Portable CMOS NMR System With 50-kHz IF, 10-μs Dead Time, and Frequency Tracking

In this work, we report the portable NMR system which provides solutions to the existing problems in miniature CMOS NMR systems. First, a higher IF of 50 kHz is achieved by exploiting separate frequencies for sample excitation and LO, thereby having higher immunity to 1/f noise. The proposed phase detector circuit aligns the experiment trigger with the constant phase difference between two frequencies. It maintains the constant RX output phase in every experiment, allowing the direct signal averaging without extra phase correction. In addition, 40X faster RX recovery with RX BW switching enables the signal acquisition with the dead time of 10
$\mu \text{s}$
. Fast settling improves the acquisition sensitivity. Lastly, a frequency calibration method based on signal peak detection is proposed. Combining the techniques with the fully integrated NMR transceivers, the system demonstrates the NMR experiments to measure the decaying time constant of less than 100
$\mu \text{s}$
. Furthermore, NMR diffusion experiments are performed with the echo spacing of
$0.2\sim 8$
ms.