Pulsed and continuous-wave magnetic resonance spectroscopy using a low-cost software-defined radio

Software-defined radios (SDRs) constitute a modern and highly adaptive realization of a radio-frequency transceiver system. This work demonstrates how a particular radio transceiver, namely, the LimeSDR board, can be configured for pulsed and continuous-wave magnetic resonance spectroscopy. As a first step, the board needed to be extended by a bit pattern generator, so as to facilitate synchronization of other spectrometer equipment. The upgraded board was incorporated into two different spectrometers, namely, into a pulsed spectrometer operating at either 30 MHz for nuclear spins or 150 MHz for electron spins and into a continuous-wave spectrometer at 150 MHz for electron spins. Pulse sequencing capabilities were approved by relaxometry and Rabi oscillations of electron spins at a time resolution of 33 ns. Upon exhaustive averaging of acquired transients, unwanted oscillations that are characteristic for fast analog-to-digital converters emerged out of the noise floor. Methods for cancellations of these oscillations are presented, in particular a new acquisition scheme with cyclic incrementation of the acquisition position. The continuous-wave spectrometer provided derivative spectra of absorption and dispersion by phase-synchronous sideband demodulation. Furthermore, this spectrometer featured a software-defined automatic frequency control to account for probe drift, which has been incorporated into the open-source spectrometer control software.