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A high resolution method of RF phase adjustment and test is proposed for the Crab Cavity system of the ILC. The method is based on beam as ultimate test instrument. To measure phase imbalance in the pair of crab cavities (<0.02deg at 1.3GHz is required), a low energy (~1GeV) beam is used. A bunch center-of-mass trajectory through the cavities spaced (n+1/2) RF wavelengths and excited as in the case of the ILC, is a straight line for phase-balanced cavities and gets a kick when unbalanced. The kick is measured by two spaced BPMs with reference to the initial trajectory angle measured by two other BPMs. The method is insensitive to a bunch arrival time jitter and RF phase Common Mode jitter. A prototype of the test bench based on the method, is proposed. Using a 10MeV beam, two simple dipole cavities and low RF power, the prototype can be utilized for mastering high resolution measurements, for adjustment and tests of low level electronics of the Crab Cavity system and RF systems of XFEL ERLs as well. The phase resolution of the prototype is estimated as 0.01deg and the amplitude resolution as 0.01%.
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