Analog-to-digital converters (ADC’s) are increasingly replacing mixers in frequency conversion schemes. To achieve superior performances, in terms of bandwidth and dynamic range, a nearly ideal ADC clock is needed, with a spectral purity higher than the reference signal of the classical mixing scheme. These requirements of spectral purity for the ADC clock are discussed by analyzing in detail the nonuniform sampling process and by characterizing an actual acquisition system. The effect of clock phase imperfections on the output is proportional to the input frequency over sampling frequency ratio. Moreover, at the output we may have a multiple folding of the phase jitter spectrum. These effects are illustrated by three sets of measurements performed using our system: transfer of spurious clock components, aliasing of these components, and transfer of clock phase noise.