FAST RECOVERY, HIGH SENSITIVITY NMR PROBE AND PRE-AMPLIFIER FOR LOW-FREQUENCIES

The design of a probe-preamplifier combination which reduces the NMR receiver recovery time following a pulse by a factor of at least twenty at 5 MHz is described. The preamplifier, which has a noise figure of 1.3 dB, employs negative feedback to damp the probe coil (a typical effective Q is 7), but maintains the signal-to-noise ratio that one would expect from an undamped coil. In addition, the use of a short phase-inverted pulse following the main transmitter pulse serves to reduce drastically the ring-down of the probe for those high voltages where the preamplifier is inoperative. Optimum utilization of transmitter power in achieving fast pulse rise times is also discussed, as well as elimination of phase-glitch" by proper tuning and the use of a broadband transmitter."