  We describe an method that measures the unloaded quality factor $Q_0$, the
external quality factor $Q_E$, and the cavity detuning $\Delta\omega$ with a
recursive least-squares algorithm. It combines a large number of consecutive
measurements to successively improve an estimate of fit parameters that
asymptotically converges to the ``real'' values. Exploiting the large amount of
data acquired by a digital low-level RF system permits us to reach this
asymptotic regime in a moderate time frame of seconds to minutes. Simulations
show that the method works both for critically coupled and over-coupled
cavities. A new calibration method adresses very tight tolerances of the method
on system parameters.
