Highly sensitive magnetic field detecting device

Abstract: An adjustment function of the bias current and modulating signal of a highly sensitive magnetic field detecting device for detecting a magnetic field by using a DC SQUID (1). An F.L.L circuit of the highly sensitive magnetic field detecting device is equipped with an external input terminal (14) or a low-frequency oscillator (13) so that a low-frequency signal can be superposed and applied to a feedback modulating coil (10) of a DC SQUID (1) from the outside. The circumstances for the adjustment and measurement need not be changed to facilitate the adjustment such a special DC SQUID (1) as will offset the disturbing magnetic field.

Description: Background of the invention 1. Field of the Invention The present invention relates to a highly sensitive magnetic field detecting device using a DC Superconducting quantum interference device (i.e., DC SQUID; DC Superconducting Quantum Interference Device) according to the preamble of claim 1, which can be used for medical or underground resource searching applications. 2. Description of the Prior Art Such a magnetic field detecting device is for example known from Journal of Low Temperature Physics, vol. 71(1988), June, pages 369 - 392, and in particular pages 385 - 386 (of Fig. 3). The magnetic field detecting device disclosed therein discloses SQUID electronics for operating a SQUID system in a low noise and high slew rate condition. The SQUID electronics is equipped with circuits allowing remote control such as an external flux modulation signal for using in order to run several SQUID systems simultaneously. Another known system according to the preamble of claim 1 is for example shown in Fig. 4, which shows a block diagram showing the structure of a highly sensitive magnetic field detecting device of the prior art. A reference signal outputted from an oscillator 5 is adjusted by an amplitude adjustor 9 and is converted into an electric current by a first voltage-current converter 6 until it is applied as a modulating signal to a DC SQUID 1. When a suitable bias current is fed to the DC SQUID 1 from a variable current source 8, a magnetic flux interlinking the DC SQUID 1 is modulated by the modulating signal and is detected as a modulated signal. This modulated signal is amplified by an amplifier 2 and is detected by a phase detector 3 using the reference signal coming from the oscillator 5. A detected signal corresponding to the output of that phase detector 3 is outputted from an integration circuit 4 and is converted into an electric current by a second voltage-current converter 7. The electric current is applied as such a feedback signal to the DC SQUID 1 as to compensate a change in the magnetic flux interlinking the DC SQUID 1. These circuits for driving the DC SQUID 1 are called the "F.L.L. (i.e., Flux Locked Loop) circuit device". And, the magnetic field can be measured by reading out that feedback signal or detected signal. The measurement of a fine magnetic field is performed under a magnetically shielded special circumstance so as to eliminate the disturbing magnetic field. In the prior art, the adjustments of the variable current source 8 and the amplitude adjustor 9 for the measurements described above are accomplished under the circumstance having the disturbing magnetic field or by using a magnetic element such as a magnet to apply noises to the DC SQUID 1 while observing the amplitude of the output coming from the amplifier 2 and synchronized with the reference signal and the output of the phase detector 3. Thus, according to the highly sensitive magnetic field detecting device of the prior art, the adjustments of the variable current source 8 and the amplitude adjustor 9 have to be accomplished under a circumstance different from that for the measurements. As a result, measurements under a different circumstance may deviate from those in an optimum state even with an optimum adjustment. If, on the other hand, the DC SQUID 1 used is exemplified by a plane type gradiometer which is developed to operate under circumstances other than the special one shielded magnetically by offsetting the influences of the disturbing magnetic field, there arises a problem that the signal hardly changes even with disturbances so that the optimum adjustment cannot be achieved. SUMMARY OF THE INVENTION It is an object of the present invention to provide a magnetic field detecting device with optimum adjusting function. In order to achieve the object, the F.L.L circuit device of the highly sensitive magnetic field detecting device is partially equipped with: a signal change-over circuit enabled to superpose a false signal for adjusting a bias current modulating signal on an signal inputted to a feedback modulating coil; and at least either an external input terminal connected with the signal change-over circuit and adapted to receive the false signal from the outside or a low-frequency oscillator for generating the false signal. According to the above-specified structure, the disturbing magnetic field to be applied to the DC SQUID 1 at the time of adjusting the bias current or the modulating signal can be applied not from the outside of the DC SQUID 1 but from the feedback modulating coil by using the false signal on the circuit. