Efficient fast-recovery scheme for NMR pulse spectrometers

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Abstract
We describe an active damping circuit which uses a 90° phase-shift feedback loop to dampen the ringing of nuclear resonance probes just after the RF excitation. The damping does not operate during the reception of the nuclear signal and there is therefore no loss in efficiency or in sensitivity as would be the case for permanent damping. The use of a high voltage FET as a gated duplexer to isolate the RF transmitter after the pulse is also described. The circuit damper is especially useful at low frequencies and we have been able to reduce the recovery time of a series resonant circuit (Q = 50) to less than 7 μs. at 3 MHz using this feedback scheme and the gated duplexer

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