JPH07294615A - Squid magnetic flux meter - Google Patents

Abstract

PURPOSE:To improve dynamic range characteristics in a SQUID magnetic flux meter having a positive feedback circuit. CONSTITUTION:The SQUID magnetic flux meter is provided with a positive feedback coil L1 to a SQUID 1, a positive feedback circuit 2 including a band pass filter consisting of an inductor L2 and a variable-capacitance capacitor 3, a sweeping voltage source for the capacitor 3, a high frequency amplifier connected to the SQUID 1, and an FLL (magnetic flux loop) circuit for the SQUID 1. In a frequency range where the FLL can operate, a magnetic flux loop is constituted according to a zero method, thereby obtaining an output from the SQUID l. On the other hand, in a frequency range where the FLL cannot operate, a positive feedback amount for a specific frequency is increased, so that the specific frequency is continuously changed and the output of the SQUID 1 at the specific frequency is obtained by the high frequency amplifier.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to SQUID (Supercon
Ducting Quantum Interference Device: A SQUID magnetometer that measures a magnetic field using a superconducting quantum interference device), and more specifically, relates to an SQUID magnetometer with improved noise resistance. Here, SQUID is a superconducting loop that is maintained in a low temperature state in a heat-insulating container (such as a cryostat) by liquid helium, liquid nitrogen, etc., and a DC current is used as a bias current in a SQUID loop that is a superconducting loop including a Josephson junction in the loop. Apply and drive,
When a magnetic flux from the outside is coupled and applied to this SQUID loop via a pickup coil, an input coil, etc., a circulating current is induced in the SQUID loop, and due to the quantum interference effect in the Josephson junction in the loop,
It is an element that measures a minute change in magnetic flux by utilizing the fact that it operates as a transducer that converts a weak change in the applied external magnetic flux into a large change in the output voltage.

[0002]

2. Description of the Related Art Conventionally, d containing two Josephson junctions.
The c-SQUID magnetometer is a dewar (or cryostat) that is an adiabatic containment vessel that stores liquid helium, which is a coolant for maintaining a low temperature environment (cooling system).
And a SQUID probe operating in liquid helium,
It is known that an amplifier and a controller that operate at room temperature are provided and that the SQUID probe in liquid helium and the amplifier at room temperature are connected by a coaxial cable. Such a SQUID magnetometer has a very high sensitivity, with a magnetic flux resolution of 10 -5 φo / Hz 1/2 (φ o in the left equation indicates a magnetic flux quantum).
The response of UID is very fast, and is characterized by operating at several GHz (gigahertz) to several tens of GHz. On the other hand, SQU
By adding a positive feedback circuit to the ID and improving the magnetic field measurement sensitivity, a low noise magnetometer can be realized with a simple circuit configuration (D. Drung, R. Cantor, M. Peters, T. Ryhane).
n, and H. Koch, "INTEGRATED dcSQUID MAGNETOMETER W
ITH HIGH dV / dB ", JEEE TRANSACTIONS ON MAGNETICS, V
See OL. 27, NO. 2, MARCH 1991). FIG. 5 is a diagram showing the configuration of the SQUID to which a positive feedback circuit is added, and FIG. 6 is a characteristic diagram of the circuit of FIG. As shown in FIG. 5, this SQ
The UID 21 has two Josephson junctions J3 and J4, and a resistor R2 and a positive feedback coil L3 having a mutual inductance Mp are connected in parallel with the SQUID 21. With such a configuration, as shown in FIG. 6, the magnetic flux voltage conversion rate at the operating point d (the slope of the waveform at the point d) is larger in the thick line with positive feedback than in the thin line without positive feedback. Has become. FIG. 7 shows a direct feedback type FLL (Flux Locked) in the configuration of the SQUID magnetometer that measures the magnetic field using the SQUID to which the positive feedback circuit as described above is added.
Loop: magnetic flux lock loop) circuit and dc-SQUID connection relationship, 21 in the figure is a dc-SQ having two Josephson junctions J3, J4 as shown in FIG.
UID and 22 are SQ including resistor R2 and positive feedback coil L3
Positive feedback circuit for UID21, 28 is a negative feedback coil for magnetic flux lock, 25 is a preamplifier with large input impedance to SQUID21, 24 is a bias current supply source for supplying a bias current to SQUID21, and 26 is an output from the preamplifier 25 is integrated. The integrator 27 is a negative feedback circuit, which is a current source. The preamplifier 25, the integrator 26, the feedback circuit 27, and the SQUID 21 form a FLL circuit. The output of the preamplifier 25 is compared with the reference potential by the integrator 26, and when the output of the integrator 26 is added to the negative feedback coil 28 for magnetic flux lock to perform negative feedback, the operating point on the Φ-V curve (for example, in FIG. 5). D point)
The magnetic field to be stable and to be measured can be obtained by monitoring the above feedback amount by the output value.
This state is referred to as "locked". The above method is called the FLL method, which is a kind of so-called “zero method”, and is characterized in that the input / output relationship is linear. The operating point d is set at the reference potential, and is usually the point where the slope of the Φ-V curve is the steepest (for example, the middle point of the slope).

