinned data. For example, antialiasing and thinning processing by a data processing device are carried out by a program (software) executed by a CPU or the like mounted on the data processing device.

When all the sampling frequencies fs of the measurement target are, for example, 3 kps or more, antialiasing by the data processing device can be omitted, and the data of the measurement target can be acquired only by the thinning process.

The functional unit that performs the thinning process by the data processing device is a function that is additionally executed when the thinning process by the thinning process unit 41 is insufficient, and is an example of the thinning process unit. For example, by making it possible to set the thinning rate of data by the thinning processing unit 41 corresponding to the sampling frequency fs of the measurement target, it is possible to eliminate the need for thinning processing by the data processing device. In this case, the thinning processing unit 41 receives, for example, frequency information indicating the sampling frequency fs of the measurement target from the data processing device, and performs the thinning process at a thinning rate according to the received frequency information.

The data processing device may store the thinned data in the storage device. The above-mentioned data processing includes various calculations using the data stored in the storage device, generation of image data, display of an image such as a waveform of a magnetic field signal indicated by the generated image data on a display device (not shown), and the like. ..

The data processing device receives an instruction from an operator who operates the magnetic field measuring device 100 via an input device (not shown), and measures the biomagnetic field of the subject based on the received instruction. The operation of the SQUID sensor 20 may be controlled. For example, the display device and the input device are included in the magnetic field measuring device 100.

As described above, in this embodiment, by selectively using a plurality of low-pass filters 32 having different cutoff frequencies fc to perform antialiasing, aliasing noise and the like are included in the data of the sampling frequency fs according to the measurement target. Can be prevented. Therefore, it is possible to provide a magnetic field measuring device 100 capable of measuring a plurality of magnetic field signals to be measured.

By selectively providing a plurality of low-pass filters 32 in the digital FLL circuit 30, the magnetic field measuring device 100 capable of measuring a plurality of magnetic field signals to be measured eliminates the need for antialiasing by the post-processing circuit 40 or the data processing device. be able to.

By thinning out the data output from the digital integrator 34 by the thinning-out processing by the thinning-out processing unit 41 and the data processing device, it is possible to acquire the data necessary for measuring the magnetic fields of a plurality of measurement targets. At this time, by setting the sampling frequency fs of the AD converter 33 to a common multiple of all the sampling frequencies fs of the measurement target, the data of the measurement target can be acquired only by thinning out the data output from the digital integrator 34. it can. In other words, it is possible to acquire the data corresponding to the sampling frequency fs of the measurement target without providing the data complementing unit that complements the data in front of the thinning processing unit 41.

As a result, one magnetic field measuring device 100 can measure the magnetic fields of a plurality of measurement targets such as the brain, spinal cord, nerve, heart, and muscle. That is, the magnetic field measuring device 100 can be used for a plurality of applications such as a spinometer, a magnetocardiography, a magnetoencephalograph, and measurement of a nerve magnetic field or a muscle magnetic field. Further, even when a plurality of low-pass filters 32 are provided in the digital FLL circuit 30, the voltage generated by the SQUID 10 can be maintained near the lock point (linear region) of the Φ-V characteristic, and the biomagnetic field signal can be accurate. You can ask for it well.

(Second Embodiment)
FIG. 3 is a block diagram showing an example of the magnetic field measuring device according to the second embodiment of the present invention. The same elements as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted. The magnetic field measuring device 100A (biomagnetic field measuring device) shown in FIG. 3 has a SQUID 10 and a SQUID sensor 20A. The SQUID sensor 20A has a digital FLL circuit 30A and a post-processing circuit 40A.

The digital FLL circuit 30A has a low-pass filter 32e instead of the low-pass filters 32a, 32b, 32c, 32d shown in FIG. Since the digital FLL circuit 30A has one low-pass filter 32e, it does not have the selection circuit 38 shown in FIG. 2, and the output of the amplifier 31 is directly connected to the input of the low-pass filter 32e. For example, in the low-pass filter 32e, the cutoff frequency fc is set to 20 kHz, similarly to the low-pass filter 32a shown in FIG.

The post-processing circuit 40A has a thinning processing unit 41 and a filter unit 42A. Also in this embodiment, since antialiasing is performed by the low-pass filter 32e at a cutoff frequency fc of 20 kHz, it is possible to eliminate the need for a low-pass filter that antialiases the data output from the digital integrator 34. As a result, in the post-processing circuit 40A, the thinning-out processing unit 41 performs the thinning-out processing of the data, so that the data necessary for the processing for each measurement target can be acquired.

