Non-Adiabatic Rapid Scan (NARS) was introduced as a method to obtain pure-absorption EPR spectra while maintaining the thermal equilibrium condition of continuous-wave EPR. Using modern digital detection, the traditional lock-in amplifier is replaced by digital filtering. For the same collection time, NARS spectroscopy has a factor of 2 in SNR compared to optimally chosen field modulation. For a spectra with disparate linewidths or narrow hyperfine features, the SNR increase is more.
Due to the relatively large 3 dB bandwidth (33 MHz) of the micro-helix, combining NARS with arbitrary-waveform generator (AWG) frequency sweeps allows for a 1 mT equivalent sweep. Additional signal averaging occurs when field steps are used and spectra are collapsed together. Segmented NARS has an added benefit of removing low frequency baseline.
Frequency sweep has a number of advantages compared to field sweep modulation: