Phase cycling

The ability to select particular coherence pathways in a multipulse experiment is key to the success of the majority of NMR (and pulsed EPR) experiments. One method of achieving this is by varying the phases of the RF pulses, the receiver, and digitizer in a predetermined pattern during signal averaging such that the desired pathway is retained while undesired routes are rejected. Phase cycling also has an important role to play in frequency sign discrimination in the indirect dimensions of multidimensional spectra and in the suppression of spectral artifacts. This article describes the theory behind phase cycling, using some simple examples, and describes how frequency discrimination schemes may be implemented. Techniques for optimizing the lengths of phase cycles are also discussed.