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

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