  An approximation to coherent sampling, also known as boot-strapped waveform
averaging, is presented. The method uses digital cavities to determine the
condition for coherent sampling. It can be used to increase the effective
sampling rate of a repetitive signal and the signal to noise ratio
simultaneously. The method is demonstrated by using it to directly measure the
fluorescence lifetime from rhodamine 6G by digitizing the signal from a fast
avalanche photodiode. The obtained lifetime of 4.4+-0.1 ns is in agreement with
the known values.
