Towards a Model-Based Field-Frequency Lock for NMR

Nuclear Magnetic Resonance (NMR) relaxometry is a powerful technique that allows to investigate the properties of materials. More advanced NMR relaxometry techniques such as Fast Field-Cycling (FFC) require the magnetic field to reach any desired value in a very short time (few milliseconds) and field oscillations to stay within few ppms. Such specifications call for the introduction of a suitable Field Frequency Lock (FFL) system. FFL relies on an indirect measure of the magnetic field which can be obtained by performing a parallel NMR experiment with a known sample. In this paper we propose a PID controller to guarantee field fluctuations to stay below the desired level and short settling time. The tuning of the controller is based on a mathematical description of the entire process, which is validated by performing real experiments. Numerical simulations show promising results that we expect to be confirmed by real experiments.