  In the Brazilian synchrotron light source, we observed that modulating the
phase of the accelerating fields at approximately twice the synchrotron
frequency suppressed remarkably well a longitudinal coupled-bunch mode of the
beam driven by a higher order mode (HOM) in one of the radiofrequency (RF)
cavities. In this work, we present the results of a set of systematic
measurements, in single and multibunch mode, aimed at characterizing the
effects of RF phase modulation on the beam. We compare those experiments with
the results of tracking simulations and of a theoretical model in which Landau
damping is the stabilizing mechanism that explains the suppression of the
longitudinal coupled bunch instability. We also measure the frequency of the
stable islands created in longitudinal phase space by phase modulation and the
longitudinal beam transfer function (BTF) as a function of the modulation
frequency and amplitude. The experimental results are in good agreement with
theoretical expectations.
