JP3014866B2 - Magnetic flux measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic flux measuring device for measuring a very weak magnetic field, and more particularly to a weak magnetic field generated from a human body, especially a human heart or brain, or an ionosphere substantially above the ground surface. Also, the present invention relates to a magnetic flux measuring device for measuring a weak magnetic field or the like generated from an oil field deep underground.

[0002]

2. Description of the Related Art A high-sensitivity magnetic flux measurement for applying an extremely weak magnetic field has hitherto been performed by using a DC superconducting quantum interference device (dc-SQUID) using a Josephson junction or a high-frequency superconducting quantum interference device (rf). -SQUID) has been used.

The output signal of such a device contains a non-zero constant called "DC offset". Therefore, when measuring the value of magnetic flux instead of the amount of change in magnetic flux,
It is necessary to perform zero magnetic flux calibration by subtracting the DC offset value from the output of the SQUID device. However, such zero flux calibration is extremely difficult to implement.

Therefore, the present inventors have conventionally developed a magnetic flux measuring device capable of obtaining the absolute value of the magnetic flux without performing zero magnetic flux calibration by using even harmonics (Japanese Patent Application Laid-Open (JP-A) no. 63-153484).

In the above magnetic flux measuring device, the input (low-frequency) magnetic flux Φ _in = (Φ ₀ / 2π) φ _in ) is converted to the frequency f = ω / 2π
_M = (Φ ₀ / 2π) φ _M = (Φ ₀ z / 2
π) cos (ωt) is applied to the Josephson junction. Φ ₀ = h / 2e is the elementary quantum flux, and z is a parameter called the modulation index. This composite magnetic flux Φ is Φ
= Φ _M + Φ _in .

Here, C _m = cos (mωt), S _m =
Assuming that sin (mωt), the magnetic flux angle is φ = φ _in + φ _M , and the critical Josephson junction current is I _J , the current flowing through the Josephson junction is as follows.

[0007] _{I / I J = sinφ = sin} (Φ in + Φ M) = sinΦ in cosΦ M + cosΦ in sinΦ M = sinΦ in (J 0 + Σ (-1) m 2J 2m C 2m) + cosΦ in (Σ (-1 ^{_{) m 2J 2m + 1 S 2m}} + 1) (1) _{here, J m = J m (z} ) is the Bessel function. Assuming that the magnetic flux angle φ _in of the input magnetic field is small, sin φ
_in will be approximately equal to zero. Therefore, the even harmonic component I
_2m is as follows.

[0008]

(Equation 1) As is apparent from the equation (2), the even harmonic component I _2m
Is proportional to φ _in . Therefore, the absolute value of the magnetic flux of the magnetic field can be obtained by selectively detecting the even harmonic component I _2m from the current flowing through the Josephson junction.

[0009]

However, the above-described conventional magnetic flux measuring apparatus has the following problems.
That is, first, in the case of a parallel resonance circuit composed of a Josephson junction, Johnson noise at a frequency of 2ω is large. Would.

In addition, for proper operation, the Josephson junction in the detection circuit must be maintained in a superconducting state. However, the current becomes larger than the critical current of the Josephson junction, and the superconducting state may be damaged. And stable measurement cannot be performed.

The present invention has been made in view of such a conventional situation, and it is possible to obtain the absolute value of the magnetic flux of a weak magnetic field without performing zero magnetic flux calibration.
An object of the present invention is to provide a magnetic flux measuring device having high stability.

[0012]

In order to achieve the above object, a magnetic flux measuring apparatus according to the present invention comprises an angular frequency ω, a modulation index z
Magnetic flux generating means for generating a modulation magnetic flux (Φ ₀ z / 2π) cos ωt, and at least one Josephson junction having a specific inductance L _J ,
Including a tank circuit, the input magnetic flux to be measured and the modulation magnetic flux are superimposed and applied to the Josephson junction,
And a detection circuit for separating and detecting a specific even harmonic component of the current flowing through the Josephson junction, the modulation index z is approximately 2 to 5, about 1 to inductance L of the detection circuit is the intrinsic inductance L _J 1/5, wherein the capacitance C of the detection circuit is set to a value that tunes to 2ω, which is twice the angular frequency ω.

