Abstract:
A signal detector has a SQUID, a bias current supply circuit for supplying a bias current to the SQUID, and a drive circuit for detecting a magnetic flux by receiving a voltage signal of the SQUID and reading out the input signal as an output voltage. The drive circuit has two systems including a flux-locked loop system for converting a voltage signal of the SQUID into a current signal and making a magnetic flux inside the SQUID constant by feeding back a signal whereto, and an output system separate from and operating independently of the flux-locked loop system for converting a voltage signal of the SQUID and reading out the converted voltage signal without feeding back a signal to the SQUID.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a signal detector for detecting weak (or small) magnetic fields, electrical currents, voltage, electromagnetic wavers, etc. using a superconducting quantum interference device, which is a highly sensitivity magnetic sensor, and to a measuring method therefor, and more particularly relates to a broadband detector for measuring a high speed signal. 
     FIG. 6 shows an equivalent circuit diagram of a direct-current superconducting quantum interference device (hereafter referred to as a DC-SQUID). The DC-SQUID forms a superconducting ring using a SQUID inductance  2  and a pair of Josephson junctions  1  connected to the two ends of the SQUID inductance  2 . A modulation coil  3  is magnetically coupled to the superconducting ring. An input coil  4  for inputting a signal is also magnetically coupled to the superconducting ring. 
     The DC-SQUID is a magnetic flux-voltage transducer for periodically converting voltage Vs on both ends of the superconducting ring to a single magnetic flux quantum (Φ 0 : 2.07×10 −‥ Wb) for a magnetic flux Φ mixed inside the superconducting ring. With a constant bias current supplied to the DC-SQUID, the magnetic flux-voltage characteristic is shown in FIG.  2 A. As shown in FIG. 2A the DC-SQUID has high sensitivity to magnetic flux, but this response is non-linear and the dynamic range is narrow. In the case of low frequency measurement, in order to obtain sufficient linearity and timing a Flux locked loop (FLL) method, being a type of null-balance method, is used to drive the SQUID. However, the FLL method constructs a closed loop circuit, which means that with broadband over 1 MHz stable drive is difficult due to going out of lock etc. With broadband measurement, it is also possible to utilize a method where the SQUID is driven by an open loop circuit having a simple circuit structure. 
     FIG. 7 is a schematic diagram of a signal detector using a superconducting quantum interference device with an open loop circuit of the related art. This detector is comprised of a bias current supply circuit  6  for applying bias current I, a broadband amplifier circuit  7  for amplifying a voltage Vs of the DC-SQUID  5 , and a modulation circuit  12  for moving operation points on the magnetic flux-voltage curve to given positions. The modulation circuit  12  supplies a modulation signal Im to the SQUID through a modulation coil  3 . At that time, using the magnetic flux-voltage curve of FIG. 2A, the magnetic flux-voltage conversion coefficient is adjusted so as to be fixed at point A on a rapidly changing part of the graph. Voltage Vo that has been amplified by the amplifier circuit  7  is output as output voltage Vout, and the magnetic flux-voltage conversion coefficient, as well as an input signal from the mutual inductance between the input coil and the SQUID loop, are obtained. 
     In the case of using an open loop circuit, the structure of a drive circuit becomes simple, and high speed signal measurement becomes easy. However, due to external causes, such as environmental magnetic noise, source noise etc., there is movement of the operating points on the magnetic flux-voltage curve. Because of this movement of the operating points, there is a problem that the magnetic flux-voltage conversion coefficient required in obtaining the input signal from the output voltage also changes. 
     Also, since there is no linear region in the magnetic flux-voltage characteristic of the SQUID, there is a problem that it is not possible to accurately obtain the input signal from the output voltage. 
     SUMMARY OF THE INVENTION 
     A drive circuit for magnetic flux detection for reading out an input signal as an output voltage is made up of two systems, one being a flux-locked loop system for converting a voltage signal of the superconducting quantum interference device to a current signal and making magnetic flux inside the superconducting ring constant by feeding back a feedback signal to the modulation coil, and another being an output system for converting a voltage signal of the superconducting quantum interference device to a voltage signal, and not feeding back to the modulation coil but reading out the converted voltage signal as an output voltage. 
     Also, the frequency band of the flux-locked loop system is narrower than the frequency band of the output system. 
     The frequency band of the flux-locked loop system can also be controlled to take a given value. Further, a method of using the inventive device comprises the steps of measuring a flux-voltage curve of the superconducting quantum interference device before measuring an input signal, and converting an output signal obtained from the output system of the drive circuit into an input signal by comparison with the flux-voltage curve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a measurement device using a superconducting quantum interference device illustrating a first embodiment of the present invention. 
