JP3063654B2 - Voltage amplifier circuit for magnetic sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage amplifying circuit for a magnetic sensor, and more particularly to a SQUA as a magnetic detecting element.
ID (Superconducting Quantum Interference Device)
More particularly, the present invention relates to an output voltage amplifying circuit for a magnetic sensor using an oxide-based SQUID formed of a so-called “high-temperature superconductor”.

[0002]

2. Description of the Related Art SQUIDs are known as ultra-high sensitivity magnetic sensors capable of measuring extremely weak magnetic fields. When an oxide-based high-temperature superconductor is used for such a SQUID, the liquid nitrogen temperature (-1) is inexpensive and easy to handle.
96 ° C), which greatly simplifies handling compared to expensive and difficult to use systems using liquid helium (-269 ° C), medical diagnostics, non-destructive testing, food testing, geology Various applications such as inspections are expected to be developed and put to practical use, and applied research and development are being actively pursued. In order to contribute to such research and development, the present inventors have devised a “magnetic sensor drive circuit device” having both functions of magnetic field measurement and element characteristic evaluation, and already filed a patent on October 17, 1996. Filed.

When the SQUID is used to measure an extremely weak magnetic field as described above, the conventional technique employs, for example, a so-called "FLL system" as shown in FIG. That is, for the SQUID sensor S including the SQUID element E and the feedback coil FC, the bias current source B, the modulation frequency oscillator M,
A drive processing circuit DP including a phase detector PD, a feedback amplifier FA, and the like is provided, and a desired magnetic flux measurement signal Vφ representing an external magnetic field is obtained from the output of the feedback amplifier FA. In this case, the amplifying means S for amplifying the output voltage Vd generated by the SQUID element E
As A, a first transformer T1 operating at liquid helium temperature or liquid nitrogen temperature or low temperature is installed, while a second transformer T2 and an amplifier A operating at room temperature are installed on the drive processing circuit DP side.

In FIG. 1, a small output voltage Vd generated by a SQUID element E in response to an external magnetic field to be detected is generated under the same cryogenic temperature environment as that of the SQUID element E, ie, liquid helium or liquid helium. The pressure is once increased to a required value by a first transformer T1 installed in liquid nitrogen, and then taken out of the sensor S. The boosted SQUID output voltage Vd is insulated by the second transformer T2 at room temperature and adjusted to a desired value, and then supplied to the amplifier A. In the prior art, as in this example,
After the SQUID output voltage Vd is amplified by the amplifying means SA which extends over the inside and outside of the sensor S, it undergoes signal processing operations after the phase detector PD.

As described above, since the first transformer T1 constituting the first stage of the amplifying means SA is placed inside the sensor S and in a constant temperature state, there is little possibility that external noise is mixed into a weak output voltage. In other words, the transformation characteristic itself of the first transformer T1 is not affected by the temperature. However, when such a structure is adopted, the sensor S becomes the first transformer T
1 is built inside, so the transformer itself needs expensive cryogenic material, and the structure is enlarged and designed.
Increase difficulties in manufacturing and practical placement. In addition to it,
The core (magnetic core) material of the transformer coil is also cooled to an extremely low temperature, so that the magnetic permeability characteristics are deteriorated and the original performance cannot be exhibited. Furthermore, since two transformers are used, it is naturally necessary to consider matching between the two transformer coils. Therefore, such things are
This is a major challenge that hinders the practical application of IDs to magnetic sensors.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide an output voltage amplifying circuit for a magnetic sensor which can solve the above various problems at once. That is, the present invention improves a circuit for amplifying a small SQUID output voltage generated by a SQUID element used as a magnetic sensing element,
Eliminates the need to incorporate a transformer into the magnetic sensor, greatly simplifies the structure of the magnetic sensor, does not cause deterioration of the core material of the transformer coil, and eliminates the need to consider matching. Things.

[0007]

According to the present invention, there is provided a magnetic sensor for amplifying and processing a voltage detection signal output from a SQUID element which is used as a magnetic sensor and receives modulation excitation based on a modulation signal. A voltage amplifying circuit, comprising: a boosting transformer for receiving the voltage detection signal from the SQUID element and boosting the signal by a predetermined factor at a normal temperature; and an electronic amplification circuit for amplifying the boosted signal. Means and a signal processing circuit for processing an amplified signal from the signal amplifying means based on the modulated signal and outputting a magnetic field measurement signal.

The idea of the present invention is to boldly reverse conventional thinking that a transformer for boosting the output voltage of a SQUID element should be installed in a sensor at an extremely low temperature, and to place this transformer outside the sensor. It was inspired by an idea that was not considered before and was provided at room temperature. In short, there is a simple solution in which one step-up transformer is placed at the first stage of a drive circuit. The present invention that solves this problem can solve all the conventional difficulties.

