The present invention relates to a highly sensitive magnetic field detecting device using a DC superconducting quantum interference device (i.e., DC SQUID) for use in medical or underground resource searching applications.
The highly sensitive magnetic field detecting device is used for detecting a micro magnetic field. FIG. 5 is a block diagram showing an example of a highly sensitive magnetic field detecting device of the prior art. This highly sensitive magnetic field detecting device comprises at least a DC SQUID 2, a bias circuit 1, a signal amplifier 3, a phase detector 4, a feedback amplifier and a modulator, including oscillator 6 and two V/I converters 7 and 8. These elements, except the DC SQUID, constitute a flux locked loop (F.L.L.) circuit. The bias circuit is composed of a voltage source and a voltage-current converter so as to feed an electric current to the DC SQUID. A modulated signal coming from the DC SQUID fed with a bias current is amplified by the amplifier 3 and detected by a phase detector 4.
The detected signal is inputted to the integrator 5, which has its output feedback signal converted into an electric currrent. This current is added to the modulated signal converted into the current and is inputted together to the DC SQUID to fix the total magnetic flux applied to the DC SQUID always at a constant value. As a result, the magnetic field can be detected by monitoring the feedback signal. The detection of a fine magnetic field utilizes the aforementioned method, but the current-to-voltage characteristics and flux-to-voltage characteristics of the DC SQUID are measured by using a SQUID measurement device, as shown in a block diagram in FIG. 6.
The measurements of the current-to-voltage characteristics of the DC SQUID using the SQUID measurement device are accomplished: by sweeping the DC voltage source by the oscillator; by measuring a voltage generated in the DC SQUID; and by displaying the current against the voltage in an oscilloscope. The measurements of the flux-to-voltage characteristics are accomplished: by feeding the DC SQUID with the feedback modulation current swept by the oscillator while feeding the DC SQUID with a bias current equal to a critical current; by measuring a voltage generated in the DC SQUID; and by displaying the magnetic flux against the voltage in an oscilloscope.
Since the highly sensitive magnetic field detecting device of the prior art cannot sweep the bias current and the feedback modulation current by itself, the SQUID measurement device has to be used for measuring the element characteristics of the DC SQUID. According to this method, if it is intended to observe the element characteristics of the DC SQUID when the DC SQUID is driven as the highly sensitive magnetic field detecting device, the DC SQUID has to be disconnected from the F.L.L circuit device and connected with the SQUID element measurement device.