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The gain of a widely used sequential-type phase/frequency detector (PFD) as a function of its input frequency is modeled adopting a continuous-time approximation. High-frequency gain degradation is described by a frequency-dependent attenuation factor characterized in terms of the propagation delays of the PFD. The attenuation factor can be readily included in the dynamics equations of phase-locked loop to account for the effect of the PFD degradation. Gain-modeling-related delay components of a complimentary metal-oxide-semiconductor PFD are also described. The high-frequency model is in good agreement with the simulation results of the PFD.