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing the structure of a first embodiment of the highly sensitive magnetic field detecting device according to the present invention. Fig. 2 is a block diagram showing the structure of a second embodiment of the highly sensitive magnetic field detecting device according to the present invention. Fig. 3 is a block diagram showing the structure of a third embodiment of the highly sensitive magnetic field detecting device according to the present invention. Fig. 4 is a block diagram showing the structure of the highly sensitive magnetic field detecting device of the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in the following in connection with the embodiments thereof with reference to the accompanying drawings. Fig. 1 is a block diagram showing the structure of a highly sensitive magnetic field detecting device according to a first embodiment of the present invention. In Fig. 1, a DC SQUID 1 is constructed of a feedback modulating coil 10 and a DC SQUID ring 11, which are magnetically coupled to each other. The feedback modulating coil 10 is connected with the outputs of a first voltage-current converter 6 and a second voltage-current converter 7, and the DC SQUID ring 11 is connected with the input of an amplifier 2 and the output of a variable current source 8. An oscillator 5 is connected with the inputs of a phase detector 3 and an amplitude adjustor 9, the output of which is connected with the input of the first voltage-current converter 6. The phase detector 3 has its inputs connected separately with the output of the amplifier 2 and the oscillator 5 and its output connected with an integration circuit 4. A signal change-over circuit 12 is composed of an adder 15, a first switch 16, a second switch 17 and a third switch 18. One input of the adder 15 is connected commonly with the one-side terminals of the first switch 16, the second switch 17 and the third switch 18. The other input of the adder 15 is connected with the output of the integration circuit 4, and the output of the same is connected with the input of the second voltage-current converter 7. The other terminal of the first switch 16 is connected with a low-frequency oscillator 13, and the other terminal of the second switch 17 is connected with an external input terminal 14, and the other terminal of the third switch 18 is connected with the ground. With the structure thus far described, the operations of the adjusting time using the low-frequency oscillator 13 will be described in the following. When the first switch 16 is closed whereas the second switch 17 and the third switch 18 are opened to bring the integration circuit 4 into a reset state having an output 0, the low-frequency signal from the low-frequency oscillator 13 is fed through the adder 15 and is converted into the electric current by the second voltage-current converter 7 until it is inputted to the feedback modulating coil 10. This low-frequency signal is converted by the feedback modulating coil 10 into the magnetic flux interlinking the DC SQUID loop 11 so that the DC SQUID 1 is given the same effect as that of the time when the disturbing magnetic field is applied to the DC SQUID 1. In this state, the variable current source 8 and the amplitude adjustor 9 are so adjusted that the amplitude of the output signal of the amplifier 2 or the output signal of the phase detector 3 may have the maximum amplitude while being observed through an oscilloscope or the like. Then, it is possible to optimize the bias current to be fed to the DC SQUID ring 11 and the modulating signal to be fed to the feedback modulating coil 10. The measurements of a magnetic field are performed by opening the first switch 16 and the second switch 17 and closing the third switch 18 to release the integration circuit 4 from its reset state. The operations at the time of measuring of the magnetic field are wholly similar to those of the example of the prior art excepting that the output of the integration circuit 4 is transmitted through the signal change-over circuit 12 to the second voltage-current converter 7. Moreover, the operations at the adjusting time using the external input terminal 14 are also similar to the aforementioned ones excepting that the first switch 16 is opened at first whereas the second switch 17 is closed to input the low-frequency signal from the outside through the external input terminal 14 to the adder 15. Fig. 2 is a block diagram showing the structure of a highly sensitive magnetic field detecting device according to a second embodiment of the present invention. In Fig. 2, a DC SQUID 1 is constructed of a feedback modulating coil 10 and a DC SQUID ring 11, which are magnetically coupled to each other. The feedback modulating coil 10 is connected with the outputs of a first voltage-current converter 6 and a second voltage-current converter 7, and the DC SQUID ring 11 is connected with the input of an amplifier 2 and the output of a variable current source 8. An oscillator 5 is connected with the inputs of a phase detector 3 and an amplitude adjustor 9, and the phase detector 3 has its inputs connected separately with the output of the amplifier 2 and the oscillator 5 and its output connected with the input of an integration circuit 4. The output of the integrator 4 is connected with the input of the second voltage-current converter 7. A signal change-over circuit 12 is composed of an adder 15, a first switch 16, a second switch 17 and a third switch 18. One input of the adder 15 is connected commonly with the one-side terminals of the first switch 16, the second switch 17 and the third switch 18. The other input of the adder 15 is connected with the amplitude