  The autocorrelation function (ACF) is a simple and useful tool that allows to
measure the average linewidth of spectra with great precision. Unlike the
cross-correlation function (CCF), the ACF can be used without having to rely on
weighted binary masks. It makes the ACF much easier to use. We analyze 7
Cepheids and present a new method to obtain very precise turbulent velocity
curves for 6 of them by using the ACF. We compare our Full Widths at
Half-Maximum (FWHM) curves as a function of the pulsational phase to those of
Nardetto et al. (2006) who used the Fe I 6056.005 {\AA} line only. We note a
significant improvement in the shape of the FWHM curves for all Cepheids by
using the ACF. From the FWHM curves, we measure microturbulence curves for 6
Cepheids by using a Gaussian approximation. Finally, we artificially degrade
the resolution of the spectra and add noise to further assess and highlight the
advantages of the ACF. The FWHM curves obtained with the ACF remain virtually
unchanged up to a degradation by a factor of 10. A degradation by a factor of
20 slightly affected the results but an average linewidth variation remains
easily detectable.
