The low-level radio frequency (LLRF) systems of FAIR synchrotrons have to support a number of longitudinal beam manipulations, such as multi-harmonic operations, barrier bucket generation and bunch compression in order to meet the desired beam quality requirements. This imposes several demands on the LLRF systems. For example, the resulting RF voltage provided by the RF systems must have a phase accuracy better than ±3 degrees under all conditions, which also includes the fast ramping. Thus, each individual component must have a much better accuracy.
I In this talk a phase calibration method of frequency-variable reference RF signals for the FAIR synchrotrons is presented. The described approach utilizes a sine-wave fit algorithm to estimate the phase of the sine-wave signals produced by reference generators in spatially separated supply areas. A detailed analysis of the measurement parameters is given and the recommended parameters for the given application are derived. The quality of the results obtained with the presented method is evaluated by means of measurement uncertainty analysis. The stability of the calibration is analyzed in order to specify the margins for the room temperature and the supply voltage of the reference generators within which their output phase stays sufficiently constant.