adjustor 9, and the output of the same is connected with the input of the first voltage-current converter 6. The other terminal of the first switch 16 is connected with a low-frequency oscillator 13, the other terminal of the second switch 17 is connected with an external input terminal 14, and the other terminal of the third switch 18 is connected with the ground. With the structure thus far described, the operations are similar to those of the first embodiment shown in Fig. 1, excepting that the low-frequency signal for establishing effects similar to those of the disturbing magnetic field applied to the DC SQUID 1 is added to the modulating signal in the adder 15 and is applied to the feedback modulating coil. Fig. 3 is a block diagram showing the structure of a highly sensitive magnetic field detecting device according to a third embodiment of the present invention. In Fig. 3, a DC SQUID 1 is constructed of a feedback modulating coil 10 and a DC SQUID ring 11, which are magnetically coupled to each other. The feedback modulating coil 10 is connected with the outputs of a first voltage-current converter 6, a second voltage-current converter 7 and a third voltage-current converter 19, and the DC SQUID ring 11 is connected with the input of an amplifier 2 and the output of a variable current source 8. An oscillator 5 is connected with the inputs of a phase detector 3 and an amplitude adjustor 9, the output of which is connected with the input of the first voltage-current converter 6. The phase detector 3 has its inputs connected separately with the output of the amplifier 2 and the oscillator 5 and its output connected with an integration circuit 4. The integration circuit 4 has its output connected with the input of the second voltage-current converter 7. A signal change-over circuit 12 is composed of a first switch 16, a second switch 17 and a third switch 18. The one-side terminals of the first switch 16, the second switch 17 and the third switch 18 are connected with the input of the third voltage-current converter 19. The other terminal of the first switch 16 is connected with a low-frequency oscillator 13, the other terminal of the second switch 17 is connected with an external input terminal 14, and the other terminal of the third switch 18 is connected with the ground. With the structure thus far described, the operations are similar to those of the first embodiment shown in Fig. 1, excepting that the low-frequency signal for establishing effects similar to those of the disturbing magnetic field applied to the DC SQUID 1 is converted into an electric current by the third voltage-current converter 19 and is applied to the feedback modulating coil. As has been described hereinbefore, according to the present invention, the disturbing magnetic field necessary for the adjustment can be falsely established by making it possible to input the low-frequency signal to the feedback modulating coil. As a result, the bias current and the modulating signal can be adjusted under the same circumstances as those at the adjusting time so that the measurements can be accomplished always in the optimum state. Even if the plane type gradiometer, which is developed to operate under circumstances other than the special one shielded magnetically by offsetting the influences of the disturbing magnetic field, is used, the low-frequency signal can be superposed on the signal inputted to the feedback modulating coil to apply the adjusting disturbing magnetic field left unoffset so that the optimum adjustment can be accomplished.

Claims: A highly sensitive magnetic field detecting device comprising: a DC superconducting quantum interference device [SQUID] (1) having a superconductive ring (11) and a feedback modulation coil (10) magnetically coupled to the superconductive ring (11), for detecting a magnetic field; and a flux locked loop circuit for driving said DC SQUID, said flux locked loop circuit comprising variable current applying means (8) for applying a variable bias current to the superconductive ring (11); amplitude adjusting means (9) for adjusting the amplitude of a modulating signal applied to the feedback modulation coil (10), CHARACTERIZED IN THAT, a change-over circuit (12) is provided forsuperposing a false signal, which simulates the effect of an applied magnetic field, on the modulating signal to enable adjustment of the variable current applying means (8) and -the amplitude adjusting means (9) under measurement conditions. A highly sensitive magnetic field detecting device according to claim 1; including a low frequency oscillator (13) for supplying a low frequency signal to the change-over circuit (12) which generates the false signal dependent on the low frequency signal. A highly sensitive magnetic field detecting device according to claim 1; including an external input terminal (14) for inputting an externally supplied low frequency signal to the change-over circuit (12) which generates the false signal dependent on the low frequency signal. A highly sensitive magnetic field detecting device according to claim 1; wherein the superconductive ring (11) generates a feedback voltage during operation of the DC superconducting quantum interference device (1); and the change-over circuit (12) includes an adder (15) for adding the false signal to the feedback voltage. A highly sensitive magnetic field detecting device according to claim 1; wherein the change-over circuit (12) includes an adder for adding the false signal to the