Design and implementation of high-performance lock preamplifier for high-field nuclear magnetic resonance spectrometers

In this paper, based on the field-frequency lock technology commonly used in the current commercial nuclear magnetic resonance spectrometers to maintain the magnetic field homogeneity and the theory of weak signal detection, a lock preamplifier at the front-end of lock channel is designed and implemented. The preamplifier consists of broadband amplifier, RF switch and attenuation circuits. In the 20-200 MHz the fabricated preamplifier exhibits a gain of 53 dB and in-band fluctuation of 1 dB. The whole noise figure of receiving channel is about 1.5 dB. The delay of RF switch is less than 220 ns. For the transmitter channel the total attenuation range is 30 dB with a 2.0 dB minimum step size. Compared with the lock preamplifier of the commercial spectrometers, the developed preamplifier shows a lower noise figure, a smaller in-band fluctuation and a shorter delay of the switch. Therefore the preamplifier can be applied to nuclear magnetic resonance spectrometers from 200 MHz to 900 MHz.