[0013]

In the magnetic flux measuring apparatus of the present invention having the above-described structure, the input magnetic flux to be measured and the modulation magnetic flux are superimposed and applied to the Josephson junction to separate a specific even harmonic component of the current flowing through the Josephson junction. By detecting, the absolute value of the magnetic flux of the weak magnetic field can be obtained without performing zero magnetic flux calibration. Further, by adjusting the modulation index z, the inductance L, and the capacitance C, it is possible to reduce the Johnson noise of the 2ω component, and the S / N
The ratio and the stability can be improved.

[0014]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of a magnetic flux measuring apparatus according to an embodiment of the present invention. In FIG. 1, reference numeral 10 denotes a tank circuit which includes a tank circuit, which superimposes a measured magnetic flux and a deviated magnetic flux and separates even harmonics. Separation circuit, 20 is superposition / separation circuit 10
A semiconductor circuit including a low-temperature preamplifier and a detector for amplifying the even harmonic separated by the above-mentioned method, and 30 is a signal generating circuit for generating a predetermined modulated magnetic flux.

The detection circuit 10 includes two Josephson junctions 11 and 12, a coil 13, a capacitor 14 and a resistor 15. By using the two Josephson junctions 11 and 12, a push-pull mode is achieved. Thus, the even harmonic current is separated from the odd harmonic current. Resistor 15 is also an R _J / 2 shunt resistor to ensure that Josephson junctions 11, 12 are in a superconducting state during operation. Further, L shown in the figure is an input inductance, and C is a total capacitance including a Josephson junction capacitance, a parasitic capacitance, and an external capacitor (tunable).

In this embodiment, the output signal is fed back to the input side by a feedback circuit (not shown) and is superimposed on the input magnetic flux. This makes it possible to stabilize the effective gain and expand the measurement range of the input magnetic flux.

The semiconductor circuit 20 includes a lamp component tank circuit and a liquid helium cooled GaAs FET (gallium arsenide field effect transistor) preamplifier. This is because a signal having a high frequency on the order of gigahertz (GHz) must be processed under conditions of high efficiency and low noise. The impedance matching with the superposition / separation circuit 10 is performed using a low-temperature GaAs varactor diode that can be easily controlled at room temperature.

In such a magnetic flux measuring device, each parameter is selected as described below.

First, it is assumed that φ _in = 2πΦ _in / Φ ₀ is expanded into a Fourier series as follows.

Φ _in = α ₀ + Σ (α _2n C _2n + β _2n S _2n ) (3) The currents I _L , I ₁ , I ₂ , I _C , and I _R shown in FIG. 1 are added, and Φ ₀ / 2πI _{Using J} = L _J ,

(Equation 2) Is obtained. Incidentally, L _J is the Josephson junction 11,1
2 is an inherent inductance.

Assuming that | φ _in | is small, sin φ
_in becomes approximately equal to φ _in0 . Using the Fourier-Bessel function equation (1) and the Fourier expansion I _in = I ₀ + I ′ ₂ C ₂ +...

(Equation 3) Is obtained. Here, each parameter is a = L _J / 2L
+ J ₀ , b = a−4ω ² CL _J , b ₊ = b + J ₄ , b ₋
= B-J ₄ , q = 1 / Q = 2ωL _J / R _J = 2ωΦ ₀ /
2πV _J0 . The solution of equation (5) is

(Equation 4) M ₁₁ = b ₊ b ₋ + q ² , M ₃₃ = ab ₊ −2J ₂
^{At 2} ,

(Equation 5) Here, if the related values are represented by the suffixes “s” and “n” using the expression (6), the following s / n square ratio is obtained.