modulating signal from the amplitude adjusting means (9). A highly sensitive magnetic field detecting device according to claim 1; wherein the amplitude adjusting means (9) includes means for outputting a modulating voltage signal and the change-over circuit (12) includes means for outputting the false signal as a false voltage signal; and further comprising a first voltage-current converter (6) coupled to the amplitude adjusting means (9) for converting the modulating voltage signal to a modulating current signal which is applied as the modulating signal to the feedback modulation coil (10); and a second voltage-current converter (7) for converting the false signal to a false current signal. A highly sensitive magnetic field detecting device according to claim 1; wherein the superconductive ring (11) generates a feedback voltage during operation of the DC superconducting quantum interference device (1); the amplitude adjusting means (9) includes means for outputting a modulating voltage signal; the change-over circuit (12) includes means for outputting the false signal as a false voltage signal; the change-over circuit (12) includes an adder (15) for adding false voltage signal to the feedback voltage; and further comprising a first voltage-current converter (6) coupled to the amplitude adjusting means (9) for converting the modulating voltage signal to a modulating current signal; a second voltage-current converter (7) for converting the added false voltage signal and feedback voltage to an added false current signal; and means for superposing the modulating current and the added false current signal to constitute the modulating signal applied to the feedback modulation coil (10). A highly sensitive magnetic field detecting device according to claim 7; including a low frequency oscillator for inputting a low frequency signal to the change-over circuit (12) which generates the false signal dependent on the low frequency signal. A highly sensitive magnetic field detecting device according to claim 7; including an external input terminal (14) for inputting an externally supplied low frequency signal to the change-over circuit (12) which generates the false signal dependent on the low frequency signal. A highly sensitive magnetic field detecting device according to claim 1; wherein the amplitude adjusting means (9) includes means for outputting a modulating voltage signal and the change-over circuit (12) includes means for outputting the false signal as a false voltage signal; the change-over circuit (12) includes an adder (15) for adding the false voltage signal to the modulating voltage signal; the superconductive ring (11) generates a feedback voltage during operation of the DC superconducting quantum interference device (1); and further comprising a first voltage-current converter (6) coupled to the adder (15) for converting the added false voltage signal and modulating voltage signal to an added false current signal; a second voltage-current converter (7) for converting the feedback voltage to a feedback current signal; and means for superposing the feedback current signal and the added false current signal to constitute the modulating signal applied to the feedback modulation coil (10). A highly sensitive magnetic field detecting device according to claim 10; including a low frequency oscillator (13) for supplying a low frequency signal to the change-over circuit (12) which generates the false signal dependent on the low frequency signal. A highly sensitive magnetic field detecting device according to claim 10; including an external input terminal (14) for inputting an externally supplied low frequency signal to the change-over circuit (12) which generates the false signal dependent on the low frequency signal. A highly sensitive magnetic field detecting device according to claim 1; wherein the amplitude adjusting means (9) includes means for outputting a modulating voltage signal; the superconductive ring (11) generates a feedback voltage during operation of the DC superconducting quantum interference device (1); the change-over circuit (12) includes means for outputting the false signal as a false voltage signal; and further comprising a first voltage-current converter (6) coupled to the amplitude adjusting means (9) for converting the modulating voltage signal to a modulating current signal; a second voltage-current converter (7) for converting the feedback voltage to a feedback current signal; a third voltage-current converter (19) converting the false voltage signal to a false current signal; and means for superposing the modulating current signal, the feedback current signal and the false current signal to constitute the modulating signal applied to the feedback modulation coil (10). A highly sensitive magnetic field detecting device according to claim 13; including a low frequency oscillator for supplying a low frequency signal to the change-over circuit (12) which generates the false signal dependent on the low frequency signal. A highly sensitive magnetic field detecting device according to claim 13; including an external input terminal (14) for supplying an externally supplied low frequency signal to the change-over circuit (12) which generates the false signal dependent on the low frequency signal.

Inventors: NAKAYAMA SATOSHI, ODAWARA AKIKAZU, SHIMIZU NOBUHIRO, NAKAYAMA, SATOSHI, ODAWARA, AKIKAZU, SHIMIZU, NOBUHIRO
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1992
Main classification: G01R33
IPC codes: H03L7, G01R33