Simple, fast recovery, low-noise receiver amplifier for pulsed NMR experiments

A fast recovery, low-noise receiver designed for use in pulsed NMR, especially high-resolution multiple-pulse NMR, is described. This receiver is easy to build from commercially available parts. Passive (but biased) voltage limiters placed before each amplifier in the cascade prevent any of the amplifiers from saturating, so it is only the limiters that ever saturate. Recovery times of 850 ns and 2.35 ms are measured with a 50-W load and a high Q (Q = 141) resonant sample probe in the circuit, respectively. The receiver has a measured overall noise figure of 6.5 dB, including crossed diodes and filter on the input. An example of the use of the receiver to measure a short proton free-induction decay at 186.5 MHz is also shown.