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A direct method for incorporating S-parameters into an finite-difference time-domain (FDTD) algorithm is presented in this paper. The frequency-domain S-parameters of an arbitrary network to be embedded within an electromagnetic system are represented as a series of modified impulse responses using a nonuniform discrete-time technique. In this way, the currents passing through the embedded network can be calculated using a convolution in the time-domain. The calculated currents are used to update the electric fields at the locations occupied by the embedded network according to Ampere's law. The simulation results using this method have been compared with the results simulated by Lumped-Element FDTD method (LE-FDTD), showing very good agreement. The method presented here is highly accurate and stable, and is suitable for use in quite general cases, especially where the embedded network cannot easily be represented by lumped elements.