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Precisely determining the gap voltage and phase in an RF cavity is essentialfor the calibration of the LLRF feedbacks. Following the conventional approach,measured RF power is converted into gap voltage, assuming a given shuntimpedance. However, power and impedance evaluations can both have largeuncertainties. Alternatively, the voltage can be obtained precisely with atechnique based on longitudinal phase-space tomography. From a set of bunchprofiles, tomography reconstructs the bunch distribution in the longitudinalphase-space. The quality of the reconstruction strongly depends on the RFvoltage and therefore allows to derive its absolute value. In this paper wedescribe the tomography-based voltage measurements performed in the CERN PSBand SPS, where this method allowed to detect significant voltage errors for themain RF systems. After applying the correction factors in the LLRF, 1\%accuracies were reached. We report here also the remarkable results achieved byusing this technique to calibrate the voltage of the SPS higher-harmoniccavities at 800 MHz, as well as their relative phases with respect to the 200MHz cavities.

High-precision RF voltage measurements using longitudinal phase-space tomography in CERN PSB and SPS