A Portable NMR System with 50-kHz IF, 10-us Dead Time, and Frequency Tracking

Abstract

This paper presents a portable nuclear magnetic resonance (NMR) system with significantly enhanced capabilities. Unlike prior works that use the same clock frequency for TX excitation and RX LO, this work uses two separate frequencies with a 50-kHz intermediate frequency (IF) that break the tradeoff between on-resonance excitation and 1/f-noise & offset suppression. It also proposes an accurate but low-cost method to ensure two clocks' phase coherence, which is necessary for NMR time-domain averaging. Moreover, this work addresses the critical limitation of long RX dead time (∼1ms) in prior works. By dynamically adjusting high-pass corner frequencies of IF filters and amplifiers, it shortens the dead time by 100 times to only 10us. Additionally, an automatic frequency tracking technique is devised to address the magnetic field drift problem. This work also reports the highest integration level achieved for an NMR transceiver, with on-chip ADC and DLL. Various measurements have been performed for system validation.