[en] The effect of the transit time of the atoms in cesium beam tube is analyzed, in the case of fast sine-wave and square-wave frequency modulations and in the case of square-wave phase modulation. Analytical expressions are given for the slope of the error signal and for the frequency stability. The effect of the modulation frequency and of the separation between the second interaction region and the atom detection zone is illustrated for the interaction time distributions encountered when optical methods are implemented for the state selection and the detection of the cesium atoms