[0003]

However, although a SQUID magnetometer with low noise can be realized with the above-mentioned structure, as shown in FIG. 6, a positive feedback is applied to improve the magnetic flux voltage conversion rate, and a characteristic curve is obtained. By steepening the slope of, the dynamic range (the range of the input magnetic flux in which the linearity is maintained) becomes smaller from Φ1 to Φ2 when the positive feedback in FIG. 6 is performed. By reducing the dynamic range in this way, the null method is established for the magnetic flux input within the range of the frequency characteristics of the SQUID control circuit, but a magnetic input with a large amplitude at a high frequency at which the control circuit cannot respond. There is a problem in that control is lost with respect to (for example, magnetic input when observing a spectrum of a magnetic signal). The present invention has been made to solve the above problems, and an object thereof is to provide an SQUID magnetometer having a dynamic range characteristic for a specific frequency in an SQUID magnetometer having a positive feedback circuit. To do.

[0004]

In order to solve the above problems, the SQUID magnetometer according to the first invention of the present application is SQ
A positive feedback circuit that includes a UID, a positive feedback coil that gives positive feedback to the SQUID by mutual inductance, and a bandpass filter that includes an inductor and a voltage-controlled variable capacitor, and that is connected in parallel to the SQUID. An SQUID magnetometer comprising a voltage-controlled variable capacitor swept reference voltage source and a high-frequency amplifier connected to the SQUID, wherein a positive feedback amount for a specific frequency is increased by the band-pass filter, The sweep voltage source is configured to continuously change the specific frequency, and the high frequency amplifier is configured to obtain an SQUID output at the specific frequency. The SQUID magnetometer according to the second invention of the present application is SQ
A positive feedback circuit that includes a UID, a positive feedback coil that gives positive feedback to the SQUID by mutual inductance, and a bandpass filter that includes an inductor and a voltage-controlled variable capacitor, and that is connected in parallel to the SQUID. A sweep control voltage source for the voltage controlled variable capacitor; a high frequency amplifier connected to the SQUID; and a negative feedback applying means for applying a negative feedback to the SQUID to form a magnetic flux lock loop by a null method. S
In the frequency domain in which the magnetic flux lock loop based on the null method can be configured, the SQ can be implemented by configuring the magnetic flux lock loop based on the null method.
In the frequency region where the output of UID is obtained and the magnetic flux lock loop by the null method cannot be configured, the amount of positive feedback for a specific frequency is increased by the band pass filter,
The sweep voltage source is configured to continuously change the specific frequency, and the high frequency amplifier is configured to obtain an SQUID output at the specific frequency.

[0005]

According to the first invention of the present application having the above structure,
A band-pass filter consisting of an inductor and a voltage-controlled variable capacitance capacitor is provided in the positive feedback circuit that gives positive feedback to the SQUID, and the voltage of the voltage-controlled variable capacitance capacitor is swept by the sweep reference voltage source. An amplifier is connected, the amount of positive feedback for a specific frequency is increased by a bandpass filter, the specific frequency is continuously changed by a sweep voltage source, and a SQUID output at the specific frequency is obtained by a high frequency amplifier. As a result, the positive feedback amount can be continuously changed while increasing only a certain specific frequency, and it becomes possible to realize an SQUID magnetometer with an improved dynamic range, and the power of the magnetic flux in the high frequency region can be realized. Spectral analysis for detection is possible. Further, according to the second invention of the present application having the above-mentioned configuration, in addition to the first invention, since negative feedback is provided to the SQUID to form a magnetic flux lock loop by the null method, zero feedback is provided. In the frequency range where the magnetic flux lock loop based on the zero method can be configured, the SQUID output can be obtained by configuring the magnetic flux lock loop based on the zero method, and in the frequency range where the magnetic flux lock loop based on the zero method cannot be configured. Can obtain the SQUID output by the configuration of the first invention.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of an SQUID chip 11 used in an SQUID magnetometer which is an embodiment of the present invention.
SQUID1 with a number of Josephson junctions J1 and J2
A positive feedback circuit 2 having a resistor R1, a positive feedback coil L1, a variable capacitor 3 and an inductor L2 is connected in parallel between the output side terminals T1 and T2. In the positive feedback circuit 2 described above, the resistor R1 and the positive feedback coil L1
The inductor L2 and the variable capacitor 3 are connected in series.

