Low-frequency NMR with a non-resonant circuit

Nuclear magnetic resonance typically utilizes a tuned resonance circuit with impedance matching to transmit power and receive signal. The efficiency of such a tuned coil is often described in terms of the coil quality factor, Q. However, in field experiments such as in well-logging, the circuit Q can vary dramatically throughout the depth of the wellbore due to temperature or fluid salinity variations. Such variance can result in erroneous setting of NMR circuit parameters (tuning and matching) and subsequent errors in measurements. This paper investigates the use of a non-resonant transmitter to reduce the circuit sensitivity on Q and demonstrates that such circuits can be efficient in delivering power and current to the coil. We also describe a tuned receiver circuit whose resonant frequency can be controlled digitally. Experimental results show that a range of common NMR experiments can be performed with our circuits.

Graphical abstractThe illustration shows a non-resonant NMR transmitter circuit based on four switches. Waveforms used to control the switches are also shown.Download high-res image (44KB)Download full-size imageResearch highlights► We use a non-resonant NMR transmitter to reduce the circuit sensitivity to coil Q. ► We show that this circuit can efficiently deliver current to the NMR coil. ► We describe a digitally-tunable low-noise receiver circuit. ► A range of common NMR measurements has been performed with these circuits.