[0023]

(Equation 6) The above equation (8β) is an optimum (maximum) value by imposing the following conditions.

(Equation 7) Will be taken.

[0024] M ₃₃ = 0, _{or, M 33 = O (q)} (9) a 2> 0, q 2> 0, b - when ² <Assuming ∞, ((s
/ N) _α ) ² is less than the optimal value. Therefore, in this case, the s / n ratio is improved by selectively using the S ₂ component β ₂ . C ₂ component α _2s ² = (2J
^₂ b _- I _{0s) 2,} by imposing the following condition, can be eliminated.

[0025] b _- = 0, _or, b - = in O (q) ² (10) condition (10), when referring to a DC input current I _s, the DC energy E _{0 s} is expressed as follows.

[0026]

(Equation 8) The minimum DC input signal is (8.o) and

(Equation 9) Suppose that Nyquist formula for related value with subscript "m"

(Equation 10) And put it as follows,

[Equation 11] Where ρ = a / 2J ₂ ,

(Equation 12) The above F (ρ) can be minimized by imposing the following condition on a.

Ρ = a / 2J ₂ = 1 (13) In order to satisfy the above equations (9), (10) and (13), FIG.
Using the values z and J shown in the graph, the circuit parameters are set as follows. Incidentally, z _o in the graph of FIG. 2 =
3.611 is the optimum value. At this time, J _{2, o} = J ₂ (z
_o ) = 2J _{4, o} = 2J ₄ (z _o ) = 0.431, J
_{0, o = J 0 (z} o) = - it is 0.3928.

[0028]

(Equation 13) As described above, the conditions (9), (10), and (13)
Each parameter and operating frequency in the magnetic flux measuring device of FIG. 1 are determined so that is satisfied. Typical _{values, I J = 25μA, R J} = 40Ω, L J = 13pH,
C = 150 pF, L = 5pH, ω = 1.5 GHz. The value of Q is selected to be 50.

As described above, the optimum value of the modulation index z is about 3.6, and the value of the modulation index z is set to at least a range of about ± 50% from this value, that is, a range of about 2 to 5. Is preferred. The optimum value of the inductance L is L = L _J /2.6, and the value of L is at least in the range of about ± 50% from this value, that is, approximately 1 to 1/1 / L _J.
Preferably, it is set to 5 times. On the other hand, the capacitance C needs to be tuned to 2ω, which is twice the angular frequency ω, so that at least the Q value (50 in this embodiment)
It is necessary to set 1 / Q, that is, in the case of this embodiment, with an accuracy of about 2%. In the present embodiment, this can be realized by a low-temperature GaAs varactor diode provided in the semiconductor circuit 20.

Thus, the circuit is tuned, and the condition (9),
(10) and (13) can actually be satisfied. This also helps to maintain the stability of the measurement system, as described below.

The denominator D shown in the linear equation (7) may be such that D = 0 for certain values of the parameters a, b, and z. By nonlinear circuit theory and simulation,
It has been found that when D ≦ 0, the magnetometer becomes an FF (flip-flop), and the output of the second harmonic is obtained even when the zero DC input I _J = 0. In this case, D> 0 is required for the zero DC offset.

Assuming that D is positive and small, D = 1
/ G> 0, G >> 1 and therefore the magnetometer has a gain of O
(G) acts as a regenerative amplifier, for which there are two mechanisms: negative inductance DC amplification and degeneration of the second harmonic by pumping the _fourth harmonic term J ₄ G in modulation. This can be explained by parasitic amplification. If the gain is large, small variations in the parameters will result in an undesired D ≦ 0.

Next, in order to emphasize the essence, a description will be given of the stability and permissible range of the parameters by using arguments whose digits are simplified.

Here, a typical value q = O (q) = 0.0
2 is quite small, so a ₀ , b ₀ , J ₂₀ , J ₄₀ are
(1).

