Design and construction of a liquid state NMR spectrometer

In this work we have designed and built a modular, customizable, low-cost and high-accuracy mixed analog-digital low-field NMR spectrometer. The operating frequency ranges between 18 and 24 MHz depending on the two kinds of magnets we have employed. This apparatus can further be improved in its abilities in future for the hyperpolarized chemical physics studies which is the next stage of the project. The developed setup has advantage of low development cost and portability with reliable signal detection and reproducible results with applications for benchtop or mobile magnetic resonance.

The design utilizes permanent magnets (0.44 T or 0.55 T) for generating static field while frequency synthesis, pulse generation and reception is performed via software-based frequency synthesizers and high speed oscilloscopes. The complete analog circuit comprises various components such as an RF transmitter with variable gain, transmitter enabling circuit and circuits for superheterodyne down conversion. The receiver chain is optimized for low-noise operation allowing the capability of picking up signals from test tubes of our samples comprising 5-10 mL of liquid, ranging in the nanovolts to microvolts regime. Free-Induction Decay (FID) from various samples are observed.