The filter unit 42A receives digital data thinned from 400 kHz to 40 kHz by the thinning processing unit 41, and filters the received digital data. For example, the filter unit 42A has a function of a low-pass filter LPF and a function of a high-pass filter HPF.

The low-pass filter LPF performs antialiasing of digital data thinned out by the thinning processing unit 41 in order to remove aliasing noise from the data for each use for applications where the sampling frequency fs is lower than 40 ksps. That is, the low-pass filter LPF of the filter unit 42A implements hardware antialiasing instead of software antialiasing implemented by the data processing apparatus of the first embodiment.

The sampling frequency fs and cutoff frequency fc for each application are the same as in Table 2. The cutoff frequency fc of 20 kHz is a frequency capable of antialiasing data corresponding to the maximum sampling frequency fs (40 kps) among a plurality of applications. In this embodiment, antialiasing can be performed by the low-pass filter 32e for the measurement of the magnetic field in the application where the sampling frequency fs shown in Table 2 is 40 kps. Therefore, the low-pass filter LPF performs antialiasing of data for applications where the sampling frequency fs is lower than 40 kps.

The high-pass filter HPF removes low-frequency noise generated for each application and DC noise generated by offset.

表３は、用途毎の生体磁場信号の計測に必要とされるサンプリング周波数ｆｓとアンチエイリアス用のハイパスフィルタのカットオフ周波数ｆｃとの一例を示している。表３に示すように、ハイパスフィルタＨＰＦのカットオフ周波数ｆｃは、用途に応じて０．１Ｈｚ〜１００Ｈｚに設定される。表３の用途に示す"共通"は、表２の"共通"と同様に、脊磁計、心磁計、脳磁計に共通に使用される項目である。

Table 3 shows an example of the sampling frequency fs required for measuring the biomagnetic field signal for each application and the cutoff frequency fc of the high-pass filter for antialiasing. As shown in Table 3, the cutoff frequency fc of the high-pass filter HPF is set to 0.1 Hz to 100 Hz depending on the application. The "common" shown in the usage in Table 3 is an item commonly used in the spinometer, the magnetocardiography, and the magnetoencephalogram, as in the case of the "common" in Table 2.

Then, the post-processing circuit 40A transfers the digital data filtered by the low-pass filter LPF and the high-pass filter HPF to the data processing apparatus. Similar to the first embodiment, the data processing apparatus further thins out the data thinned to 40 kHz by the thinning processing unit 41 according to the application, and then performs the data processing.

For example, when the post-processing circuit 40A is mounted in a programmable circuit such as an FPGA (Field-Programmable Gate Array), the filter unit 42A has a low-pass filter LPF and a high-pass filter HPF specialized for each measurement target. You may. Further, when the post-processing circuit 40A is mounted on a logic circuit such as an ASIC (Application Specific Integrated Circuit), even if a low-pass filter LPF and a high-pass filter HPF having filter characteristics corresponding to the application are mounted on the filter unit 42A. Good. Alternatively, a plurality of types of low-pass filter LPF and a plurality of types of high-pass filter HPF corresponding to a plurality of uses may be provided in the filter unit 42A, and a filter to be used may be selected according to the use.

As described above, the same effect as that of the first embodiment can be obtained in the second embodiment as well. For example, by thinning out the data by the thinning processing unit 41 and the data processing device, it is possible to acquire the data necessary for measuring the magnetic fields of a plurality of measurement targets. At this time, by setting the sampling frequency fs of the AD converter 33 to a common multiple of all the sampling frequencies fs of the measurement target, the data of the measurement target can be acquired only by thinning out the data output from the digital integrator 34. it can. As a result, it is possible to provide a magnetic field measuring device 100A capable of measuring a plurality of magnetic field signals to be measured.

Further, in the second embodiment, a low-pass filter 32e having a cutoff frequency fc corresponding to the maximum sampling frequency fs among the plurality of measurement targets is provided. This makes it possible to eliminate the need for a low-pass filter that antialiases the data output from the digital integrator 34. As a result, in the post-processing circuit 40A, the thinning-out processing unit 41 performs the thinning-out processing of the data, so that the data necessary for the processing for each measurement target can be acquired.

(Third Embodiment)
FIG. 4 is a block diagram showing an example of the magnetic field measuring device according to the third embodiment of the present invention. The same elements as those in FIGS. 1 and 3 are designated by the same reference numerals, and detailed description thereof will be omitted. The magnetic field measuring device 100B (biomagnetic field measuring device) shown in FIG. 4 has a SQUID 10 and a SQUID sensor 20B. The SQUID sensor 20B has a digital FLL circuit 30A shown in FIG. 3 and a post-processing circuit 40 shown in FIG.

