Suppression of ringing in the tuned input circuit of a SQUID detector used in low-fieldNMR measurements

Abstract

In low-field NMR measurements, we employ a high temperature superconductingquantum interference device (SQUID) as a detector with an inductively coupledliquid-nitrogen-cooled LC tuned input circuit. However, ringing across the LCcircuit appears after the sudden switch-off of the prepolarizing magnetic field. Thisringing leads to instability of the SQUID readout and prevents the acquisition ofshort-relaxation-time signals. We developed and tested two simple and effective FET-basedQ switch circuits with adjustable parameters which suppress the ringing. Each of theseQ switches makes it possible to record free induction decay signals with a Larmor frequencyof 1.2 kHz and an effective relaxation time constant of 30 ms. A gradually changing currentcaused by the release of charges stored in the p–n junction of the FET, which delays theQ value recovery of the LC circuit, can only be observed by the SQUID because of itsfrequency-independent sensitivity.