The SQUID 1 described above converts a magnetic flux into an electric signal at a magnetic flux voltage conversion rate dV / dΦ. Positive feedback coil L
1 has the value of the mutual inductance Mp, SQUID
A part of the output of 1 serves to give positive feedback to SQUID 1 in the form of magnetic flux. The resistor R1 functions to limit the amount of positive feedback to SQUID1. Further, the inductor L2 has an inductance value L and limits a current having a certain frequency or higher.

The inductor L2 and the variable capacitor 3 constitute an LC resonator and operate as a band pass filter (band pass filter) having a center frequency fo. The variable capacitor 3 is of the voltage control type and, as shown in FIG. 2, is composed of a capacitor C1, a diode D and a capacitor C2 connected in series between the connection terminals T3 and T4, and the terminal T3 is connected to the resistor R1. , And terminal T4 is connected to the inductor L2. A sweep voltage source 9 described later is connected to the terminals T5 and T6 of the diode D (see FIG. 3), and a reverse bias voltage VD is applied to generate a depletion layer in the diode D. Is controlled by the reverse bias voltage VD, the circuit 3 as a whole operates as a variable capacitor. Further, the capacitors C1 and C2 are SQ reference voltage sources for applying a reverse bias voltage to the variable capacitors.
It is a capacitor for insulating from UID1.

FIG. 3 is a SQUI for measuring a magnetic field using a narrow band SQUID to which the positive feedback circuit 2 as described above is added.
Direct feedback type FLL (Flux Loc) in the configuration of the D magnetometer
kedLoop: Magnetic flux lock loop circuit and dc-SQUI
11 shows a connection relationship of D, and 11 in the figure is a dc-SQUID1 having two Josephson junctions J1 and J2,
The SQUID chip is formed by connecting in parallel a positive feedback circuit 2 having a resistor R1, a positive feedback coil L1, a variable capacitor 3 and an inductor L2. 4 is SQU
A bias current supply source that supplies a bias current to ID1;
Reference numeral 5 is a preamplifier having a large input impedance with respect to SQUID 1, 6 is an integrator for integrating the output from the preamplifier 5, 7 is a negative feedback circuit as a current source, and 8 is a negative feedback coil for magnetic flux lock. 9 is the variable capacitor 3 described above.
And a high-frequency amplifier 10 for extracting the magnetic signal discriminated by SQUID1. Further, C3 to C5 are capacitors (C3 and C4 are high cut capacitors), and 12 is an offset voltage source for determining an operating point on the .PHI.-V curve (for example, point d in FIG. 5).

Next, the operation of the SQUID magnetometer shown in FIG. 3 will be described. LC composed of inductor L2 and variable capacitor 3 of positive feedback circuit 2 shown in FIG.
The center frequency fo of the resonator is expressed by the following equation (1).

[Equation 1] Given in.

Since the impedance becomes zero at the frequency fo, the amount of positive feedback at this time is determined only by the value of the resistor R1. Therefore, the SQUID of the above SQUID magnetometer
Assuming that the unique magnetic flux voltage conversion rate of 1 is dV / dΦ and the effective magnetic flux voltage conversion rate is dV / dΦex, the effective magnetic flux voltage conversion rate dV / dΦex is expressed by the following equation (2).

[Equation 2] Can be expressed as

In the above equation (2), βa is a parameter expressing the positive feedback, and the following equation (3)

[Equation 3] It is represented by. Here, Rp represents the resistance value of the resistor R1.

