This paper demonstrates that Ramp-Tracking Filter (RTF) can affect the frequency characteristics of power system stabilizers, in both magnitude and phase, over the frequency range of electromechanical power swings. In particular, the phase response impact of RTF needs more attention. Factors that influence the RTF effect are investigated in this paper. These factors include RTF order, loading condition of generator and its governor-turbine system, and the inertia-dependent parameter of PSS. Potential situations where a significant alteration of frequency response can occur are identified.