First, in equation (9), it is assumed that M ₃₃ = σ. In general, M ₃₃ ＯO (1) applies and the equation (8.
β) becomes ((s / n) β) ² = O (q ² ) (17.β). M ₃₃ = 0, or in the case of M ₃₃ = O (q), by the formula (8.β), is obtained ((s / n) β) 2 = O (1). Therefore, according to M ₃₃ = O (q), ((s /
n) β) is greatly improved to 1 / O (q ² ) = O (Q ² ) to 2500 times.

[0036] Next, b _- When _{= 0, M 11 = b -} b +
Since + q ² and b ₊ = O (1), b ₋ = O (q ² ) is required to keep M ₁₁ = O (q ² ).

Regarding the variance δp, considering equation (13), F (p) = 1 + (δp) ² / 2− (δp) 3/2 +
P = 1 + (δp), which is..., Is 10% (+ 0.4d
For increases below B), F (p) <1.1, -0.36
<Δp <+0.56, which is less stringent than the requirements O (q) and O (q ² ) in equations (9) and (10).

Next, the denominator will be described. In equation (7),
Substituting b ₋ = O (q ² ) and M ₃₃ = O (q) gives D =
a ₀ q ² + O (q ³ ) is obtained, which is based on D
= Aq ² is extremely stable with respect to parameter variations. M ₃₃ ＯO (q) greatly improves the stability of D while maximizing ((s / n) β) ² . This is considered to be the result of an improvement in the s / n ratio as compared with the predetermined value.

[0039]

As described above, according to the present invention,
Thus, it is possible to provide a highly stable magnetic flux measurement device with little noise, which can obtain the absolute value of the magnetic flux of a weak magnetic field without performing zero magnetic flux calibration.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a magnetic flux measuring device according to one embodiment of the present invention.

FIG. 2 is a graph of a Bessel function.

[Explanation of symbols]

 Reference Signs List 10 superposition / separation circuit 11, 12 Josephson junction 13 coil 14 capacitor 15 resistance 20 semiconductor circuit 30 signal generation circuit

──────────────────────────────────────────────────の Continued on the front page (73) Patentee 000004237 NEC Corporation 5-7-1, Shiba, Minato-ku, Tokyo (72) Inventor Eiichi Goto 3-9-305, Tsujido East Coast, Fujisawa-shi, Kanagawa ( 72) Inventor Geng ▲ Chu ▼ All 3-14-6-101 Kitano, Niiza City, Saitama Prefecture (58) Field surveyed (Int.Cl. ⁷ , DB name) G01R 33/035 ZAA H01L 39/22 ZAA

Claims (4)

(57) [Claims] 1. A modulation magnetic flux (Φ) having an angular frequency ω and a modulation index z.
₀ and _z / 2 [pi) modulating the magnetic flux generating means for generating a cos .omega.t, as well as provided with at least one Josephson junction having an intrinsic inductance L _J, it includes a tank circuit,
A detection circuit that superimposes an input magnetic flux to be measured and the modulation magnetic flux, applies the superposed magnetic flux to the Josephson junction, and separates and detects a specific even harmonic component of a current flowing through the Josephson junction; approximately 2 to 5 exponent z, the inductance L is said of the detection circuit intrinsic inductance L stands for _J 1 to 1 /
5. The capacitance C of the detection circuit is equal to the angular frequency ω
The magnetic flux measuring device is set to a value that tunes to 2ω twice as large as 2. The magnetic flux measuring device according to claim 1, wherein the detection circuit includes an even number of Josephson junctions arranged to operate in a push-pull mode. 3. A pre-Symbol flux measuring apparatus according to claim 1, further comprising a semiconductor circuit having a varactor diode for modulating the capacitance C. 4. A magnetic flux measuring apparatus according to claim 1, further comprising a semiconductor circuit having an amplification means using a gas Riumuhi containing field effect transistor.