Phase enhancement for precise determination of small radiofrequency phase angles

For multiple-quantum NMR, digital phase shifters generate the nonquadrature phase shifts. Though the required phase increments must be rather small (ca. 1.4°) to separate very high-order coherences, these increments must also be set to great precision since, compared to a single-quantum coherence, an n-quantum coherence is n times more sensitive to any phase error. This paper indicates qualitatively the effect of digital phase shifter errors on multiple-quantum spectra and demonstrates a highly accurate method for measuring the relative phase of small rf phase increments. The method relies on frequency multiplication to enhance these small phase shifts.