[en] At DELTA, a third generation 1.5 GeV synchrotron radiation facility, we use common schemes to damp longitudinal coupled bunch mode instabilities, including a bunch-by-bunch feedback system. We also utilize an RF-phase modulation of the master-RF at two times the synchrotron frequency, providing strong additional damping capabilities and a significant increase in beam lifetime of approximately 20%, due to bunch lengthening and reduced Touschek effect. Despite the apparent influence on beam lifetime and longitudinal beam stability the actual correlation between the phase modulation of the master-RF and its influence on the time dependent accelerating gradient has not been studied sufficiently yet. The presentation will give an overview on the existing modulation scheme together with first studies to provide a better understanding of the RF phase modulation impact on the small bandwidth accelerating cavity.