We demonstrate the applications of discrete-time signal-processing (SP) techniques for the purpose of generating accurate plane waves in the finite-difference time-domain (FDTD) method. The SP techniques are used either to reduce the high-frequency content of the source excitation or to compute more precise incident-field values in the computational domain. The effects of smoothing windows of various lengths, digital lowpass filters of various bandwidths and characteristics, and polynomial interpolation schemes of various orders are investigated. Arbitrary signals with multifrequency content are considered.