That is, based on such a hint, the present inventors have used an oxide-based SQUID element formed of a high-temperature superconductor and placed at a temperature of -196 ° C. as an element for magnetic field detection. Numerous experiments were conducted under various conditions on a drive circuit in which the output voltage from the SQUID element was directly received by a step-up transformer placed at room temperature. After confirming that it is negligibly small and has no practical problem at all, the invention as described in the claims has been completed.

Thus, the step-up transformer is connected to the SQUID
Since the necessity of being built in the magnetic sensor can be eliminated, the sensor has a very simple structure and is miniaturized. For example, in the practical stage of measuring the distribution state of an extremely weak magnetic field using a SQUID sensor, several tens to several hundreds, or, depending on the application, thousands of SQUID sensors are arranged in a narrow measurement area, Outputs from these many sensors are processed in a multi-channel manner. In such a case, according to the present invention, these multiple SQUIS
Each of the D magnetic sensors does not require a built-in step-up transformer and is extremely compact, so that it can be arranged very easily and with high density.

In the present invention, since the step-up transformer is not built in the sensor, there is no possibility that the transformer core will be deteriorated by being exposed to cryogenic temperatures.
There is an advantage that the specifications such as the turns ratio and the material can be extremely easily changed as necessary, and it is possible to flexibly cope with various design changes.

Further, according to the present invention, as compared with the one using two transformers, the function of both transformers is provided simply by using one booster transformer operating at normal temperature in the first stage of the drive circuit. Therefore, various advantages can be obtained in circuit design and function. For example, it is possible to eliminate problems such as matching between the two transformer coils, and not only to amplify the voltage, but also to add a function as a high-pass filter that effectively passes a high-frequency modulation component, as described later. it can.

[0013]

FIG. 2 is a schematic block diagram of a voltage amplifying circuit for a magnetic sensor according to an embodiment of the present invention. The magnetic sensor 1 includes a SQUID element 11 and a feedback coil 12, and a voltage amplifying circuit 2
Includes a current bias circuit 21, a boost amplification circuit 22, and a signal processing circuit 23.

The SQUID element 11 is formed on a substrate made of SrTiO ₃ by, for example, laser vapor deposition.
This is an oxide-based high-temperature superconducting SQUID having a thin film formed thereon. The superconducting junction, which is the heart of the SQUID element, is formed, for example, by providing a step of 0.2 μm on the surface of a substrate and growing a superconducting film thereon. The performance of such a SQUID element 11 has, for example, a high resolution of about 1 / 50,000,000 of the geomagnetism.

When such a SQUID element 11 is used as a magnetic sensing element, it is placed in, for example, liquid nitrogen to operate in an environment of -196 ° C., ie, 77K, and a feedback coil. 12 is operatively attached. This feedback coil is made of, for example, a single-turn thin conductor layer surrounding the SQUID element 11, and contributes to simplification of the sensor structure.
Note that the feedback coil 12 is connected to the signal processing circuit 23.
, An excitation signal is supplied to the SQUID element 11, and a feedback magnetic field based on the detection result of the modulated predetermined magnetic field and the output voltage Vd of the SQUID element 11 is provided.

The magnetic sensor 1 is driven and the SQUID element 11
The voltage amplifying circuit 2 for processing the voltage detection signal representing the output voltage Vd generated by the above-mentioned circuit forms a sensor circuit of the flux modulation type magnetic flux lock system commonly called "FLL system" by the circuits 21 to 23 described above. It is an electronic circuit. The current bias circuit 21 generates, for example, an AC bias current having a relatively low frequency f ₁ of ₁ kHz or a predetermined DC bias current, and supplies the same to the SQUID element. The boosting amplifier 22 boosts and amplifies the voltage detection signal (Vd) from the SQUID. The signal processing circuit 23 that processes the amplified signal includes the modulated signal oscillation circuit 2
3M, a phase detection circuit 23D, a feedback integration amplification circuit 23F, and the like.

The modulation signal oscillation circuit 23M is, for example, 4
A modulation signal having a relatively high frequency f ₀ of ₀ kHz is generated, and a feedback coil 12
To apply a predetermined magnetic field to the SQUID element 11.
The phase detection circuit 23D performs phase detection based on the modulated signal. The reset switch 23 having an integrating function
The feedback integration amplifier 23F locked or reset by fs excites the feedback coil 12 via the feedback resistor 23fr according to the detection result of the output voltage Vd, and applies a feedback magnetic field to the SQUID element 11.