The above parameter βa satisfies the condition of 0 <βa <1, and the positive feedback amount does not satisfy the oscillation condition of βa ≧ 1. Therefore, in the above equation (2), the coefficient 1 / (1-βa) is 1 / (1-βa)> 1, so the value of the effective magnetic flux voltage conversion rate dV / dΦex is S.
It becomes larger than the magnetic flux voltage conversion rate dV / dΦ peculiar to the QUID. And, the closer the value of βa is to 1, the denominator (1-
βa) becomes smaller, the effective magnetic flux voltage conversion rate dV /
The value of dΦex is the magnetic flux voltage conversion rate dV / d peculiar to SQUID.
It becomes larger than Φ. The above coefficient 1 / (1-βa)
Represents the discrimination rate of the magnetic signal in the pass band centered on the center frequency fo. FIG. 4 is a graph showing the above characteristics. The value dV / dΦ1 of the magnetic flux voltage conversion rate at the portion a of the center frequency fo is given by the above equation (2). Further, the value dV / dΦ2 of the magnetic flux voltage conversion rate in the portion b other than the pass band is the unique magnetic flux voltage conversion rate of SQUID1.

In the SQUID magnetometer shown in FIG. 3, S
The QUID 1, the preamplifier 5, the integrator 6, the negative feedback circuit 7, and the negative feedback coil 8 for magnetic flux lock constitute a magnetic flux lock loop (FLL) circuit that is a negative feedback applying means that operates by the null method in the low frequency region. . Therefore, with respect to the DC magnetic field, the output of the preamplifier 5 is compared with the reference potential of the offset voltage source 12 in the integrator 6, and the output of the integrator 6 is added to the negative feedback coil 8 for magnetic flux lock to perform negative feedback. The magnetic field that is stable at the operating point on the Φ-V curve (for example, point d in FIG. 5) and should be measured can be obtained by the “zero-point method” in which the feedback amount is monitored by the output value. The operating point d is set by the offset voltage source 12, and is normally fixed to the steepest point of the Φ-V curve (for example, the midpoint of the gradient).
ID1 is a certain level range determined by the degree of positive feedback as shown in FIG. 6 (for example, Φ 1, Φ in FIG. 6).
2 and so on. ) Linearly operates with respect to the magnetic input signal. The range of the input magnetic flux in which this linearity is maintained is narrowed by performing positive feedback to make the slope of the characteristic curve steep, and the amplitude level exceeding the range is limited. SQ above
The FLL circuit including the UID 1, the preamplifier 5, the integrator 6, the negative feedback circuit 7, and the magnetic flux locking negative feedback coil 8 operates when the magnetic field to be observed is in a high frequency region (for example, a magnetic signal of several KHz or more). do not do.

A magnetic input signal (for example, several KHz or more) in the frequency region in which the FLL circuit including the SQUID 1, the preamplifier 5, the integrator 6, the negative feedback circuit 7, and the negative feedback coil 8 for magnetic flux lock does not operate in the zero position method. 3), the variable capacitance value of the diode D shown in FIG. 2 changes when the variable capacitor sweep reference voltage source 9 of FIG. 3 is swept in a sawtooth shape. Since the center frequency fo of the LC resonance circuit composed of the capacitance capacitor 3 changes, this center frequency fo is changed to the above SQ.
The center frequency fo can be set by setting the frequency range (for example, several KHz or more) of the magnetic signal in which the FLL circuit including the UID 1, the preamplifier 5, the integrator 6, and the negative feedback circuit 7 does not operate.
SQUI equivalent to a magnetic signal in the frequency band centered on
The output voltage of D1 is output through the high frequency amplifier 10. Therefore, the sweep reference voltage source 9 of the variable capacitor
Since the voltage of 1 corresponds to the center frequency fo of the output voltage output from the high frequency amplifier 10, if the voltage of the variable voltage source 9 is plotted on the vertical axis and the output voltage of the high frequency amplifier 10 is plotted on the horizontal axis, It can be operated as a spectrum analyzer for magnetic input signals in the region. That is, to increase the dynamic range for a magnetic input signal (for example, a magnetic signal of several KHz or more) in a frequency region in which the FLL circuit including the SQUID 1, the preamplifier 5, the integrator 6, and the negative feedback circuit 7 does not operate. Is possible.

The present invention is not limited to the above embodiment. The above-mentioned embodiment is an exemplification, has substantially the same configuration as the technical idea described in the scope of the claims of the present invention, and has any similar effect to the present invention. It is included in the technical scope of the invention.

For example, in the embodiment of FIG. 3 above,
An example in which the sweep reference voltage source 9 of the variable capacitor 3 is swept in a sawtooth wave has been described, but the present invention is not limited to this and may be swept in a triangular wave or a sine wave.

