Magnetic field detecting circuit

Abstract: A magnetic field detecting circuit for measuring weak magnetic field in a high sensitivity. The circuit has a relaxation oscillator in which a coil (3) and a resistor (2) connected in series with each other are connected in parallel with a Josephson device (1), and of a magnetic field input circuit (4) magnetically coupled with the coil of the oscillator circuit for applying the detected magnetic field into the oscillator circuit, whereby oscillation frequency is shifted corresponding to the detected magnetic field. An optimum design of the magnetic field detecting circuit is achieved independently for a parameter of the relaxation oscillator and a parameter of a receiving circuit respectively, and a multi-channelization of the detecting circuit is achieved easier by the structure.

Description: Background of the invention The invention relates to a magnetic field detecting circuit according to the preamble of claim 1 which uses a Josephson device applied for a high sensitive magnetic sensor, a current meter, a displacement detector, a high-frequency signal amplifier and the like. Such a magnetic field detecting circuit is for example known from Applied Physics A, solids and surfaces, vol. A 47 (1988), no. 3, pages 285-289, and is schematically represented in Fig. 2 of the present application. Fig. 2 is a circuit diagram of an SQUID (RO-SQUID) using a conventional relaxation oscillation, where a resistor 2 and a coil 3 connected in series with each other are connected in parallel with a DC-SQUID 7. A magnetic field signal from an input coil 4 enters the DC-SQUID 7 to be converted into a critical current value, and an oscillating frequency is shifted, and then a magnetic field is detected. However, because the conventional RO-SQUID is incorporated directly into the oscillator circuit with the DC-SQUID 7, then a design must be performed in consideration of both characteristics, an oscillating condition of the RO-SQUID and a sensitivity of the DC-SQUID. This requires a complicated design and results in an unsatisfactory sensitivity. A large inductance of the DC-SQUID arisen in the above case increases a difficult of an oscillation control from an external control line with respect to a design thereof, the series connection for realizing the multi-channelization is impossible, and each channel must be connected independently for a plurality of channels. A problem arises because of the increases of the number of signal lines, an occurrence of a cross talk between signal lines, and a difficulty to carry out an accurate measurement. SUMMARY OF THE INVENTION It is an object of the present invention to provide a magnetic field detection circuit with an easier design of the circuit, an easier oscillation control from the external, a satisfactory high sensitivity capable of series connection of a Josephson devices and easy improvement for a multi-channelization of the magnetic field detecting circuit. To achieve the foregoing object, this invention has a structure including a magnetic field input circuit magnetically coupled with a coil in a relaxation oscillator, so an optimum design of the magnetic field detecting circuit is achieved independently for a parameter of the relaxation oscillator and a parameter of a receiving circuit of a magnetic field respectively. A control line for controlling an oscillation magnetically coupled with a Josephson device in the oscillator is added to enable a multi-channelization by a series connection of the Josephson devices. In the magnetic field detecting circuit as described above, a signal is input from a magnetic field input circuit magnetically coupled with a coil of a relaxation oscillator oscillated by a bias current, and converts an oscillating frequency to detect a value of a input magnetic field. Through setting the bias current at a suitable value, a critical current value of the Josephson device is varied by a control line magnetically coupled to the Josephson device, thereby the control of the oscillation is available. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram of a first embodiment according to the present invention. Fig. 2 is a conventional circuit diagram of the RO-SQUID. Fig. 3 is a circuit diagram of a second embodiment according to the present invention. Fig. 4 is an embodiment of a magnetic field detecting system according to the present invention. Fig. 5 is a circuit diagram of a third embodiment according to the present invention. Fig. 6 is a circuit diagram of a fourth embodiment according to the present invention. Fig. 7 is a circuit diagram of a fifth embodiment according to the present invention. Fig. 8 is a circuit diagram of a sixth embodiment according to the present invention. Fig. 9 is a circuit diagram of a seventh embodiment according to the present invention. Fig. 10 is an embodiment of a multi-channel magnetic-field detecting system according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments according to the invention are described with reference to the drawings as follows. Fig. 1 is a circuit diagram of a first embodiment according to the invention. A oscillating frequency "f" generated across the Josephson device satisfies the following equation, f ≒ (R/L)/(ln {(Ib-Icmin)/(Ib-Icmax) } ) where Ib represents a bias current 5 flowing into a circuit, Icmax and Icmin represent a maximum critical current and a minimum critical current of the Josephson device 1 respectively, L represents an inductance of a coil 3, and R represents a resistance of a resistor 2. When a magnetic flux enters to the coil 3 from an input coil 4, the oscillating frequency "f" is varied depending on an amount of the flux, the magnetic flux being detected as a frequency variation. An oscillation amplitude is of a gap voltage of the Josephson device with a constant value. Each device can easily be produced through the use of a thin film and a photolithography processes. Fig. 3 shows a second embodiment according to the invention, where can be provided a higher sensitivity compared to the first embodiment through being applied by an RF-SQUID 9 formed of the Josephson device 6 and a superconductive loop, and by a feedback coil 8 respectively, in addition to the input coil 4 as a magnetic field input circuit. In the second embodiment, a design can be performed separately for a parameter of the oscillating circuit and for a parameter of an RF-SQUID for detecting the magnetic field. This facilitates the design and realizes an optimum design with a high sensitization compared to the conventional RO-SQUID. Fig. 4 is an example of a magnetic field detecting system according to the invention, where the RF-SQUID 9 is applied a feedback operation by a feedback coil 8 for obtaining a larger dynamic range. A magnetic field inputted from a detecting coil 11 enters the input coil 4 to be input to the coil 3 through the RF-SQUID 9, thus an oscillating frequency of a relaxation oscillator can be varied. Oscillation wave forms are converted into a DC voltage through a head amplifier 12 and an F-V converter 13, after which only an amount of variations of the frequency is taken out by an integration amplifier 14 as an output signal and a part of the output signal is fed back. Fig. 5 shows a third embodiment according to the invention, showing a circuit in which a plurality of Josephson devices 1 of the second embodiment are connected in series with each other. Since an output of the relaxation oscillator is of a gap voltage of the Josephson device, the larger number of those connected in series with each other produces a larger amplitude to facilitate detection of a signal. This method is applied to the circuit of the first embodiment. An embodiment of multi-channelization according to the invention will be described as follows. In a circuit of the invention, more multi-channels can be easily obtained by connecting a plurality of relaxation oscillators in series with each other through being additionally applied with a control line 10 for controlling an oscillation magnetically coupled with the Josephson device 1. Fig. 6 is a circuit diagram of a fourth embodiment, where the circuit of the first embodiment in Fig. 1 is multi-channelized. Two of the same magnetic-field detecting circuits are connected in series with each other to form two channels. An oscillation condition is, Icmax < Ib < {(Rn+R)/R} Icmin where Rn represents a normal conductive resistance of the Josephson device. When normally oscillating, a current is allowed to flow into the control line 10 to set a bias current 5 for obtaining Icmax < Ib. To turn OFF the oscillation, by reducing the current flowing into the control line 10 a value Icmax of the Josephson device 1 is raised to satisfy Icmax > Ib. A method according to the invention is to easily control, by the current of the control line 10, an oscillation of the circuit having multi-channels connected in series with each other, thus a magnetic field of each channel can be detected by sequentially monitoring a frequency of a voltage output there across in a manner of operating channel by channel. Wiring separately at every channel basis is not necessary, the number of wirings can be reduced, a cross talk between channels is prevented, this enables a more accurate measurement. Fig. 7 shows a fifth embodiment according to the invention, where a circuit is incorporated with a DC-SQUID 7 instead of the Josephson device 1 of the fourth embodiment (Fig. 6). The DC-SQUID 7 provides an easier control of the oscillation due to a smaller inductance and a larger variation amount of Icmax because it controls only an oscillation different from the conventional example in Fig. 2. Fig. 8 shows a sixth embodiment according to the invention, where is illustrated an example of the multi-channelization of the second embodiment (Fig. 3). Fig. 9 shows a seventh embodiment according to the invention, where a circuit is incorporated with a DC-SQUID 7 instead of the Josephson device 1 of the sixth embodiment. The DC-SQUID 7 is the same as in the fifth embodiment (Fig. 7). Fig. 10 shows an example of a multi-channel magnetic-field detecting system. A method of detecting a signal is the same as in Fig. 4, but there is added a shift register 15 in which the control line 10 for controlling the oscillation is controlled by an external clock signal 16. The shift register 15 can be produced by the superconductive circuit using the Josephson device, to be used in a superconductive state together with the magnetic field detecting circuit, and to be produced unitarily on the same substrate. In this system, an operation of each channel can be switched by the clock signal 16 to reduce the number of signal lines. As hereinbefore fully described, according to the invention, since the multi-channel magnetic-field detecting circuit connected in series with each other can be controlled their oscillations for one channel at a time to detect the signal, the magnetic field can be detected by less signal lines at a high efficiency. The embodiments has been shown and described for two channels, but more than two channels can be produced in a practical use. According to the invention, as hereinbefore described, compared to the conventional RO-SQUID, an input of the magnetic field is carried out from the coil 3 of the relaxation oscillator, this provides an optimum design and an easier control of the oscillation by the control line. Therefore, a multi-channelization can be realized in a high efficiency, a cross talk between the channels is prevented, and a result is that a more sensitive detection of the magnetic field can be provided.

