For physical processes which express themselves as a frequency, for example magnetic field measurements using optically-pumped alkali-vapor magnetometers, the precise extraction of the frequency from the noisy signal is a classical problem. We describe herein a frequency measurement system based on an inexpensive commercially available computer sound card coupled with a software single-tone estimator which reaches Cram\'er--Rao limited performance, a feature which commercial frequency counters often lack. Characterization of the system and examples of its successful application to magnetometry are presented.
@article{arxiv.physics/0506068,
title = {A sound card based multi-channel frequency measurement system},
author = {S. Groeger and G. Bison and P. E. Knowles and A. Weis},
journal= {arXiv preprint arXiv:physics/0506068},
year = {2009}
}