1. Field of the Invention
The present invention relates to a digital magnetic flux measuring apparatus using a superconducting quantum interference device. More particularly, it relates to a digital magnetic flux measuring apparatus using a superconducting quantum interference device (hereinafter referred to as a SQUID) for detecting external magnetic flux by using a SQUID ring to measure the number and direction of output pulse signals and t digitally process the output of the SQUID.
2. Description of the Related Art
In general, a highly sensitive fluxmeter utilizing a SQUID device has in recent years been used for the measurement of a very weak magnetic field generated from living bodies and the like. In particular, a measurement of magnetic field distribution in the brain and the heart makes it possible to estimate the source of electric current that generates the magnetic field, offering very meaningful data for diagnosis and contributing to clarifying nerve activities in the living body system. In order to measure such very weak magnetic flux, it is necessary to operate the SQUID device which is a sensor, by maintaining stability at all times. However, the superconducting SQUID device often produces a phenomenon (called flux trapping) in which the SQUID itself traps magnetic flux at a time when it shifts from the normally conducting state to the superconducting state or due to excess external noise even when it is in a superconducting state, causing the operation of the SQUID to become unstable. In a multi-channel SQUID fluxmeter in which a plurality of SQUID sensors are arranged to measure a magnetic field distribution at one time, it is very important that the individual SQUIDs operate stably, and a digital SQUID fluxmeter is necessary to compensate for the effect of flux trapping.
The fundamental operation of the above digital SQUID fluxmeter has been mentioned in, for example, N. Fujimaki et al., "A Single-Chip SQUID Magnetometer", IEEE Trans. Electron Device, Vol. 35, No. 12, 1968, pp. 2412-2418. This literature also refers to a single-chip SQUID in which the counter unit and the feedback unit consist of superconducting circuitry.
As a matter of fact, however, since the magnetic flux trapped by the SQUID ring is compensated, the counter generates a corresponding offset quantity and thereby the counting range or the dynamic range of the counter is narrowed. Moreover, since the magnetic flux is trapped, it becomes difficult to correctly measure the external magnetic flux.
The object of the present invention therefore is to provide a digital SQUID fluxmeter which is newly equipped with a circuit that cancels the quantity which is produced by the counter and which is equal to the compensation of flux trapping, maintaining a maximum dynamic range, improved reliability, and stability at all times.
Another conventional SQUID fluxmeter picks up magnetic flux by using a pickup coil and transfers it to the SQUID via a flux transformer.
Examples of this kind of digital SQUID include a SQUID consisting of a two-junction quantum interference element which operates on AC bias to produce pulses (see Japanese Unexamined Patent Publication (Kokai) No. 63-29979) and a DC SQUID which operates in an analog manner and in which the voltage thereof is applied to a superconducting converter or to a one-bit A/D converter to obtain pulse outputs ( Drung, Cryogenics, Vol. 26, pp. 623-627, 1986).
Therefore, the feedback circuit is provided with an offset voltage control circuit that detects the trapped amount counted by the up-down counter and an offset voltage generator that applies an offset voltage to the feedback circuit in response to a signal from the offset voltage control circuit, in order to always compensate the offset amount caused by the flux trapping using the offset voltage control circuit and thereby to prevent the dynamic range from decreasing.
According to the above prior art, however, when the amount of magnetic flux to be fed back per pulse is decreased in an attempt to decrease the effect of quantization noise and to improve the sensitivity of the fluxmeter, it becomes no longer possible to compensate the trapped magnetic flux .PHI..sub.t despite the feedback current being fed from the D/A converter to the feedback circuit at the full-range. After the power source circuit is closed, therefore, the feedback operation is not carried out and the fluxmeter fails to work.