This work presents a prototype system for detecting nuclear magnetic resonance (NMR) in the homogeneous air gap of a permanent magnet with a magnetic flux density of 0.3 T. The system integrates hardware and firmware: a Si5351A-based carrier source with a 26 MHz TCXO operated in integer-PLL mode and providing two quadrature outputs; a measurement probe with a tuned LC resonator; a passive diode mixer for I/Q conversion; a low-noise amplification and filtering chain. The static field is modulated at 70 Hz, with the sinusoid synthesized by a 12-bit DAC using a lookup table. The firmware controls the frequency, and computes the magnetic field. The RF section is mounted in an aluminium enclosure. An upgrade is proposed toward autonomous detection, and tracking of the resonance based on the error signal from synchronous detection, which would reduce the influence of magnet drift.
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