Self-switching Q-damping circuit for reducing transmitter ringdown time in high power pulse NMR

In this paper we describe a circuit for reducing the transmitter ringdown time and thereby improving the recovery time in pulse NMR experiments. The circuit uses only solid state devices and requires no external switching. For transmitter voltages less than 0.5 V peak to peak the effective resistance in parallel with the transmitter coil in a crossed coil spectrometer is only 6 {} whereas for the larger voltages the shunting resistance is of the order of 1-2 {}. This circuit thus has the effect of significantly squaring the envelope of an rf pulse. It has the extra advantage of suppressing noise generated by the transmitter during the time interval between pulses.