RF probe recovery time reduction with a novel active ringing suppression circuit

Abstract

A simple Q-damper device for active probe recovery time reduction is introduced along with a straightforward technique for the circuit's component value optimization. The device is inductively coupled to a probe through a coupling transformer positioned away from the main coil, which makes the design independent of the coil type being used. The Q-damper is a tuned circuit, which is resonant at the same frequency as the probe and can be actively interrupted. When the circuit is interrupted, it is detuned and, thereby, is uncoupled from the probe, which operates normally. Turning the device on leads to re-coupling of the circuits and causes splitting of the probe's resonance line, which can be observed through its drive port. A resistance of an appropriate value is introduced into the Qdamper circuit, resulting in smoothing of the resonance splitting into one broad line, representing the coupled system's low-Q state, in which the energy stored in the main coil is efficiently dissipated. The circuit's component values are optimized by monitoring the shape of this low-Q state. Probe recovery time reduction by, approximately, an order of magnitude has been obtained with this device. Application of the device during an NQR experiment led to an increase in the signal-to-noise ratio by a factor of 4.9.

Cu nuclear magnetic resonance study of charge and spin stripe order inLa1.875Ba0.125CuO4

“…The second important issue is ringing of the LC resonant circuit after the pulses, which often completely obscures the signal for tens of µs. Ringing can be suppressed in a number of ways, with several active damping schemes envisaged previously 27,28 . However, most of them do not operate quickly enough for our purposes, the only possibility thus being passive damping i.e.…”

TNT detection with 14N NQR: Multipulse sequences and matched filter

“…This is an effective but very primitive realization and calls for an improvement. We are in a process to build and active coil-ringdown suppression circuit [20], with same or shorter dead time, but larger Q . We believe that for TNT an optimal bandwidth is around 15 kHz, so that the target resonant circuit is Q $ 56.…”

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“…An example of the appropriate Q -switching design is described in Ref. [3], but we shall not go further into the details of Q -switching.…”

“…The problem is very pronounced at low x 0 and remedies in the form of active damping just after the RF pulse [2,3] are quite complicated. Second, the probe bandwidth is proportional to x 0 =Q and becomes very narrow at high Q , eventually, in the ''super-Q " limit [4] even narrower than the resonance line under investigation.…”