A Broadband NMR Magnetometer System With Field Searching and Automatic Tuning Function

In this article, we present a CMOS-based broadband nuclear magnetic resonance (NMR) magnetometer system with magnetic field searching and automatic tuning capabilities. The NMR front-end consists of a custom-designed NMR transceiver application-specific integrated circuit (ASIC) and an external $LC$ resonance circuit formed by an NMR coil and a digitally tunable capacitor (DTC). With a 1-mm NMR coil, the system is designed to measure the magnetic fields between 0.54 and 2.54 T with nanotesla resolution. To this end, the system automatically sweeps its excitation frequency to acquire an NMR signal from a known sample inside the NMR coil. The measured frequency of the NMR signal, together with the known Larmor frequency of the sample, can then be used to determine the unknown magnetic field with very high precision. The presented system provides an optimum sensitivity over its entire frequency range by automatically tuning the $LC$ resonant circuit to the current Larmor frequency using the DTC. With a 0.78- $\mu \text{L}$ sample of vegetable oil, the presented NMR magnetometer achieves an excellent field resolution of 1.77 nT inside a 1.45-T benchtop MR magnet, corresponding to a relative resolution of 1.2 ppb, which is limited by the field stability of the magnet. With its intrinsic frequency noise of 15 mHz, measured using a synthetic input signal, the presented sensor can support magnetic field resolutions down to 242 pT within a 1.45-T base field.