In this embodiment, instead of the filter processing by the filter unit 42A shown in FIG. 3, the filter processing by the data processing device to which the data is transferred from the post-processing circuit 40 is performed. That is, the data processing apparatus has an antialiasing function by the filter unit 42A (low-pass filter LPF and high-pass filter HPF) of FIG. The function of the filter circuit by the data processing device may be realized by software or hardware. Then, the data processing device thins out the anti-aliased data according to the application, and performs data processing using the thinned-out data.

As described above, also in the third embodiment, the same effects as those in the first embodiment and the second embodiment can be obtained.

Although the above-described embodiment has been described as an example of application to a biomagnetic field measuring device such as a magnetoencephalograph, a spinometer, and a magnetocardiography, it may be applied to a magnetic field measuring device other than the biomagnetic field measuring device.

Although the present invention has been described above based on each embodiment, the present invention is not limited to the requirements shown in the above embodiments. With respect to these points, the gist of the present invention can be changed without impairing the gist of the present invention, and can be appropriately determined according to the application form thereof.

10 SQUID
20, 20A, 20B SQUID sensor 30, 30A Digital FLL circuit 31 Amplifier 32a, 32b, 32c, 32d Low-pass filter 33 AD converter 34 Digital integrator 35 DA converter 36 Voltage current converter 37 Feedback coil 38 Selection circuit 40, 40A Post-processing circuit 41 Thinning processing unit 42A Filter unit 100, 100A, 100B Magnetic field measuring device

Patent No. 4133934

Claims (10)

An amplifier that amplifies the voltage output from the superconducting quantum interference device according to the strength of the magnetic field,
A plurality of low-pass filters having filter characteristics corresponding to a plurality of measurement targets for which a magnetic field is measured and antialiasing the signal output from the amplifier, and a plurality of low-pass filters.
A selection circuit that selects one of the plurality of low-pass filters according to the plurality of measurement targets, and
A digital including an AD converter that converts an analog signal from the low-pass filter selected by the selection circuit into a digital signal by using a sampling frequency higher than a plurality of sampling frequencies corresponding to the plurality of measurement targets. A magnetic field measuring device having an FLL circuit. The magnetic field measuring device according to claim 1, wherein the sampling frequency of the AD converter is set to a common multiple of the plurality of sampling frequencies. The magnetic field measuring apparatus according to claim 2, further comprising a thinning processing unit for thinning out a digital signal output from the AD converter to a number of samples corresponding to a sampling frequency for each measurement target. The magnetic field measuring device according to any one of claims 1 to 3, wherein each of the plurality of measurement objects is any one of a brain, a spinal cord, a nerve, a heart, and a muscle. The digital FLL circuit is
An integrator that integrates the digital values output by the AD converter, and
A DA converter that converts the integrated value output by the integrator into a voltage, and
A voltage-current converter that converts the voltage output by the DA converter into a current,
The invention according to any one of claims 1 to 4, wherein the superconducting quantum interfering element has a feedback coil that generates a magnetic field based on a current output by the voltage-current converter. Magnetic field measuring device. An amplifier that amplifies the voltage output from the superconducting quantum interference device according to the strength of the magnetic field,
A low-pass filter having filter characteristics capable of antialiasing a signal corresponding to at least the maximum frequency among a plurality of sampling frequencies corresponding to a plurality of measurement targets to which a magnetic field is measured by receiving a signal output from the amplifier.
A magnetic field measuring device comprising a digital FLL circuit including an AD converter that converts an analog signal from the low-pass filter into a digital signal using a sampling frequency higher than the plurality of sampling frequencies. The magnetic field measuring device according to claim 6, wherein the sampling frequency of the AD converter is set to a common multiple of the plurality of sampling frequencies. The magnetic field measuring apparatus according to claim 7, further comprising a thinning processing unit for thinning out a digital signal output from the AD converter to a number of samples corresponding to a sampling frequency for each measurement target. The magnetic field measuring device according to any one of claims 6 to 8, wherein each of the plurality of measurement objects is any one of a brain, a spinal cord, a nerve, a heart, and a muscle. The digital FLL circuit is
An integrator that integrates the digital values output by the AD converter, and
A DA converter that converts the integrated value output by the integrator into a voltage, and
A voltage-current converter that converts the voltage output by the DA converter into a current,
The invention according to any one of claims 6 to 9, wherein the superconducting quantum interfering element has a feedback coil that generates a magnetic field based on a current output by the voltage-current converter. Magnetic field measuring device.

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