Goryashko, V.A. Dancila, D.S. Gajewski, K.J. Hermansson, L. Li, H. Nicander, H. Ruber, R.J.M.Y. Santiago Kern, R. Teerikoski, S. JACoW Geneva, Switzerland 978-3-95450-178-6 10.18429/JACoW-SRF2015-TUPB089 English 810-813 TUPB089 cavity coupling impedance network SRF Contribution to a conference proceedings 2015 2015-12 http://dx.doi.org/10.18429/JACoW-SRF2015-TUPB089 http://srf2015.vrws.de/papers/tupb089.pdf In this paper we propose a high-precision method of measuring Q0 of SRF cavities. A common way to study the performance of an SRF cavity is to build an oscillator around it that is referred to as a self-exciting loop. In the standard approach, by tuning the loop phase for a maximum field level in the cavity and measuring forward and reflected waves, one finds the cavity coupling. Then, performing a time-decay measurement and finding the total quality factor, one gets Q0. However, this approach suffers from a deficiency originating from a single data-point measurement of the reflection coefficient. In our method by varying the loop phase shift, one obtains amplitudes of the reflection coefficient of the cavity as a function of its phases. The complex reflection coefficient describes a perfect circle in polar coordinates. Fitting the overdetermined set of data to that circle allows more accurate calculation of Q0 via the least-squares procedure. The method has been tested at the FREIA Laboratory on two cavities from IPN Orsay: a single spoke and a prototype ESS double spoke.