A method of operating a high-resolution NMR spectrometer comprising a DDS generator containing an NCOL for generating an LO frequency, wherein the frequency of the NCOL is defined by inputting a numerical value Z, is characterized in that this numerical value Z may assume only values which satisfy the equation Z=n·N/m, wherein Z, n, N, and m are integer and positive numbers, N is a power of 2 with a positive integer exponent, wherein said exponent represents the maximum number of bits during the calculation process, m is approximately 2·fs/&Dgr;B, n approximately m·fout/fs and m additionally a common integer divisor of n·N and fs is the clock frequency of the NCOL, &Dgr;B is the desired bandwidth with high spectral purity and fout is the output frequency of the NCOL. This method allows the use of a DDS generator even in the case where very high spectral purity is required, wherein in particular quantization noise is largely eliminated over the frequency range relevant for NMR measurements.