Claims: A magnetic field detecting circuit having an oscillator circuit (31) in which a coil (3) and a resistor (2) are connected in series with each other and a Josephson device (1) is connected in parallel with the coil (3) and the resistor (2) connected in series, comprising; a magnetic field input circuit (32) for applying a detected magnetic field into said oscillator circuit (31), whereby oscillation frequency of the oscillator circuit (31) is shifted corresponding to said detected magnetic field, characterized in that said magnetic field input circuit (32) is magnetically coupled with said coil (3) of said oscillator circuit (31). A magnetic field detecting circuit according to claim 1, wherein the magnetic field circuit (32) comprises an input coil (4), a feedback coil (8), and an RF-SQUID (9) having a superconductive ring with a further Josephson device (6). A magnetic field detecting circuit according to claim 1, wherein a plurality of said magnetic field detecting circuits are connected in series with each other, and a plurality of control lines (10) are magnetically coupled with each of said Josephson devices (1) for varying critical current of the Josephson devices (1) to control oscillation of each of said oscillator circuits (31).

Inventors: CHIBA NORIO, KIRYU SHOGO, SHIMIZU NOBUHIRO, CHIBA, NORIO, KIRYU, SHOGO, SHIMIZU, NOBUHIRO, Kiryu, Shogo, c/o Agency of Industry &amp; Technology
Applicants: AGENCY IND SCIENCE TECHN, SEIKO INSTR INC, AGENCY OF INDUSTRIAL SCIENCE AND TECHNOLOGY, SEIKO INSTRUMENTS INC.
Application year: 1992
Main classification: G01R33
IPC codes: G01R33, H01L39