Investigating the effectiveness of frequency measurement methods for spin echo enhanced proton precession magnetometers

The incorporating frequency measurement technique has a very important role in the determination of achievable sensitivity using NMR-based magnetometers for earth field measurements. This is while, the selection of the frequency calculation algorithm, determines the required resources of the digital processing hardware for such magnetometers. We have previously developed a proton precession magnetometer equipped with a spin echo mechanism that could measure the gradient of the magnetic field as well as the magnitude of the magnetic field. In this magnetometer, a switching field is used for making the spin echo which decreases the effective spin-spin relaxation time constant. The reduction of this time constant increase the decay rate of the NMR signal and so the frequency calculation for obtaining the Larmor frequency should be accomplished in a shorter time. In this work, different frequency calculation methods are investigated for achieving high-sensitivity earth field measurements. In this regard, both time domain and frequency domain analysis of free induction decay (FID) frequency calculation is implemented and compared. The effect of SNR and decay time constant of FID signal on the accuracy of Larmor frequency calculation is discussed. It is shown that frequency domain analysis is much preferred for frequency calculation especially when using the spin echo enhanced proton precession magnetometer.