     FIG. 2A is a magnetic flux-voltage characteristic of a DC-SQUID, and FIG. 2B is a magnetic flux-voltage characteristic after amplification. 
     FIG. 3 is a frequency characteristic of a measurement device using a superconducting quantum interference device of the first embodiment of the present invention. 
     FIG. 4 is a schematic diagram of a measurement device using a superconducting quantum interference device illustrating a second embodiment of the present invention. 
     FIG. 5 is a frequency characteristic of a measurement device using a superconducting quantum interference device of the second embodiment of the present invention. 
     FIG. 6 is an equivalent circuit diagram of a DC-SQUID. 
     FIG. 7 is a schematic diagram of a superconducting quantum interference magnetic flux meter of the related art. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention will be described in the following with reference to FIG.  1 . 
     Embodiment 1 
     FIG. 1 is a schematic diagram of a signal detector using a superconducting quantum interference device illustrating a first embodiment of the present invention. A DC-SQUID  5  is comprised of a pair of Josephson junctions  1 , a SQUID inductance  2 , a feedback-modulation coil  3 , and an input coil  4 . A superconducting ring is formed using a SQUID inductance  2  and the pair of Josephson junctions  1  connected to the two ends of the SQUID inductance  2 . The feedback-modulation coil  3  is magnetically coupled to the superconducting ring. The input coil  4  for inputting magnetic flux is also magnetically coupled to the superconducting ring. The magnetic flux-voltage characteristic acquired with a constant bias current supplied to the DC-SQUID is shown in FIG. 2A. A DC-SQUID drive circuit is comprised of a bias current supply circuit  6  for applying bias current Ib, a drive circuit for magnetic flux detection  11  for reading out an input signal as an output voltage, and a modulation circuit  12  for moving operation points on the magnetic flux-voltage curve to given positions. 
     The drive circuit for magnetic flux detection  11  has two systems, namely a flux-locked loop system  13  and an output system  14 . 
     The flux-locked loop system  13  comprises an amplifier circuit AMP  7 - 1 , an integrator INT  8 , a reference signal generating circuit REF  9 , and a feedback circuit FB  10 , and has a function of keeping the flux Φ inside the superconducting ring constant by converting a voltage signal Vs of the DC-SQUID  5  into a current signal and feeding back to the modulation coil  3  as a feedback signal If. The output system  14  consists solely of a broadband amplifier circuit AMP  7 - 2 , and has a function of amplifying a voltage Vs of the DC-SQUID  5  and converting it to a voltage signal Vo. The output of the output system  14  is not fedback to the modulation coil  3 . The modulation circuit  12  generates a modulation signal Im, and applies magnetic flux to Φm to the SQUID through the modulation coil  3 . At that time, with the magnetic flux-voltage curve of FIG. 2A, magnetic flux is applied to the SQUID using the modulation circuit  12 , and the operating point is adjusted so as to become point A of the magnetic flux-voltage conversion coefficient. 
     FIG. 2B shows a magnetic flux-voltage characteristic measured after amplification by the amplifier circuit  7 - 1 . In the flux-locked loop system  13 , a signal Vs fed from the DC-SQUID  5  is amplified by the amplifier circuit  7 - 1  to a voltage Va, a change portion ΔV of a difference (Va−Vr) between that voltage Va and a reference signal Vr fed by the reference signal generating circuit  9  is stored as Vi by the integrator  8 , and converted to a feedback current If proportional to Vi by the feedback circuit  10 . Operation is then carried out so that the magnetic flux inside the superconducting ring is always Φa. The cut-off frequency fcf for the response frequency of the feedback signal If for the input signal Iin of the flux-locked loop system is represented by the following equation 1.                f     c                 f       =         V   φ          G   a          M   f         2        πτ   i          R   f                 Equation   .              1                                
     The magnetic flux-voltage conversion coefficient of the DC-SQUID  5  is VΦ, gain of the amplifier circuit  7  is Ga, time constant of the integrator  8  is τi, feedback ratio of the feedback circuit  10  is Rf, and mutual inductance between the modulation coil  3  and the SQUID inductance  2  is Mf. 
     When an input signal Iin having a frequency below fcf is input, magnetic flux inside the SQUID loop is kept constant by the flux-locked loop system  13 , and is fixed at point A on the magnetic flux-voltage curve. As a result, for an input signal Iin having a frequency below fcf there is no output through the output system  14 . On the other hand, for an input signal Iin having a high frequency above fcf an output voltage Vout is obtained through the output system  14 . In FIG. 2A, if a change in magnetic flux inside the SQUID loop due to a high speed input signal is made ΔΦ, the voltage change of the DC-SQUID  5  becomes ΔVs. The output voltage Vout is a value obtained by amplifying ΔVs using the amplifier circuit  7 - 2 . FIG. 3 shows a frequency response characteristic acquired using this embodiment. The horizontal axis represents frequency, and the vertical axis represents output voltage Vout (units have been normalized). A low frequency region up to the cut-off frequency fcf of the flux-locked loop system  13  is cancelled out. fca is the cut-off frequency of the output system. 
