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In this paper a study of the effect of integral pulse frequency modulation (IPFM) on single-input single-output feedback control is attempted. For zero input such systems can be reduced to a nonlinear discrete system. Lagrange stability concepts are used for the stability study of such systems. A step-by-step procedure is devised for the construction of the state trajectories of the IPFM system. This has been applied to a second-order plant where it is shown that instability, asymptotic stability in the large, and asymptotic stability in the Lagrange sense are exhibited by such systems. It is also shown that in IPFM systems, the periodic oscillation that exists depends on the initial state. The equivalence concepts of such systems are reviewed critically, and the limitations of the method are pointed out. Further research in this area of feedback modulation is proposed and discussed.