Extremely rapid SQUID electronics

SQUIDs (Superconducting QUantum Interference Devices) are generally used for the sensitive measurement of small magnetic fields, but may very well also be used for the measurement of smallest currents. Thus, they are used, e.g. in medical diagnostic systems for the detection of the magnetic signals of the heart or the brain or as preamplifiers for certain types of radiation detectors such as microcalorimeters, which are operated at low temperatures as are the SQUIDs themselves. In addition to the SQUID chip, the superconductive, integrated sensor circuit, which is normally cooled with liquid helium, one requires readout electronics which amplifies the SQUID output signal at a low noise level and “feeds it back” into the SQUID as magnetic flux, so that the periodic flux voltage characteristic of the SQUIDs is linearised. In this way, the dynamic range of the SQUID which otherwise constitutes only fractions of a magnetic flux quantum, is extended to 10 to 100 flux quanta needed for typical applications. The flux-locked loop (FLL) thus set up determines the bandwidth of the sensor system. By using modern commercial FLL electronics, it has been possible up to now to attain closed-loop bandwidths up to 20 MHz. The bandwidth limitation results thereby from the signal propagation on the connection lines between the SQUID chip – which is located in the helium bath of a cryostat – and the electronic unit which is usually mounted at a distance, on the cryostat cover. Distinctly reduced line lengths and thus higher bandwidths can be obtained if also the FLL electronics is operated in liquid helium in the immediate vicinity of the SQUID chip. That means, however, that the electronics must be rated for a temperature of 4.2 K, which entails considerable effort in the development of the circuit.

At PTB, an FLL circuit cooled with liquid helium has now been developed which has, when operating with a PTB SQUID sensor chip, reached a system bandwidth of 350 MHz and has opened new perspectives for special applications, e.g. in the signal amplification of superconducting hot-spot photon counters. The new electronics, too, is to be made commercially accessible to users.