     With this embodiment, low frequency noise due to external causes, such as environmental magnetic noise, noise from commercial power supply etc., is cancelled by the flux-locked loop, and it is possible to stably extract only a high speed signal for which measurement is desired. 
     Operation points on the magnetic flux-voltage curve also do not vary due to external causes such as environmental magnetic noise and noise from commercial power supply, which means that by performing fitting using a magnetic flux-voltage curve measured before actual measurement, it is possible to accurately obtain an input signal from the output voltage. 
     Embodiment 2 
     FIG. 4 is a schematic diagram of a signal detector using a superconducting quantum interference device illustrating a second embodiment of the present invention. 
     A DC-SQUID  5  is comprised of a Josephson junction  1 , a SQUID inductance  2 , a feedback-modulation coil  3 , and an input coil  4 . A superconducting ring is formed using a SQUID inductance  2  and a pair of Josephson junction  1  connected to the two ends of the SQUID inductance  2 . The feedback-modulation coil  3  is magnetically coupled to the superconducting ring. The input coil  4  for inputting magnetic flux is also magnetically coupled to the superconducting ring. A DC-SQUID drive circuit is comprised of a bias current supply circuit Ib, a drive circuit for magnetic flux detection  11  for reading out an input signal as an output voltage, and a modulation circuit  12  for moving operation points on the magnetic flux-voltage curve to given positions. The drive circuit for magnetic flux detection  11  has two systems, namely a flux-locked loop system  13  and an output system  14 . The flux-locked loop system  13  comprises an amplifier circuit  7 - 1 , an integrator  8 , a reference signal generating circuit  9 , a feedback circuit  10 , and a filter  15 , and has the function of keeping the flux inside the superconducting ring constant by converting a voltage signal Vs of the DC-SQUID  5  into a corresponding current signal and feeding it back to the modulation coil  3  as a feedback signal If. It is also possible to control the frequency band of the feedback signal If using the filter  15 . The output system  14  has a broadband amplifier circuit  7 - 2 , and has the function of amplifying a voltage Vs of the DC-SQUID  5  and converting to a voltage signal Vo. Output of the output system  14  is not fedback to the modulation signal Im, and applies magnetic flux Φm is applied to the SQUID using the modulation circuit  12 , and the operating point is adjusted so as to become point A of the magnetic flux-voltage conversion coefficient. 
     When the filter  15  is a band pass filter passing only a main power frequency f 50 , and input signal I in a f 50  is cancelled by the flux-locked loop system  13 . FIG. 5 shows a frequency response characteristic acquired using this embodiment. The horizontal axis represents frequency, and the vertical axis represents output voltage Vout (units have been normalized). Frequency components of f 50 , being the power frequency, are cancelled. fca is the cut-off frequency of the output system. 
     With this embodiment, detection of extremely small signals hidden in extremely large noise sources such as noise from a commercial power supply can be measured stably and across a broadband region. 
     The present invention has been implemented as described above, and has the following effects. 
     A drive circuit for magnetic flux detection for converting an input signal to a corresponding output voltage is made up of two systems, one being a flux-locked loop system for fixing operation points of a superconducting quantum interference device at given positions on a magnetic flux voltage curve by converting a voltage signal of the superconducting quantum interference device into a corresponding current signal and feeding it back to a modulation coil as a feedback signal, and an output system for converting a voltage signal of the superconducting quantum interference device into a corresponding voltage signal, and not feeding it back to the modulation coil but outputting it as an output voltage, and in this way noise components are cancelled and it is possible to stably extract only a signal that it is desired to measure. 
     Also, by making the frequency band of the flux-locked loop system narrower than the frequency band of the output system, it is possible to cancel low frequency noise caused by external factors such as environmental magnetic noise and noise from a commercial power supply, which is effective for extremely small, high speed signal detection. 
     Also, by controlling the frequency band of the flux-locked loop system to a given value, detection of an extremely small signal hidden in extremely large noise sources having specific frequency components, such as noise from a commercial power supply, can be measured stably and across a broadband region. 
     Operation points on the magnetic flux-voltage curve also do not vary due to external causes such as environmental magnetic noise and noise from commercial power supply, which means that by performing fitting using a magnetic flux-voltage curve measured in advance and before actual measurement, it is possible to accurately obtain an input signal from the output voltage.