Absolute value of the modulation frequency around the lines of constant spin phase () as a function of time for the pulse sequences from Fig. 2. Horizontal error bars mark the edges of the ten-pulse samples used for f₃ calculation. Vertical error bars show the nominal frequency determination error taking into account the zero-padding. For each subsample, only f₃ with the highest S/N is plotted (see text for details on S/N estimates). Only peaks that were stronger than 99.5% of the simulated sample are shown. The vertical lines mark the mode edges. All points here correspond to negative except for the empty point for session L99010. For the PRAO sessions, the absence of f₃ points near the observation end is due to pulsar signal attenuation.