[0018]

As described above, according to the first invention of the present application having the above-mentioned configuration, the band composed of the inductor and the voltage controlled variable capacitance capacitor is provided in the positive feedback circuit for giving the positive feedback to the SQUID. A pass filter is provided, the voltage of the voltage controlled variable capacitor is swept by the sweep reference voltage source, a high frequency amplifier is connected to the SQUID, the amount of positive feedback for a specific frequency is increased by the band pass filter, and the Since the specific frequency is continuously changed and the SQUID output at the specific frequency is obtained by the high frequency amplifier, it is possible to increase the positive feedback amount only for a certain specific frequency and continuously change it, thereby to obtain a dynamic range. Improved SQ
It becomes possible to realize a UID magnetometer, and it becomes possible to perform spectrum analysis for detecting the power of magnetic flux in a high frequency region. Further, according to the second invention of the present application having the above-mentioned configuration, in addition to the first invention, since negative feedback is provided to the SQUID to form a magnetic flux lock loop by the null method, zero feedback is provided. In the frequency range where the magnetic flux lock loop based on the zero method can be configured, the SQUID output can be obtained by configuring the magnetic flux lock loop based on the zero method, and in the frequency range where the magnetic flux lock loop based on the zero method cannot be configured. Can obtain the SQUID output by the configuration of the first invention.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an SQUID chip used in an embodiment of an SQUID magnetometer according to the present invention.

FIG. 2 is a circuit diagram showing a configuration of a variable capacitor in the SQUID chip shown in FIG.

FIG. 3 is an SQUI using the SQUID chip shown in FIG.
It is a block circuit diagram which shows the structure of a D magnetometer.

FIG. 4 is a diagram illustrating the operation of the SQUID magnetometer shown in FIG.

FIG. 5 is a SQUID used in a conventional SQUID magnetometer.
It is a circuit diagram which shows the structure of a chip.

FIG. 6 is a diagram illustrating the operation of the SQUID shown in FIG.

7 is a block circuit diagram showing a configuration of a conventional SQUID magnetometer using the SQUID chip shown in FIG.

[Explanation of symbols]

 1 SQUID 2 Positive Feedback Circuit 3 Variable Capacitor 4 Bias Current Supply Source 5 Preamplifier 6 Integrator 7 Negative Feedback Circuit 8 Negative Feedback Coil for Flux Lock 9 Sweep Reference Voltage Source 10 High Frequency Amplifier 11 SQUID Chip 12 Offset Voltage Source 21 SQUID 24 Bias Current supply source 25 Preamplifier 26 Integrator 27 Negative feedback circuit 28 Negative feedback coil for magnetic flux lock C1 to C6 Capacitor D Diode J1 to J4 Josephson junction L1 and L3 Positive feedback coil L2 Inductor R1 and R2 Resistor T1 and T2 SQUID output terminal T3 ~ T6 connection terminal

Claims (2)

[Claims] 1. A SQUID, a positive feedback coil that gives positive feedback to the SQUID by mutual inductance, and a bandpass filter including an inductor and a voltage controlled variable capacitance capacitor, and the SQUID is connected in parallel to the SQUID. A SQUID magnetometer comprising: a positive feedback circuit; a voltage reference for sweeping the voltage-controlled variable capacitor; and a high-frequency amplifier connected to the SQUID, the positive feedback amount for a specific frequency by the bandpass filter. The SQUID magnetometer is characterized in that the specific frequency is continuously changed by the sweep voltage source, and the SQUID output at the specific frequency is obtained by the high frequency amplifier. 2. A SQUID, a positive feedback coil for giving positive feedback to the SQUID by mutual inductance, and a bandpass filter composed of an inductor and a voltage-controlled variable capacitance capacitor, and connected in parallel to the SQUID. Positive feedback circuit, swept reference voltage source of the voltage controlled variable capacitor, high frequency amplifier connected to the SQUID, and negative feedback providing means for performing negative feedback on the SQUID to form a magnetic flux lock loop by a null method. And a SQUID magnetometer including: in a frequency range in which a magnetic flux lock loop by the null method can be configured, an output of the SQUID is obtained by configuring a magnetic flux lock loop by the null method, In the frequency region where the magnetic flux lock loop by the null method cannot be constructed, An SQUID magnetic flux, characterized in that the amount of positive feedback for a specific frequency is increased by an overfilter, the specific frequency is continuously changed by the sweep voltage source, and the SQUID output at the specific frequency is obtained by the high frequency amplifier. Total.