As is well known, the FLL type electronic circuit 2 uses a null method technique. That is, by feeding back the excitation signal corresponding to the voltage detection signal (Vd) from the feedback integration amplifier circuit 23F to the feedback coil 12, a magnetic field that cancels out the magnetic field externally applied to the SQUID element 11 is applied to the SQUID element 11. Feedback, SQUID
The device 11 is automatically balanced so that the operating point of the device 11 is locked at the position of the valley (or peak) of the magnetic field (Φ) -voltage (V) characteristic curve. Therefore, the excitation signal output from the feedback integration amplifier circuit 23F represents a magnetic field applied from the outside to the SQUID element 11, and therefore, the output measurement terminal 24 of the voltage amplification circuit 2 is desired by the action of the FLL circuit. The magnetic field measurement signal Vφ can be obtained.

Here, according to the present invention, the above-described boosting amplifier circuit 22 includes a boosting transformer (step-up transformer) 2.
2T and an electronic amplifier circuit 22A, the voltage detection signal (Vd) from the SQUID element 11 is increased by a predetermined factor by the step-up transformer 22T, and the increased voltage signal is further electronically amplified by the amplifier circuit 22A. Thus, a required processing signal is obtained. The step-up transformer 22T is configured, for example, by winding a primary coil and a secondary coil around a ferrite core at a predetermined winding ratio, and this winding ratio corresponds to the specification of the SQUID element 11 and the subsequent amplifier circuit, for example, The value is selected from several to several hundred.

As described above, the step-up transformer 22T is provided by the present inventors at a distance from the SQUID element and at a fluctuating room temperature. It has been confirmed that the content is so small as to be negligible and there is no practical problem.

Considering one of the reasons, generally, the thermal noise due to the resistance component is proportional to the square root of the absolute temperature. On the other hand, in the conventional sensor structure with a built-in transformer, a normal SQ operating at least at -269 ° C. = 4K is used.
It is considered that the UID was considered, whereas the oxide-based high temperature superconductor SQUID element 11 of the present invention
Operates at 196 ° C. = 77K. Therefore, the output voltage of the SQUID element 11 is itself equal to the above-mentioned normal SQUID.
Since the thermal noise component already contains about four times [(77/4) ^1/2 ≒ 4] as compared with the above, the step-up transformer has such an output even when separated from the element and at room temperature. It is considered that the voltage transformer does not affect the voltage as much as a noise source, and therefore can function effectively even when the boosting transformer 22T is on the drive circuit side in a room temperature state.

The step-up transformer 22T can not only have a voltage amplifying function but also add a function as a high-pass filter that effectively passes a high-frequency modulation component by setting a circuit constant to an appropriate value. . Let's explain this high-pass filter function.

In the voltage amplifying circuit 2 which performs signal processing by the magnetic flux modulation method, the output voltage Vd of the SQUID element 11 including the frequency component f ₀ of the modulation signal generated by the modulation signal oscillating circuit 23 M After being increased by the step-up transformer 22T and further amplified by the amplifier circuit 22A, it is delivered to the signal processing circuit 23. In the signal processing circuit 23, the required frequency component f _{0 is} detected by the phase detector 23D.
Only those that are to be removed.

Here, the current bias circuit 21 generates, for example, an AC bias current having the frequency f _{1. When the} AC bias current is applied to the SQUID element 11, the SQUID element 11 includes a frequency component f ₁ to the output voltage Vd of. In particular, when the AC bias current of a rectangular wave, not only the frequency components f _1, the harmonic component nf ₁ also included. In such a case, only the frequency component f ₀ is passed and the frequency component f
_1, furthermore, when it is possible to remove the harmonic component nf _1, is indeed useful. In this regard, in the present invention,
By setting the circuit constant of the step-up transformer 22T to an appropriate value, a circuit configuration capable of removing such harmonic components can be obtained.

Here, the internal resistance of the SQUID element 11 is Rs, the circuit resistance between the SQUID element 11 and the step-up transformer 22T is Rc, and the internal resistance of the step-up transformer 22T is Rs.
t, and the coil inductance of the step-up transformer 22T is Lt, and the transformer internal resistance Rt is equal to the element internal resistance Rt.
Assuming that it is sufficiently smaller than s and the circuit resistance Rc, a circuit including the SQUID element 11 and the step-up transformer 22T is represented by an equivalent circuit in FIG. As seen from this equivalent circuit, the step-up transformer 22T can fulfill the function of a high-pass filter, and the cut-off (cut-off) frequency fc of this high-pass filter is represented by fc = (Rs + Rc) / (2πLt) (1) We can see that we can do it. Therefore, this (1)
From the equation, the coil inductance Lt is represented by Lt = (Rs + Rc) / (2πfc) (2)

[0026] In the present invention, in the above case, the cutoff frequency _{fc, f 0> fc»f 1 (} 3) and so as to set to selecting a desired coil inductance Lt of the step-up transformer 22T. The above f ₀ = 40 kHz
In the case of z and f ₁ = 1 kHz, for example, when fc is set to 10 kHz based on the equation (3), from this frequency numerical value and given resistance values Rs and Rc, (2)
The desired coil inductance value Lt is obtained by the equation.

[0027] Incidentally, the high-pass filter function of such step-up transformer 22T, when included is sufficiently low lower frequency noise than the modulation signal frequency f ₀ to the output voltage Vd from the SQUID element 11, which is effective against the noise .
Therefore, not only in the case where the current bias circuit 21 generates the AC bias current (frequency f ₁ ) as described above, but also in the case of the DC bias, another low-frequency noise included in the output voltage Vd can be used. The leverage works effectively.

As can be understood from the above description, according to the present invention, in designing and manufacturing a magnetic sensor, various restrictions are imposed by incorporating a step-up transformer in a small and cryogenic sensor, so that handling is easy. Inexpensive magnetic sensor device can be provided. FIG. 4 is a schematic block diagram showing an example of a circuit configuration in a case where the voltage amplifier circuit according to the present invention is applied to the sensor circuit of the above-mentioned patent application “magnetic sensor drive circuit device” utilizing such features. ing. In this example, too, the advantages described above are utilized.

The circuit shown in FIG.
A sensor circuit for measuring the magnetic field by obtaining a magnetic field measurement signal Vφ by driving the sensor as a magnetic measurement sensor, and a voltage signal V
, The magnetic field signal Φ or the current signal I is obtained in response to the switching operation indicated by “i / φ”, and Φ−V of the SQUID element 11 is obtained.
The main element is an evaluation circuit for evaluating characteristics such as characteristics and IV characteristics. In this circuit, the feedback coil 12 is also used as a coil for applying these characteristic evaluation magnetic fields when the evaluation circuit operates. The main function of this circuit is to completely control the power supply itself of each circuit by a measurement / evaluation switching circuit that responds to the operation switching operation so that either circuit can operate reliably. See the above-mentioned patent application for details.

[Brief description of the drawings]

FIG. 1 is a diagram showing a conventional SQUID voltage amplifier circuit.

FIG. 2 is a diagram showing a schematic circuit block diagram of a voltage amplifier circuit for SQUID according to one embodiment of the present invention.

FIG. 3 is a diagram showing an equivalent circuit of a voltage detection signal input unit in FIG. 2;

FIG. 4 is a diagram showing an application example of a voltage amplifier circuit for SQUID according to the present invention.

[Explanation of symbols]

S, 1 SQUID sensor, E, 11 SQUID element, FC, 12 feedback coil, SA Amplifying means consisting of first transformer T1, second transformer T2 and amplifier A, DP current bias source B, phase detector PD, modulation frequency oscillator M, a drive processing circuit provided with a feedback amplifier FA, etc., 2 voltage amplifying circuit 21 current bias circuit, 22 boost amplifying circuit, 22T boost transformer for receiving element output voltage Vd, 22A electronic amplifier circuit, 23 signal processing circuit. 23D phase detection circuit, 23M modulation signal oscillation circuit, 23F feedback integration amplifier circuit, 23fs reset switch, 23fr feedback resistor.

Continuation of the front page (56) References JP-A-4-268471 (JP, A) JP-A-7-84020 (JP, A) JP-A-7-311250 (JP, A) JP-A-8-102559 (JP) , A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01R 33/00-33/18

Claims (5)

(57) [Claims] 1. A voltage amplifying circuit for a magnetic sensor for amplifying and processing a voltage detection signal output from a SQUID element used as a magnetic sensor and receiving modulation excitation based on a modulation signal, wherein the voltage detection from the SQUID element is performed. A step-up transformer for receiving a signal and boosting the signal by a predetermined factor at a normal temperature; and a voltage signal amplifying unit including an electronic amplifying circuit for amplifying the boosted signal; and amplifying the signal from the signal amplifying unit. A voltage processing circuit for a magnetic sensor, comprising: a signal processing circuit that processes the modulated signal and outputs a magnetic field measurement signal. 2. The voltage amplifying circuit for a magnetic sensor according to claim 1, wherein the SQUID element is a high-temperature superconductor oxide-based SQUID element. 3. A signal processing circuit, comprising: a modulation signal source for generating the modulation signal and applying modulation excitation to the SQUID element; a phase detection circuit for detecting the amplified signal based on the modulation signal; 3. The voltage amplifying circuit for a magnetic sensor according to claim 1, further comprising a feedback amplifying circuit for providing feedback excitation based on the phase detection signal to the SQUID element. 4. The voltage amplifying circuit for a magnetic sensor according to claim 1, further comprising a bias current source for supplying an AC or DC bias signal to the SQUID element. 5. The magnetic sensor according to claim 1, wherein a circuit constant of the step-up transformer is set so as to function as a high-pass filter that passes the modulation frequency component. Voltage amplification circuit.